Hubble Space Telescope

The Hubble Space Telescope (HST) is a space telescope jointly developed by NASA and ESA and named after the astronomer Edwin Hubble. It operates in the electromagnetic spectrum from the infrared range to visible light and the ultraviolet range. The mirror diameter is 2.4 meters.

Type	Space telescope
Country	United States Europe
Operator	NASA ESA
COSPAR ID	1990-037B
Mission data
Mass	11,600 kg
Size	Length: 13.1 m Diameter: 4.3 m
Start	April 24, 1990, 12:33 UTC
Launch location	Kennedy Space Center, LC-39B
Launch vehicle	Space Shuttle Discovery
Status	active
Orbit data
Period	95.4 min
Orbital inclination	28,5°
Apogee height	549 km
Perigee height	545 km

The HST was launched on April 24, 1990, on the Space Shuttle mission STS-31 and released the next day from the hold of the Discovery. It was the first of four NASA space telescopes as part of the Great Observatory program. The other three space telescopes are Compton Gamma Ray Observatory, Chandra X-Ray Observatory and Spitzer Space Telescope.

After the telescope was suspended, the image quality did not meet expectations. An error in the primary mirror resulted in images that were practically unusable. Three years later, in 1993, the error was successfully corrected using the COSTAR mirror system (Corrective Optics Space Telescope Axial Replacement). After this first repair mission STS-61, there were other maintenance missions: STS-82, STS-103, STS-109 and STS-125. With the fifth and final maintenance mission in May 2009, the COSTAR correction became superfluous, as all instruments had their own system for correcting the mirror error.

Mission objectives of the Hubble Space Telescope

The Hubble Space Telescope was created primarily to circumvent the constraints of Earth’s atmosphere. The molecules of the atmosphere limit the resolution of telescopes on the Earth’s surface, and various spectral ranges are blocked. The space telescope was to achieve an unprecedented resolution. The mission objectives are therefore extremely broad and include practically all essential objects and phenomena of the universe:

• Planets in the Solar System and Beyond (exoplanets)
• Stars
• Fog of all kinds
• Black holes and their surroundings
• Galaxies at almost any distance and age
• Dark Matter and Dark Energy
• Age of the universe

Prehistory

The first serious concept of a scientific telescope in Earth orbit was presented by Lyman Spitzer, then a professor at Yale University, in 1946. In the scientific publication Astronomical Advantages of an Extra-Terrestrial Observatory, he described the then inevitable disturbances caused by the Earth’s atmosphere, which limited the resolution of any capable ground-based telescope. In addition, the atmosphere also absorbs all X-rays, making it impossible to observe very hot and active cosmic events. As a solution, he proposed a telescope in Earth’s orbit outside the atmosphere.

Sometime later, the National Academy of Sciences approached Spitzer, who was now teaching at Princeton University, to serve as head of an ad hoc committee to design a Large Space Telescope. “Large Space Telescope”). During the first meeting in 1966, extensive studies were made for the use of such a telescope. Three years later, a paper entitled Scientific Uses of the Large Space Telescope was published. “Scientific Uses of the Large Space Telescope”), in which the committee called for the construction of such a telescope because it could make a “significant contribution to our knowledge of cosmology”.

To realize this project, NASA was turned to because no other organization had the means and capabilities for such an ambitious project. It had already carried out several internal studies, including under the direction of Wernher von Braun, on space telescopes, all of which, however, were planned with smaller mirrors. The decision to build the Space Shuttle in the mid-1960s gave the necessary flexibility to further develop the existing designs. In 1971, NASA Director George Low created the Large Space Telescope Science Steering Group to conduct the first feasibility studies.

Meanwhile, the Orbiting Astronomical Observatory satellites achieved significant successes, giving a boost to proponents of a large space telescope. The satellites worked mainly in the ultraviolet range and had telescopes with 30.5 to 97 cm large primary mirrors. In 1983, IRAS was launched as a telescope with a mirror diameter of 60 cm for infrared observation. The technical predecessors of the Hubble telescope are the KH-11 Kennan spy satellites of the National Reconnaissance Office; they were launched from 1976 to 1988 and have a primary mirror comparable to the Hubble telescope.

Next, government funding for the project had to be secured. Due to the high price of 400 to 500 million (about two billion according to today’s value) US dollars, the first application was rejected in 1975 by the Budget Committee of the House of Representatives. Intensive lobbying began under the leadership of Spitzer, John N. Bahcall, and another leading astronomer from Princeton. In addition, ESRO (a predecessor organization of ESA) was approached to finance the solar cells, in return for observation time and scientific assistance. In the same year, the latter announced its consent. By additionally reducing the size of the primary mirror from 3.0 to 2.4 meters, the price could be reduced to about 200 million US dollars. The new concept was approved by Congress two years later, allowing work on the new telescope to begin.

In 1978, the most important contracts were awarded: PerkinElmer was to design the optical system including the primary mirror, Lockheed was responsible for the structure and the satellite bus, whereby the solar cells and an instrument (the Faint Object Camera) were to come from European production. Due to the importance of the primary mirror, PerkinElmer was also instructed to hire a subcontractor to produce a spare mirror in case of damage. Eastman Kodak was chosen, which opted for a more traditional manufacturing process (PerkinElmer used a new laser and computer-aided grinding process). Although both mirrors later turned out to pass faulty quality control, according to some scientists, the Kodak brand was the better one.

Nevertheless, PerkinElmer decided to use his own mirror. Originally, the telescope was to be launched in 1983. This deadline could not be met due to delays in the construction of the optics, the final readiness for launch was reached in December 1985. In the meantime, in 1983, the Space Telescope Science Institute was founded at Johns Hopkins University, which was to take over the operation of the new telescope as part of the Association of Universities for Research in Astronomy. In the same year, it was renamed the Hubble Space Telescope (HST) after Edwin Hubble, the discoverer of the expansion of the universe.

Start

After internal problems had delayed the launch by two years, the new launch date of October 1986 could not be met. The reason for this was the Challenger accident on 28 January, in which all seven astronauts died due to material failure on one of the solid boosters. Since Hubble was to be transported by the Space Shuttle, the launch was delayed for another four years due to the extensive improvement measures on the other space shuttles.

On April 24, 1990, at 12:33 UTC, the space shuttle Discovery finally launched with the telescope on board from launch complex 39B of the Kennedy Space Center in Florida. The mission, designated STS-31, went smoothly despite the record altitude of 611 km, the telescope was successfully deployed the next day and was activated on schedule.

The Primary Mirror Fault

Although quality assurance measures were taken after the primary mirror was manufactured, massive image errors were discovered at first light (see picture). According to the specifications, for a point target (for example, a star), 70% of the light should be concentrated within 0.1 angular seconds. In fact, it was distributed over 0.7 angular seconds, which massively lowered the scientific value of the telescope. Subsequent measurements with the help of the Wide Field/Planetary Camera, the Faint Object Camera and the wavefront sensors of the three Fine Guidance Sensors showed with high certainty a strong spherical aberration due to unevenness on the primary mirror.

When it became clear that this was a large and complex error, NASA Director Richard Harrison Truly ordered the formation of a Hubble Space Telescope Optical Systems Board of Investigation to further narrow down and correct the error. The investigations focused on an instrument used in manufacturing for quality control that should have indicated spherical aberration: the null corrector, a fairly simple optical instrument that projects a special wavefront onto the mirror which, if ground correctly, is reflected as an exactly circular pattern. On the basis of deviations from this circular pattern, it can be seen whether and to what extent polishing and grinding work is still necessary.

For reliable results, however, the lenses installed in the zero corrector must be aligned and adjusted with high precision. When examining the original corrector, which had been stored after the delivery of the primary mirror, it was found that one lens was 1.3 mm too far away from another. Subsequently, computer simulations were carried out that calculated the effects of this error on the primary mirror. The type and extent of the results matched the observed imaging errors of the telescope in orbit very precisely, so the wrong lens distance in the corrector was ultimately responsible for the primary mirror error.

In the further course of the investigations, a large number of omissions and obstructive structures were uncovered in the area of quality assurance:

• Quality assurance personnel were not integrated into the project team.
• There were no independent audits by neutral field offices.
• For the tests, there were no documented criteria for distinguishing between failure and success.
• Quality assurance employees had no access to large parts of the proofreader’s production.

Because PerkinElmer used some new and largely untested computer-based techniques in the production of the primary mirror, NASA had commissioned Kodak to produce a reserve mirror made by more traditional means. However, since the spherical aberration at PerkinElmer was not detected before launch, the Kodak mirror remained on Earth. After discovering the error, it was therefore considered to recapture Hubble with a shuttle and replace the mirror with the Kodak brand. However, this proved to be extremely complex and expensive, which is why a correction system was developed that corrects the primary mirror error before the collected light reaches the instruments.

It is called the Corrective Optics Space Telescope Axial Replacement (COSTAR) and was installed two and a half years after launch during the first service mission. Only after this mission was the telescope able to start its scientific operation without significant problems. However, COSTAR occupied one of five instrument bays that was actually intended for scientific systems (specifically, the High-Speed Photometer (HSP) had to be removed during installation). Therefore, all subsequent instruments were equipped with their own internal correction systems, so that they could obtain the light directly from the primary mirror without detour via COSTAR. During the last fourth service mission, the system was removed and replaced by a scientific instrument, the Cosmic Origins Spectrograph (COS). Since then, the primary mirror error no longer plays a role.

The Service Missions of the Hubble Telescope

	Start	SM 1	SM 2	SM 3A	SM 3B	SM 4
Date	Apr 1990	Dec 1993	Feb 1997	Dec 1999	Mar 2002	May 2009
Mission Shuttle	STS-31 Discovery	STS-61 Endeavor	STS-82 Discovery	STS-103 Discovery	STS-109 Columbia	STS-125 Atlantis
Orbital height Reboost	618 km	590 km + 8 km	596 km +15 km	603 km	577 km + 6 km	567 km
Instr. 1	WF/PC	WFPC2				WFC3
Instr. 2	GHRS		STIS			STIS (R)
Instr. 3 (axial pos.)	HSP	COSTAR				COS
Instr. 4	FOC				ACS	ACS (R)
Instr. 5	FOS		NICMOS		NICMOS cooler
Gyroscopes	6	4 (R)	2 (R)	6 (R)	2 (R)	6 (R)
Photovoltaics	SA1	SA2			SA3

The Hubble telescope was designed from the outset for maintenance in orbit, allowing a total of five space shuttle missions for repair and upgrade. In the following, these are listed and described, the exact technical modifications can be found in the correspondingly linked sections.

Service Mission SM 1

• Mission Number: STS-61
• Period: December 2, 1993 (09:27 UTC) to December 13 (05:25 UTC)
• Number of EVAs: 5
• Total EVA time: 28.5 hours

The primary goal of the first service mission was to correct the optical error of the primary mirror. For this purpose, the high-speed photometer instrument was removed and replaced by the COSTAR lens system, which could provide all other instruments with a correct and error-free image. However, the New Wide Field and Planetary Camera 2, which replaced its predecessor, already had its own correction system and was therefore not dependent on COSTAR. This should be removed in the long run in order to be able to use the space scientifically again, which is why all subsequent newly installed instruments were equipped with their own design to correct the primary mirror error.

In addition, several other technical systems were replaced, modernized and maintained. New solar wings were installed, as the old ones deformed too much under frequent temperature changes. In the area of attitude control, two magnetic field sensors, two measuring systems for the gyroscopes and their fuses were replaced. In addition, the main computer received an additional coprocessor system.

Service Mission SM 2

• Mission Number: STS-82
• Period: February 11, 1997 (08:55 UTC) to February 21 (08:32 UTC)
• Number of EVAs: 5
• Total EVA time: 33.2 hours

The primary goal of the second service mission was to replace two sensors. On the one hand, the Goddard High-Resolution Spectrograph was replaced by the Space Telescope Imaging Spectrograph, on the other hand, the Faint Object Spectrograph was expanded for the installation of the Near Infrared Camera and Multi-Object Spectrometer. As a result, the resolution and spectral accuracy could be massively increased and it was possible for the first time to carry out observations in the infrared range.

Extensive modernization and maintenance work was also carried out on the technical systems. In the area of attitude control, a fine guidance sensor was replaced by a newly certified and calibrated model, the OCE-EK system was retrofitted to better maintain alignment accuracy and one of the four reaction wheel assemblies was replaced. In addition, two of the three tape storage systems have been serviced, and the third has been replaced by a much more powerful solid-state recorder. Furthermore, a data interface unit and the alignment system for one of the two solar wings were replaced. Finally, the insulation of the telescope was repaired unscheduled during the last spacewalk, after considerable damage had been detected. Here, reserve materials were used, which were actually intended for a possible repair of the solar wings.

Service Mission SM 3A

• Mission Number: STS-103
• Period: December 20, 1999 (00:50 UTC) to December 28 (00:01 UTC)
• Number of EVAs: 3
• Total EVA time: 26.1 hours

Originally, there was to be only one mission called “SM 3”, in which improved scientific instruments were to be installed again. However, the RWAs needed for alignment proved to be unexpectedly unreliable. After the third of a total of six gyroscopes failed, NASA decided to split the mission in two. In the first SM-3A mission, mainly new gyroscopes were to be installed, in the second SM-3B mission, the installation of the new instruments was planned. On November 13, 1999, just over a month before the planned launch of the first mission, the onboard electronics put the telescope into a safety state that only guaranteed the operation of the most important technical systems. The reason was the failure of a fourth gyroscope, so that only two pieces were functional. For proper operation, however, at least three copies were necessary, so the scientific operation of the telescope was no longer possible.

During the first spacewalk, all three reaction wheel assemblies and a fine guidance sensor were immediately replaced with new models, making Hubble operational again. In addition, other technical systems were later maintained or upgraded. For example, the old DF-224 central computer was replaced by a much more powerful model, and another tape drive was replaced by an advanced solid-state recorder. Voltage/Temperature Improvement Kits have also been installed on the accumulators to improve the charging process. Also, a defective S-band transmitter was replaced with a new one, which was a very time-consuming and complex operation, as such an exchange was never intended and was not part of the ORU concept. Finally, the improvised thermal shielding of Mission SM 2 was removed and replaced by two newly manufactured devices.

Service Mission SM 3B

• Mission Number: STS-109
• Period: March 1, 2002 (11:22 UTC) to March 12 (09:32 UTC)
• Number of EVAs: 5
• Total EVA time: 35.7 hours

After only repair and maintenance work had been carried out on the SM 3A mission, the telescope also received a new scientific instrument with the SM-3B mission: the Advanced Camera for Surveys. It replaced the Faint Object Camera and extended Hubble’s spectral range into the distant ultraviolet range. In order to restore the capacities in the infrared range, the NICMOS instrument was equipped with an additional cooling system that works permanently and does not become ineffective after a certain time. With the installation of new, much more efficient solar wings, the telescope also had about a third more electrical energy at its disposal, allowing four instead of two scientific instruments to work in parallel. To make this possible, the Power Control Unit, which is used for central power distribution, also had to be replaced. In addition, a SHEV was replaced again and another device for insulating the telescope was installed.

Service Mission SM 4

• Mission Number: STS-125
• Period: May 15, 2009 (18:01 UTC) to May 24 (15:39 UTC)
• Number of EVAs: 5
• Total EVA time: 36.9 hours

During this last service mission, extensive upgrade and lifetime extension measures were once again taken to ensure the operation of the telescope for as long as possible. Thus, the Wide Field Planetary Camera 2 was replaced by a modernized model called Wide Field Camera 3, which allowed the COSTAR system to be removed, as all instruments now had internal methods for correcting the mirror error. At its position, the Cosmic Origins Spectrograph was installed, giving the telescope a dedicated spectrograph again.

In addition, repairs were necessary to two other instruments: the Advanced Camera for Surveys, which had been virtually unusable since July 2006 due to a failure in the internal electronics, and the Space Telescope Imaging Spectrograph, whose power system failed in August 2004. Both instruments could have been easily removed as a whole, but it was decided to try to repair them in space, even if this was not provided for in the design. Despite the complex processes – at the ACS alone, 111 screws had to be loosened, partly with specially made tools – both repairs were successful, so that the instruments can work again (whereby one of the three sensors of the ACS was not repaired and is still defective).

In addition to the instruments, many technical systems were maintained. All six gyroscopes and all three accumulator modules were replaced by new models. Finally, the last three remaining NOBL protective panels and a soft capture mechanism were installed on the outer skin. The latter is located at the rear of the telescope and allows easy docking of another autonomous spacecraft. In this way, a targeted and safe re-entry into the Earth’s atmosphere should be made possible after the telescope is switched off at the end of its lifetime.

The future of the Hubble Telescope

After more than 30 years of operation, the Hubble telescope is now showing clear signs of wear. Some components have now failed and you had to switch to the backup systems. Hubble will likely be able to be used for research purposes until at least 2026.

As a replacement for the Hubble telescope, the James Webb Space Telescope was launched on 25 December 2021 and has been in operation since July 2022. It has a mirror more than five times as large and has considerably larger capacities than Hubble, especially in the infrared range, which makes it easier to study objects behind dense nebulae and dust clouds or at extreme distances with strong redshift.

In September 2022, NASA announced that it was conducting a study to test the feasibility of another manned service mission for the Hubble telescope using SpaceX’s Crew Dragon spacecraft. In such a mission, on the one hand, the orbit of the telescope would be raised by acceleration, and on the other hand, damaged components would be replaced in extravehicular vessels. This would extend Hubble’s scientifically usable lifetime by several years to decades and allow it to continue operating in parallel with the James Webb telescope.

The Nancy Grace Roman Space Telescope will cover the optical range in the future. It achieves a similar resolution to Wide Field Camera 3 but has a 100 times larger field of view. This telescope will not go into operation until 2026 at the earliest.

In order to be able to study the ultraviolet region in the future, a concept called Advanced Technology Large-Aperture Space Telescope (ATLAST) was presented; this has since been further developed into LUVOIR. This is a space telescope with an 8 to 16 meter mirror with instruments for the visible and ultraviolet spectral range.

Hubble Telescope’s technology and structure

The following exploded view illustrates the main structure of the Hubble telescope. The graphic is reference-sensitive, a click on the respective component leads to the corresponding section. Short quick information appears when the mouse rests over the object for a short time.

General structure

The Hubble Space Telescope is generally a cylindrical construction with a length of 13.2 m, a diameter of up to 4.3 m and a weight of 11.11 tons. Most of the volume is taken up by the optical system, at the end of which the scientific instruments are housed in the Focal Plane Structure (FPS). These two components are enclosed by several interconnected cylinders, the so-called “Support Systems Module” (SSM). This also includes a hollow ring in the middle of the telescope, which houses the majority of all technical systems for its control. The required electrical energy is generated by two shade sails, which are also installed in the middle. For communication, two booms, each with a high-performance antenna, are attached to the SSM.

At the front end of Hubble is a flap with a diameter of 3 m, which can be used to completely close the opening of the optical system if necessary. It is designed in the aluminum-honeycomb construction and is equipped with a reflective coating on the outside to protect against sunlight. This is continuously monitored by several sensors, as too high a level of incident light could damage the highly sensitive scientific instruments. If the sun is less than 20° from the telescope’s alignment axis, this system automatically closes the flap within less than 60 seconds, unless manually switched off from ground control.

The flap itself is attached to a 4 m long light protection cylinder (baffle). This consists of magnesium in corrugated iron form, which is protected by an insulation layer from the strong temperature changes during an orbit. On the outside, in addition to grab handles for the astronauts and the fasteners for securing in the cargo bay of the Space Shuttle, there are the following components: a low-gain antenna, two magnetometers and two sun sensors.

The next cylinder is also 4 m long, made of aluminum and stiffened by additional struts and support rings. As with the light protection cylinder, there are several devices for attaching the telescope, whereby a particularly stable mechanism is attached here, to which the robot arm of the Space Shuttle can dock. On the outside, in addition to four magnetic gate crossers, there are also brackets for the two booms with high-gain antennas. Also in this section, insulation materials are applied to the surface to reduce thermal stress.

The next component is the most important of the entire Support Systems module: the Equipment section. This is a doughnut-shaped ring that completely encloses the telescope. It contains about 90% of all technical systems in a total of ten individual bays. Each of these bays measures approximately 0.9 m × 1.2 m × 1.5 m and is easily accessible from the outside through a flap. These are designed in honeycomb construction and each has its own insulation on the surface. The individual bays are occupied as follows:

• Bay 1: Data processing (central computer and DMU)
• Bay 2: Power supply (accumulator module and two timing oscillators)
• Bay 3: Power supply (accumulator module and a DIU)
• Bay 4: Power distribution (PCU and two PDUs)
• Bay 5: Data storage and transmission (communication system and two E/SDRs)
• Bay 6: Attitude control (SHEV)
• Bay 7: mechanical systems for solar sail alignment and a DIU
• Bay 8: Data storage and emergency systems (E/SDRs and PSEA)
• Bay 9: Attitude control (SHEV)
• Bay 10: Data processing (SI C&DH and a DIU)

The telescope is completed by a last 3.5 m long cylinder at its rear. As with the previous section, this one is also made of aluminum and stiffened by struts. Between this cylinder and the equipment ring, there are also four bays for the installation of the three FGS and the radial scientific instrument (No. 5). The other four instruments are located in an axial position behind maintenance flaps within the structure. At the end of the cylinder is a final aluminum honeycomb plate with a thickness of 2 cm. A low-gain antenna is attached to it, which has openings for several gas valves and electrical connectors. The latter enable the operation of internal systems during service missions via charging cables from the Space Shuttle when the solar cells have to deactivate their own electricity production.

Energy supply

All the electrical energy used to operate the telescope is generated by two wing-like solar panels developed and built by ESA. The original silicon-based modules delivered an output of at least 4550 watts (depending on the orientation to the sun), measured 12.1 m × 2.5 m each and weighed 7.7 kg each. Since the telescope itself, like the payload bay of the Space Shuttle, is round in cross-section, the two wings could not be easily folded in as usual. Instead, the individual panels were applied to a surface of glass fibers and Kapton, the cabling was realized by an underlying silver thread matrix, which was finally protected by another layer of Kapton. This combination was only 0.5 mm thick and could thus be rolled up onto a drum, which in turn could be folded in to save space.

However, problems quickly became apparent due to high bending forces caused by the intensive thermal load during entry and exit from the Earth’s shadow. Due to the rapid change between light and shadow, the panels were heated from −100 °C to +100 °C in a very short time and also cooled down again, which led to unwanted twisting and deformation and thus to vibrations of the entire telescope. Therefore, during the SM 1 service mission, they were replaced with newer models where this problem no longer occurred. Nine years later, advances in solar cell technology enabled the installation of better, gallium arsenide-based solar modules during the SM 3B service mission, which provide about 20% more energy despite a 33% reduction in area. The smaller area of the wings also provides lower atmospheric resistance, so that the telescope loses altitude less quickly.

Due to the low orbit of the telescope, the solar panels are only illuminated about two-thirds of the time, as the Earth’s shadow blocks solar radiation. In order to continue to supply the systems and instruments with energy during this time, six nickel-hydrogen accumulators were integrated, which are charged as soon as sunlight hits the solar panels, with the charging process using about a third of the electrical energy generated. The original accumulators could each store about 75 Ah, which is sufficient for a total of 7.5 hours of continuous operation or five full orbits.

The power consumption of the telescope is about 2,800 watts. This overcapacity is needed because when the telescope is pointed at some of the objects to be observed, the solar modules cannot be optimally positioned in relation to the Sun and therefore deliver less power. The accumulators have their own systems for charge, temperature and pressure control and consist of 22 individual cells. Three accumulators are organized in one module so that they can be safely replaced by astronauts in open space. Such a module has approximately the dimensions 90 cm × 90 cm × 25 cm and weighs 214 kg.

In order to compensate for the natural aging of the accumulators, they were equipped with a Voltage/Temperature Improvement Kit (VIK) during the SM 3A service mission, which reduces thermal stress and overcharging problems in particular through improved charging control systems. During the SM 4 service mission, the six old accumulators had already been in operation for 13 years and were replaced. The new models are much more robust due to better manufacturing processes and have an increased capacity to 88 Ah, of which only 75 Ah can be used due to thermal limitations. However, this overcapacity offers greater wear reserves, which ensures a further increased service life.

The energy is distributed centrally by the Power Control Unit (PCU), which weighs 55 kg and is installed in bay 4 of the equipment section. This unit supplies the onboard computers with a constant voltage of 5 volts. Four Power Distribution Units (PDUs), each weighing 11 kg, are connected to this and to which the bus systems of the instruments are connected. They also include monitoring instruments and overcurrent protection devices. During the SM 3B service mission, the PCU was replaced by a new model in order to be able to take full advantage of the increased energy production of the solar cells, which were also new. The totality of all power supply systems is referred to as the Electrical Power Subsystem (EPS). In SM4, the PCU was replaced again as a precaution.

On June 16, 2021, it was reported that the computer used to control the scientific instruments had shut down due to an error. At first, a defective memory module was assumed to be the cause of the error, but switching to one of the other three memory modules did not eliminate the problem. This was followed by several unsuccessful attempts to restart the computer or switch to the backup computer. On July 13, the fault could be narrowed down to the Power Control Unit (PCU). A control circuit monitors the onboard voltage and, in the event of deviations upwards or downwards, sends a signal to the computer, which switches it off for safety.

There is a redundant PCU that could take over this function, but the full activation of all necessary backup components took several days until the regular operation was possible again. In some cases, redundant systems were switched on and put into operation for the first time since the beginning of the mission. On July 17, 2021, scheduled scientific operations were resumed, and on July 19, 2021, new images could be received again. The observations that have not been made up until then are to be made up for at another date.

Electronics and data processing in the Hubble Telescope

NSSC-1 computers from 1974 were used to control the spacecraft until 2001. Since 2001, one of these computers has been replaced by a computer built in the 1980s with CMOS memory as RAM. In 2009, the computer installed in 2001 was replaced again.

All systems for data processing and storage are organized in the Data Management Subsystem (DMS). Until the SM 3A service mission, its core was a DF-224 central computer, which was responsible for the higher-level control of all technical and scientific systems. This contained three identical 8-bit processors clocked at 1.25 MHz, whereby only one was used at a time, the other two served as a reserve in the event of a defect. The memory is organized into six modules of 192 kBit each. The internal bus is designed with triple redundancy, the connection to the external systems is doubly redundant. The computer measures 40 cm × 40 cm × 30 cm, weighs 50 kg, and was programmed in an assembly language specific to it.

Just a few years after launch, two of the six memory modules failed (three are at least necessary for operation), so an additional coprocessor system was installed on the SM 1 service mission. This consists of a dual-redundant combination of Intel 80386 processor and 80387 coprocessor (32-bit x86 architecture “IA-32”), eight shared memory modules with a capacity of 192 KiBits each and 1 MiB of memory exclusively for the x86 processor. The programming of the coprocessor system was done in C.

During the SM 3A service mission, the entire computer system, including the coprocessor, was removed and replaced by the much more powerful Advanced Computer. It has three Intel 80486 processors with a clock frequency of 25 MHz. These are about 20 times faster than those of the DF-224 computer. Each processor is housed on its own board with 2 MiB SRAM and a 1 MiB EPROM. The entire system measures 48 cm × 46 cm × 33 cm and weighs 32 kg.

The central element for distributing data within the computer is the Data Management Unit (DMU). In addition to routing, the approximately 38 kg DMU is responsible for distributing the system-wide time, for which it is connected to two redundant, high-precision oscillators. Most systems are directly connected to the DMU, but some components are only connected to them via four Data Interface Units (DIUs), each weighing 16 kg.

The Science Instrument Control and Data Handling Unit (SI C&DH) installed in Bay 10 is responsible for controlling the scientific instruments. This is a complex of several electronic components that control the instruments, read their data and format it. The core element of this system is the Control Unit/Science Data Formatter (CU/SDF). It formats commands and requests from the ground station into the appropriate format of the target system or instrument. In the opposite direction, it also translates data streams from the connected components into a format suitable for the ground station.

The NASA Standard Spacecraft Computer (NSCC-I) is responsible for interpreting the formatted data and commands. It has eight memory modules with a capacity of 148 kbit each, in which command sequences can be stored. This allows the telescope to work even if it has no contact with the ground station. The commands generated or retrieved by the NSCC-I itself are then transmitted back to the CU/SDF via Direct Memory Access. In addition, all components of the SI C&DH are designed redundantly, so that an identical reserve module is available in the event of a failure.

Due to an error suspected in the SI C&DH, the instruments could not receive a signal for synchronization on October 23, 2021. The protection electronics of the instruments then put them into safety mode. A reset of the instruments did not bring the desired success and the error occurred again on October 25th. Since then, various strategies have been worked on to solve the problem. On November 1, the defective NICMOS instrument, which has been inactive since 2010, was put back into operation to monitor the synchronization signals. In this way, knowledge could be gained without endangering any of the active instruments.

Next, on November 7, the ACS instrument was put into operation on a trial basis. Due to its design, it is the system that is least affected by a malfunction in synchronization. On November 23, Wide Field Camera 3 was put back into operation after the error has not recurred since November 1. At the same time, work is being done on a software modification of the instruments that will allow scientific operations to continue in the absence of synchronization signals. The simulations and the test are estimated to take a few weeks. On November 28, 2021, Cosmic Origins Spectrograph, the third instrument, was put back into operation. The fourth instrument, Space Telescope Imaging Spectrograph (STIS), resumed regular operations on 6 December.

Three Engineering/Science Data Recorders (E/SDRs ) are available to store data that cannot be transmitted to Earth in real-time. At launch, these were tape drives, each with a capacity of 1.2 GBit, a weight of 9 kg each, and dimensions of 30 cm × 23 cm × 18 cm. Since magnetic tapes have to be moved by means of electric motors for reading and writing, one copy was already replaced by a flash-based memory known as a solid-state recorder (SSR) during the SM 2 service mission. This has no mechanical components and is therefore much more reliable with a longer service life. In addition, the SSR with 12 GBit has about ten times the capacity and enables parallel read and write access.

In addition to the redundant design of important components, there is a software and hardware security system for the operational safety of the telescope. The software system is a series of programs that run on the central computer and monitor various operating parameters. If any but not highly dangerous malfunction is detected, all scientific instruments are switched off and the telescope is kept in its current orientation. This mode can only be canceled by the intervention of ground control after the fault has been corrected. However, should serious deviations occur in the energy system, the telescope is aligned so that the solar sails are illuminated by the sun in the best possible way in order to produce as much electricity as possible. In addition, measures are taken to keep all components at their operating temperature to ensure a rapid resumption of scientific investigations after the lifting of the safety mode.

In the event of highly critical system failures or malfunctions, there is another safety system called Pointing/Safemode Electronics Assembly (PSEA). This is a 39 kg complex of 40 special printed circuit boards, on which there are programs that are intended exclusively to ensure the survival of the telescope. In contrast to the software-based security system in the central computer, these are hardwired into the PSEA hardware, which makes them considerably more robust against interference. The PSEA system is activated when one or more of the following situations occur:

• Malfunction of the central computer
• Two of the three RGAs fail.
• The accumulators lose more than 50% of their charge.
• Failure of the DMS

After activation, the wired programs ensure that the solar sails are aligned with the sun in the best possible way and that all non-vital components are switched off. The temperature control is controlled in such a way that all systems are kept above their temperature necessary for survival. In order to remain able to act even in the event of severe damage to the main systems, the PSEA complex is connected to the critical telescope components with its own data lines. In order to compensate for a failure of the RGAs, three reserve gyroscopes are also available, which, however, are much less accurate and can only ensure a rough alignment, which does not allow scientific operation. The PSEA system can thus work completely autonomously, a connection to the ground station is only necessary for troubleshooting itself.

Communication

For communication, Hubble has two high- and two low-gain antennas (referred to as HGA and LGA, respectively). The two high-gain antennas are designed as parabolic antennas in honeycomb construction (aluminum honeycombs between two CFRP plates) and mounted on two separate 4.3 m long booms, which also serve as waveguides due to their box-shaped construction. They have a diameter of 1.3 m and can be swiveled up to 100 degrees in two axes, allowing communication with a TDRS satellite in any position. Since the high data rate via the HGAs is achieved thanks to their strong directivity, this property is important in order to transmit very extensive scientific image and measurement data in an acceptable time.

The signals to be sent are generated by the S-Band Single Access Transmitter (SSAT). This transceiver has a transmission power of 17.5 watts and achieves a data rate of up to 1 Mbit/s by means of phase modulation. In total, about 120 GBit of data per week are sent to the ground station in this way, using the frequencies 2255.5 MHz and 2287.5 MHz. As a reserve, a second, identical SSAT is available, which had to be put into operation after the failure of the primary transceiver in 1998. In December 1999, this was replaced by a functional model during the SM 3A service mission.

Two low-gain antennas are available for the transmission of technical data and for emergencies. These have a very wide antenna pattern and are immobile. In combination, communication with the telescope is possible even if its HGAs are not correctly aligned. However, the low directivity severely limits the data rate, so that only short technical control commands and status data can be transmitted. The frequencies here are 2106.4 and 2287.5 MHz. Two redundant transceivers called multiple access transmitters (MAT) are used to generate signals. Commands are received at 1 kBit/s, data can be sent at up to 32 kBit/s.

Position control

Since Hubble is designed to observe objects with a very high resolution, the entire telescope must be aligned and tracked extremely precisely. The system responsible for this, called Pointing Control Subsystem (PCS), can align the telescope with an accuracy of 0.01″ and track an object for 24 hours with an accuracy of at least 0.007″. If Hubble were located in San Francisco, it could illuminate a moving 10-cent coin over Los Angeles, about 600 km away, with a narrow beam of light. To achieve such high-precision alignment, a total of five different sensor complexes are used.

A total of four Coarse Sun Sensors (CSSs), two of which are located at the bow and stern, determine the orientation to the Sun, two Magnetic Sensing Systems (MSSs) on the telescope cover determine the orientation relative to the Earth by measuring the Earth’s magnetic field, and three-star sensors, known as Fixed Head Star Trackers (FHSTs), each record the orientation relative to a specific guide star. The movements in the three spatial axes are recorded by three-rate gyro assemblies (RGAs). Each RGA has two rate sensing unit (RSU) that can detect and measure acceleration along its respective axis. Hubble now has a total of six gyroscopes at its disposal, with at least three required for operation. Since these showed a high degree of wear relatively quickly after take-off, two to six of them were replaced during each service mission.

The actual core system that enables the high precision of the telescope is the complex of the three Fine Guidance Sensors (FGSs). They draw their light from the edge areas of the illumination area of the main optics and thus work coaxially and temporally parallel to the scientific instruments. Since the optical aberrations are greatest in the edge region, each FGS has a large field of view, so the probability is high to still find a suitable guide star. Once found, it is precisely detected and focused by means of a complex system of small electric motors, prisms and mirrors in order to direct its light onto two interferometers, which in turn consist of two photomultipliers.

These complexes capture the phase of the incident light, which is exactly the same when the guide star is exactly in the middle of the field of view. If this moves towards the edge of the image due to the movements of the telescope, there is a phase shift between the two interferometers, which is detected by a computer system. This calculates the necessary alignment correction and sends the corresponding commands to the attitude control system. Since the complex is able to detect deviations as early as 0.002 8″, the correction maneuvers can be initiated before significant deviations occur (from 0.005″).

However, an FGS can only detect the deviation in one spatial dimension, which means that at least two of them are needed for correct alignment, the third system also measures the angular position of the star. Each FGS is 1.5 m long, has a diameter of 1 m and weighs 220 kg. During the SM 2, SM 3A and SM 4 service missions, one sensor was replaced by a newly calibrated and certified model. In addition, a system called the Optical Control Electronics Enhancement Kit (OCE-EK) was installed during the SM 2 mission. It allows minor adjustments and calibrations of the FGSs without external intervention, allowing their accuracy to be maintained to a certain extent without new service missions.

The movements requested by the control systems are primarily implemented by four Reaction Wheel Assemblies (RWAs). These each contain two reaction wheels which, when their rotational speed changes, transfer angular momentum to the telescope and thus realign it. Each wheel has a diameter of 59 cm, weighs 45 kg and can rotate at a speed of up to 3000 revolutions per minute.

Hubble has a total of six of these wheels, with only three necessary for operation, the rest being kept in reserve. In addition, four magnetic doors transverse are used for position control. These are electromagnets that interact with the Earth’s magnetic field and can thus control the speed of the inertial wheels by means of impulse transmission. In the event that the SHEVs fail completely, these torquers allow the telescope to reach a position where it can point the solar panels at the sun so that electricity continues to be generated.

Optical system

The optical system (referred to as the Optical Telescope Assembly, OTA for short) is the actual heart of Hubble, as it collects the light required for scientific investigations and distributes it to individual instruments. It is a Ritchey-Chrétien-Cassegrain construction consisting of only two mirrors. The first is the primary mirror, which is responsible for capturing the light.

It has a diameter of 2.4 m and is hyperbolically shaped, whereby the incident light is reflected on the 30 cm secondary mirror. This reflects on the scientific instruments and the three FGSs. A special feature of the Hubble telescope is that all instruments receive a fixed part of the collected light and can therefore work at the same time. Otherwise, it is usual to “switch” between different sensors, so that only one measurement can be active at a time. The 6.4 m long optical construction thus achieves a focal length of 57.6 m at an aperture of ƒ/24.

Hubble’s primary mirror was manufactured by PerkinElmer (now part of Raytheon), using a special type of glass from Corning that hardly deforms when temperature changes, thus preserving imaging performance. A 3.8 cm thick front surface was made of it, which was additionally stabilized by a honeycomb structure also made of this glass with a thickness of 25.4 cm. This design reduced the weight to a moderate 818 kg, a conventional, solid glass body would have weighed about 3600 kg to achieve the same performance. In order to guarantee the body’s absolute freedom from tension, it was cooled very slowly from its casting temperature (1180 °C) to room temperature over three months before being taken to PerkinElmer for final production.

There, the front surface was first brought into an almost hyperbolic shape with diamond-studded grinding machines, with about 1.28 cm of material being sanded off the front surface. Subsequently, experienced opticians used manual tools to remove another 7.6 mm. Finally, a computer-aided laser system was used, which formed the desired surface profile with a deviation of less than 31.75 nm (if the mirror were the size of the Earth, a deviation in the ratio would be at most 15 cm high).

Despite accurate manufacturing and quality control, there was a significant deviation that was only detected in orbit, rendering the telescope virtually useless (details above). Only the installation of a special correction system called COSTAR in the SM 1 service mission three years later made the planned scientific investigations possible. The actual reflection properties of the mirror are determined by a 100 nm thick aluminum layer, which is protected from environmental influences by an additional 25 nm magnesium fluoride.

In addition, this layer increases the reflectance of the mirror (to over 70%) in the area of the Lyman series, which is of great importance for many scientific investigations. In the visible spectrum, reflexivity is more than 85%. Behind the primary mirror is a special beryllium support structure containing several heating elements and 24 small actuators. The former ensure that the mirror is kept at its optimum temperature of about 21 °C, whereby with the help of the actuators the shape of the mirror can be readjusted minimally by a control command from the ground. The entire construction is in turn held in position by a 546 kg hollow titanium support ring with a thickness of 38 cm.

The primary mirror is shaped in such a way that all collected light hits the 30 cm secondary mirror. Its reflective coating also consists of magnesium fluoride and aluminum, but glass of the Zerodur variety was used for the even more hyperbolic mirror body. This is held by a highly stiffened CFRP construction in the middle of the telescope near the aperture. This has also been coated with multilayer insulation to further minimize deformations caused by temperature differences. This is very important for the proper operation of the telescope, as even a position deviation of more than 0.0025 mm is sufficient to produce serious aberrations. In addition, as with the primary mirror, there are six actuators with which the orientation can be corrected to a small extent. The light is then directed to the instruments through a 60 cm hole in the middle of the primary mirror.

To protect against scattered light, which essentially comes from the Earth, the moon and the sun, three baffles are available. These are elongated cylindrical constructs whose inner wall is provided with a deep black, finely and coarsely fluted structure. This absorbs or disperses light that comes from objects that are in the vicinity of the targeted target and could thus interfere with the investigations. In terms of purpose, it resembles a lens hood, but the structure, which is often found in many commercially available cameras in the area around the sensor and rarely on the front part of the lens, is located inside the telescope.

The largest primary baffle is attached to the edge of the primary mirror, is made of aluminum and extends to the aperture of the telescope, resulting in a length of 4.8 m. Another 3 m long central baffle is mounted in the center of the mirror to shield the light reflected by the secondary mirror, on which such a construction was also mounted. All parts of the optics are connected and held together by a skeletal construction made of CFRP. This is 5.3 m long and weighs 114 kg.

Insulation and temperature control

Due to the low orbit, the telescope passes through the Earth’s shadow very frequently and for a long time. This results in very high thermal loads when it emerges from the shade again and is immediately intensively illuminated by the sun. To reduce this stress, Hubble’s entire surface is surrounded by various insulation materials. With a share of 80%, Multilayer Insulation (MLI) is the most important component. This consists of 15 aluminum-vaporized Kapton layers and a final glued-on layer of the so-called “Flexible Optical Solar Reflector” (FOSR).

This is an adhesive Teflon film vaporized with either silver or aluminum, giving Hubble its typically shiny appearance. It was also used to protect surfaces that were not additionally protected by an MLI layer, the largest areas being the front cover flap and the lateral surfaces of the telescope (these are less intensely illuminated by the sun than the upper and lower parts). Since the scientific instruments have different optimal temperature ranges, insulation materials are also available between the four axial instrument bays to create individual temperature zones.

Although Teflon is a very stretchy and robust material, minor cracks in the FOSR material were already apparent during the inspection during the first service mission. By the time of the next SM 2 mission, these had expanded massively within just three years, with over 100 cracks with a length of more than 12 cm. Already during the mission itself, the first improvised repairs were carried out unscheduled using FOSR adhesive tapes. In order to solve the erosion problem of FOSR film safely and definitively, a new cover was developed: the New Outer Blanket Layer (NOBL). This is a construction consisting of a specially coated stainless steel panel inserted into a steel frame.

This frame is individually adapted to a specific bay of the equipment section, where a NOBL module is installed over the old, damaged insulation to protect it from further erosion. In addition, some modules are also equipped with a radiator for improved cooling. This was necessary because as the telescope continued to be modernized, more and more powerful electronics were installed, which produced more heat than their predecessor systems, which in turn affected Hubble’s heat balance. A total of seven of these protective panels were installed during the outboard operations of the SM 3A, SM 3B and SM 4 missions.

In addition to the passively acting insulation materials, the telescope has a system for active temperature control. This is detected internally and externally by more than 200 sensors, whereby an optimal thermal environment can be created for each important component. This is done through the use of individually placed heating elements and radiators.

Scientific instruments in the Hubble Telescope

Currently

The following five instruments are installed and are used for scientific investigations except for the defective NICMOS. Since no further service missions are planned since SM4, all instruments will remain on board.

Advanced Camera for Surveys (ACS)

This instrument has been designed for the observation of large spatial areas in the visible, ultraviolet and near-infrared spectrum. This generally allows a wide range of applications. In particular, galaxies that formed shortly after the Big Bang and thus have a high redshift will be investigated. The instrument was installed on the SM 3B service mission, displacing the Faint Object Camera from Instrument Bay No. 3. Three different subsystems are available for investigations: a High-Resolution Channel (HRC), a Wide Field Channel (WFC) and a special channel for the ultraviolet spectral range (Solar Blind Channel, SBC).

In addition, 38 different filters are available to enable targeted examinations and special optics to correct the primary mirror error without the help of COSTAR. Due to electronic failures in July 2006 and January 2007, the HRC and WRC channels were not operational until the SM 4 service mission. During maintenance, only the WRC channel was repaired, the damage to the HRC channel was too extensive, which is why it is no longer usable.

The WFC channel has two silicon-based back-exposed CCD sensors. Each has 2048 × 4096 pixels and is sensitive in the range of 350–1100 nm, with the quantum yield up to 800 nm at about 80% and then dropping evenly below 5% at 1100 nm. With a pixel size of 225 μm² and a field of view of 202″ × 202″, the channel achieves a resolution of 0.05″/pixel. The high-resolution HRC channel, on the other hand, has a much narrower field of view of 29″ × 26″ and, despite a smaller CCD sensor with 1024 × 1024 pixels, achieves a resolution of about twice as high as 0.027″/pixel.

In addition, it already has a quantum yield of about 35 % from 170 nm, which increases to up to 65 % from 400 nm and, as with the WFC channel, drops continuously from about 700 nm to 1100 nm. Both sensors are otherwise identical in design and operate at a temperature of −80 °C. A distinctive feature of the HRC channel is the ability to observe faint objects near strong light sources.

For this purpose, a special mask (coronagraph) is inserted into the beam path so that light from the bright source is blocked. For observations in the ultraviolet spectrum, the SBC channel is available, which uses the optical construction of the HRC channel. The cesium iodide-based sensor is a spare part for the STIS instrument. It has 1024 × 1024 pixels with a size of 25 μm² each, which achieves a quantum yield of up to 20% in the range of 115–170 nm. With a field of view of 35″ × 31″, the channel achieves a resolution of 0.032″/pixel.

Wide Field Camera 3 (WFC3)

The Wide Field Camera 3 (WFC3) enables the observation and imaging of an extended area of space with high resolution and large spectral bandwidth (200–1700 nm). In the visible and infrared range, their performance is only slightly below the level of the Advanced Camera for Surveys, so the WFC3 can be used as an alternative in the event of their failure. In the ultraviolet and visible range, on the other hand, it is clearly superior to all other instruments in the fields of field of view and bandwidth, which predestines it for large-scale investigations in this spectral range. The observation goals are correspondingly diverse and range from the investigation of nearby star-forming regions in the ultraviolet range to extremely distant galaxies using infrared. The instrument was installed during the SM 4 service mission in the axial instrument bay No. 5, where the Wide Field/Planetary Camera 2 was previously located.

The WFC3 has two separate channels for near-infrared (IR) and ultraviolet/visible (UVIS) imaging. The latter uses two combined 2051 × 4096 pixels silicon-based CCD sensors, which are kept at a temperature of −83 °C by a four-stage Peltier cooling. They achieve a quantum efficiency of 50 to 70%, with a maximum of about 600 nm. By combining 225 μm² pixels with a field of view of 162″ × 162″, this channel achieves a resolution of about 0.04″/pixel in the spectral range from 200 to 1000 nm. The square HgCdTe CMOS sensor of the near-infrared channel, on the other hand, is only 1 megapixel in size and delivers only a resolution of 0.13″/pixel despite its smaller field of view of 136″ × 123″.

On the other hand, its quantum yield of almost continuous 80% over the entire spectrum (900–1700 nm) is significantly better. Since infrared detectors react particularly unfavorably to heat, it is also equipped with a stronger six-stage cooling system, which enables an operating temperature of −128 °C. Both channels also have a variety of filters (62 for UVIS and 16 for IR) to investigate specific properties of the observed region. Of particular interest are three lattice prisms (one for UVIS, two for IR), which allow both channels to produce classical spectra for an object lying in the middle of the image. Although these have only a low resolution (70–210), they range in combination over the spectrum of 190–450 nm and 800–1700 nm.

Cosmic Origins Spectrograph (COS)

The COS is essentially a spectrometer, so it usually does not provide images, but readings at a single targeted point. In this way, the structure of the universe as well as the evolution of galaxies, stars and planets will be investigated. The measuring range (90 to 320 nm) overlaps with that of the STIS instrument, although it is about ten times more sensitive for point targets. For examinations, you can choose between a far-ultraviolet (FUV) and a near-ultraviolet (NUV) channel.

Both sensors are preceded by one of a total of seven special optical gratings, which splits the incident light and deflects it to varying degrees according to its wavelength. Portions with a low wavelength hit the downstream CCD sensor rather in the middle, while long-wave components tend to hit the edge area. From the position and charge of the pixels, an intensity spectrum can be made as a function of the wavelength, which in turn allows conclusions to be drawn about the chemical structure of the observed object. The instrument was installed during the SM 4 service mission and displaced the COSTAR system, as at that time all other instruments were equipped with internal correction mechanisms and it was no longer needed.

In the FUV channel, two adjacent CCD sensors based on cesium iodide with combined 16,384 × 1024 pixels are used for measurement. A quantum yield of up to 26% is achieved at 134 nm, the spectral resolution and bandwidth of the spectrum is mainly determined by the optical grating used. Two pieces are optimized for a high resolution (about 11,500 to 21,000 in the range 90 to 178 nm), while the broadband grating can work on a wide wavelength range of 90 to 215 nm, but only has a low resolution of 1500 to 4000.

The situation is similar in the NUV channel, where there are three narrowband but high-resolution gratings (16,000 to 24,000 at a bandwidth of about 40 nm) and a broadband grating that achieves a resolution of only 2,100 to 3,900 in the range of 165 to 320 nm. However, a different CCD chip is used in this channel. It is based on a cesium-tellurium compound and has 1024 × 1024 pixels that achieve a quantum yield of up to 10% at 220 nm. The square design also allows an imaging measurement mode for this channel, which achieves a resolution of 0.0235″/pixel at a field of view of 2″. Since strong vignetting already occurs from a viewing angle of 0.5″ away from the image center, only small and compact objects can be reliably observed.

Space Telescope Imaging Spectrograph (STIS)

The STIS instrument is a spectrograph that covers a wide range of ultraviolet to infrared radiation (115 to 1030 nm). In contrast to the COS instrument, which specializes in single targets, STIS can be used to create spectra at up to 500 points of an image, which enables the rapid investigation of extended objects. However, the measurement results are less accurate than those of the COS instrument but are particularly suitable for the search and analysis of black holes and their jets.

A total of three channels are available for observations: the CCD channel with a wide bandwidth (ultraviolet to infrared) and the NUV and FUV channel for the near and far ultraviolet spectrum. The spectra are formed by means of optical gratings analogous to the COS instrument. The instrument was installed during the SM 2 service mission in Instrument Bay No. 1, where it replaced the Goddard High-Resolution Spectrograph. Between August 2004 and May 2009, STIS was not operational due to a failure in the internal power supply. Since the installation of a new circuit board during the SM 4 service mission, the instrument has been working without interference again.

For the formation of spectra, the STIS has two similarly constructed MAMA sensors. They each have 1024 × 1024 pixels with a size of 625 μm². A field of view of 25″ × 25″ results in a resolution of 0.025″/pixel each. The difference between the two sensors lies in their spectral bandwidth and quantum efficiency. The CsI sensor in the far ultraviolet (FUV) channel is sensitive in the range of 115 to 170 nm and has a quantum efficiency of up to 24%, the CsTe sensor in the far ultraviolet (FUV) channel operates at 160 to 310 nm with an efficiency of only 10%.

A large number of optical gratings are available for the formation of spectra. These achieve a resolution of 500 to 17,400 with a bandwidth of about 60 or 150 nm. By means of echelle grids and special data processing techniques, resolution values of over 200,000 can be achieved with similar bandwidth. In addition to the two MAMA sensors, a CCD chip is available for measurements. This is also one megapixel in size, but its spectrum is much wider at 164–1100 nm and offers a wider field of view (52″ × 52″). In addition, the quantum efficiency is almost consistently over 20%, reaching its maximum of 67% at 600 nm. The total of six optical gratings enable a resolution of 530 to 10630 with a bandwidth of 140 to 500 nm.

Near Infrared Camera and Multi-Object Spectrometer (NICMOS)

The NICMOS is a relatively specialized instrument, which is mainly due to its focus on the near-infrared spectral range (800–2500 nm). In return, all three existing measurement channels (with slightly different viewing ranges) can be used simultaneously, so internal switching for different examination methods is not necessary. Another unique feature is the elaborate cooling system. For the observation of the near-infrared spectrum, the lowest possible temperature of the sensors is crucial, as their own thermal noise would otherwise superimpose almost all signals collected by the primary mirror. Therefore, these are housed in an elaborate, quadruple-insulated Dewar vessel, which takes up a good half of the available volume within the instrument.

The cooling was only carried out by means of a supply of 109 kg of solid nitrogen. During the SM 3B service mission, a closed cooling system was installed because the nitrogen was used up after almost two years of operation. After a good six years of operation, this could no longer be started reliably after a software update, so the operation of the instrument has been suspended since the end of 2008 due to too high sensor temperature. Before the failure, the instrument was particularly well suited for observing objects inside or behind dense dust and gas clouds due to its spectrum extending very far into the infrared, as these absorb short-wave radiation very strongly in the visible and ultraviolet range in contrast to infrared light. The NICMOS was already installed in Instrument Bay No. 2 during the SM 2 service mission, where it replaced the Faint Object Spectrograph.

Each of the three measurement channels (NIC 1 to 3) has an identical HgCdTe-based sensor, each with 256 × 256 pixels. The channels differ only in a few aspects:

Canal	Field of vision(“)	Resolution (“/pixels)	Specifics
NIC 1	11×11	0,043	Polarization measurement at 800–1300 nm
NIC 2	19×19	0,075	Polarization measurement at 1900–2100 nm, coronagraph with 0.3″ radius
NIC 3	51×51	0.20	3 lattice prisms

In total, NICMOS has 32 filters, 3 grating prisms and 3 polarization filters to enable specific investigations. All these components are mounted on a CFRP construction in the innermost part of Dewar’s vessel. This complex, together with a supply of frozen nitrogen, was located in a shell that was kept at a temperature of about 60 K by its cold gases. To further improve insulation, this complex is surrounded by two Peltier-cooled shells before the dewar is closed off by an outer pressure vessel.

The supply of frozen nitrogen was originally intended to ensure sufficient cooling of the sensors for about four and a half years. However, during its melting process, ice crystal formation and unexpectedly strong deformation occurred, so that the deep-frozen CFRP carrier construction came into contact with the innermost shell of the Dewar. This led to a significantly increased heat flow, which on the one hand led to even greater deformations and in turn caused an increased need for nitrogen cooling. The result was the halving of the instrument’s mission time and a strong defocusing of the three measurement channels due to the resulting deformations. The latter could be reduced to an acceptable level, at least for NIC 3, by an internal compensation system.

In order to make all channels of the NICMOS operational again, a closed cooling system was installed in the rear area of Hubble during the SM 3B service mission. This has a powerful air conditioning compressor that works with neon as a coolant. The resulting heat is conducted via a pump to a radiator on the outer structure of the telescope, where it is radiated into open space. The compressed neon, on the other hand, is expanded in a heat exchanger, whereby it cools another neon gas circuit via the effect of the enthalpy of vaporization.

This leads via a special interface, which was originally intended for continuous cooling of the instrument during ground tests, into the innermost part of the Dewar, which ultimately cools the sensors. The complex is only operated periodically, as it requires a lot of energy with 375 watts of electrical power. Since the dewar is still very well insulated despite deformation, the cooling lasts a long time, so the system rarely needs to be activated, while the sensor temperature is kept at a stable 77 Kelvin.

After an observation and cooling break in September 2008, the cooling system surprisingly could no longer be put into operation. Although the cooling compressor worked, the closed neon gas circuit of the Dewar required an additional coolant pump, which no longer started. The reason is assumed to be an accumulation of water ice in their housing. In order to liquefy it again, the instrument was not cooled for several weeks. On 16 December, this measure was successful, as the pump could initially be put back into operation. However, it failed again four days later. Further attempts in 2009 were also largely unsuccessful, which is why it was decided to shut down the instrument completely indefinitely.

Historical

The following instruments were removed during the service missions and returned to Earth with the help of the Space Shuttle. Most of them are now on public display.

Corrective Optics Space Telescope Axial Replacement (COSTAR)

COSTAR is not a scientific system in the strict sense, but a correction system for neutralizing the primary mirror error. For this purpose, small correction mirrors have been developed, which are also not perfectly shaped and reflect the incident light unevenly. However, the deviations have been calculated in such a way that they are exactly inverse to those of the primary mirror. Thus, after being reflected by two irregular mirrors, the light is back in the correct shape and can be used for scientific investigations. In principle, the system is similar to conventional glasses, but mirrors are used instead of lenses.

After installation during the SM 1 service mission, three mechanical booms were used to position them in front of the entrance openings of the following instruments: Faint Object Camera, Faint Object Spectrograph and Goddard High-Resolution Spectrograph. Since these instruments have more than one measuring channel, a total of ten correction mirrors with a diameter of about 1.8 to 2.4 cm had to be used. With the SM 4 service mission, COSTAR was then expanded again, as all new instruments now had their own correction mechanisms. It is now on public display at the National Air and Space Museum in Washington.

The entire development, production and verification of COSTAR took only 26 months, with in many areas assigning a single task to two completely separate teams with different approaches to eliminate further errors such as the design of the primary mirror. Thus, the measurement of its errors was determined on the one hand by examining the still completely preserved production plant, on the other hand by calculations based on distorted images transmitted by Hubble. Both groups came to practically identical measurement results, which means that this step was carried out correctly with a high degree of certainty.

The correction mirrors produced afterward were also checked for accuracy by two independent teams. For this purpose, COSTAR was first installed in a special test system called COSTAR Alignment System (CAS), which checked these mirrors through special tests. The Hubble Opto-Mechanical Simulator (HOMS) was developed to prevent errors in the CAS from leading to incorrect results. This simulated the deviations of the primary mirror so that the correction mirrors could be verified according to their output image. The HOMS system was also tested by two independent groups, with ESA also contributing by providing the engineering model of the Faint Object Camera. A final comparison of the test systems and COSTAR with images from Hubble finally showed the correctness of the correction mirrors.

Faint Object Camera (FOC)

This camera was Hubble’s telephoto lens, as it achieved the highest image resolutions of any instrument. It covered much of the ultraviolet and visible spectrum with high sensitivity. In return, however, the field of view had to be greatly reduced, so that a picture can only depict a small area of space. This profile makes the instrument particularly interesting for the investigation of small objects and fine structures. The field of view and the associated resolution can be influenced by the choice between two separate measurement channels, whereby the detectors are identical.

Due to the good performance values, the FOC remained on board Hubble for a very long time and was only replaced by the Advanced Camera for Surveys during the penultimate service mission SM 3B. The instrument was a major ESA contribution to the project and was built by Dornier. After removal and return transport, it was therefore handed over to the Dornier Museum in Friedrichshafen, where it is now on public display.

Both measuring channels are optically designed in such a way that they enlarge the image of the primary mirror by twice or four times. This focal length extension reduces the f-number, which therefore serves as the naming of the two channels: ƒ/48 for double magnification and ƒ/96 for quadruple magnification (primary mirror f-number: ƒ/24). With the installation of COSTAR, the optical formula has been significantly changed, so the real f-numbers amount to ƒ/75.5 and ƒ/151. The fields of view vary accordingly by double with 44″ × 44″ or 22″ × 22″.

The detectors, on the other hand, are identical in both channels and are sensitive to a spectrum of 115 to 650 nm. In order to detect even weak signals, the FOC has three image intensifiers connected in series, which increase the original electron flow generated by the magnesium fluoride window by about 10,000 times. Subsequently, the electrons are converted back into photons through a phosphor window, which are directed by an optical lens system onto a plate with silicon diodes. These are then read out by an electron beam and interpreted in such a way that a 512 × 512 pixel image can be stored at the end. In the ƒ/96 channel, resolutions of up to 0.014″/pixel can be achieved.

Faint Object Spectrograph (FOS)

This highly sensitive spectrograph was used for the chemical study of distant and faint objects. The instrument proved to be particularly helpful in the study of black holes, as it could be used to precisely measure the velocities and movements of the surrounding gas clouds, which made it possible to draw conclusions about the black hole itself. Two independent measurement channels are available for investigations, which differ only in terms of their spectral ranges covered. Combined, both can cover a range of 160 to 850 nm (far ultraviolet to near-infrared). The instrument was displaced by NICMOS during the SM2 service mission and is now on public display at the National Air and Space Museum in Washington.

The two detectors are called blue and red channels according to their spectral ranges. Both have line sensors, each with 512 silicon photodiodes, which are “bombarded” with electrons by different photocathodes. In the blue channel, Na 2-K-Sb is used as cathode material, in the red channel cesium was additionally added (yields Na 2-K-Sb-Cs). Due to this variance, the spectral sensitivity has changed significantly: the blue channel is highly sensitive in the range 130 to 400 nm (quantum efficiency 13-18%) and loses efficiency at about 550 nm, while the red channel works best in the range 180 to 450 nm (23-28% efficiency) and only has its upper limit at 850 nm.

Regardless, both detectors achieve a resolution of up to 1300 with a field of view of 3.71″ × 3.66″ (after installation of COSTAR, previously 4.3″ × 4.3″). Due to the primary mirror error and errors in the construction of the instrument (one mirror was dirty and the shielding of the photocathodes was inadequate), first observations were only possible with significant limitations. It was only through the installation of COSTAR and a complex recalibration that the instrument’s capabilities could be used almost fully.

High-Speed Photometer (HSP)

This instrument is specialized in the study of variable stars, especially Cepheids, and is therefore relatively simple (no moving parts). By means of five separate detectors, the brightness and polarization can be measured up to 100,000 times per second, which means that even extremely high-frequency fluctuations can be detected. The stars in question are mainly located in the far UV spectrum, but measurements can be made up to the near-infrared. Since the HSP could not make a significant contribution to many research objectives of the mission due to its strong specialization, it was expanded during the first service mission to make room for the COSTAR correction system. It has been on public display at the Space Place of the University of Wisconsin–Madison since 2007.

Four of the five detectors are used to measure brightness, two of which consist of Cs-Te-based photocells and magnesium fluoride photocathodes and another two of bipotassium photocells (similar to those from the FOS) with quartz glass cathodes. The former cover a spectral range of 120 to 300 nm, the latter the range of 160–700 nm. Three of the detectors, as well as a GaAs photomultiplier, are used for photometry, the remaining one is used for polarimetry, whereby the quantum efficiency is extremely low at only 0.1 to 3%. The aperture of the optical system can be reduced to up to one angular second in order to focus the measurement as accurately as possible by hiding the background and neighboring objects. In order to precisely limit the wavelength to be measured, 23 filters are also available, whose filtering effect is very strong according to the purpose.

Wide Field/Planetary Camera (WFPC)

This camera system was designed for the multispectral detection of relatively large spatial areas and is therefore suitable for a variety of scientific investigations. Particularly useful here is the broad spectrum covered from the far UV to the near-infrared range. In addition, there are also some filters and optical gratings with which spectrographic measurements can be carried out to a limited extent.

The instrument has two measurement channels: the wide-angle channel (Wide Field), which has a particularly large field of view at the expense of resolution, and the Planetary Camera, which has a smaller field of view, but which can make full use of the resolution of the primary mirror. The WFPC was housed in the only axial instrument bay (No. 5) at launch but was replaced with an improved model (WFPC2) during the second service mission. After returning, the instrument was disassembled in order to be able to recycle structural parts for the third camera generation (WFPC3).

Both channels each have four back-exposed CCD sensors, each with 800 × 800 pixels. These are 15 μm in size and use silicon as a semiconductor material, whereby an upstream layer of corons is also present, which converts UV light into visible photons and thus makes it detectable. The measurable spectrum ranges from about 130 to 1400 nm, the quantum efficiency is generally close to these limits at 5 to 10%, but increases constantly in the range of 430 to 800 nm and reaches the maximum of 20% at 600 nm. In order to reduce the dark current, a two-stage cooling system was integrated.

The sensor is cooled by means of a silver plate and a Peltier element, which then transfers the heat via a heat pipe filled with ammonia to an externally mounted radiator, where it is radiated into space. In this way, a sensor can be cooled down to −115 °C. Due to the different tasks of the channels, they use different optical configurations. While the wide-angle channel uses a field of view of 2.6′ × 2.5′ (angular minutes) and an aperture of f /12.9, these values are 66″ × 66″ and f /30 for the wide-angle channel. Thus, a resolution of 0.1 and 0.043″/pixel is achieved. In order to be able to observe particularly bright objects without overload symptoms, several light-weakening filters are available, which are mounted on a wheel. In addition, spectra can also be generated with the help of a total of 40 optical gratings and grating prisms.

Wide Field/Planetary Camera 2 (WFPC2)

The WFPC2 is an improved version of the WFPC, which it replaced on the SM 3B service mission in the only radial instrument bay No. 1. The research objectives of the instrument remained unchanged: the investigation of relatively large spatial areas with good resolution and a broad spectrum. In contrast, the camera is relatively insensitive to extreme UV and infrared radiation and does not reach peak resolution values.

The most important improvement over the predecessor camera is an integrated correction system to compensate for the primary mirror error. Thus, the WFPC2 is no longer dependent on COSTAR, which brought its expansion one step closer. Due to a tight budget, the design could not be comprehensively improved. The detectors are based on the same design, but were manufactured differently. There were only significant performance increases in the areas of dark noise (eight times lower), readout noise (about twice as smaller) and dynamic range (twice as large). In order to save costs, only four instead of eight CCDs were produced, which halved the recording area. In addition, the sensors are no longer backlit, which somewhat deteriorated the signal-to-noise ratio and reduced the resolution. The other parameters are essentially identical to the WFPC.

Goddard High-Resolution Spectrograph (GHRS)

This instrument is the telescope’s first spectrograph. It works exclusively in the ultraviolet range from 115 to 320 nm, as the measuring range has been significantly limited by the COSTAR correction system. The spectra are generated with optical (echelle) gratings and then measured by two detectors with a resolution power of up to 80,000. The instrument can also produce images in the UV range, but it is not optimized for this task, so the performance values are rather low. The GHRS was removed during Service Mission 2 and replaced by the STIS, which has improved performance values.

Two digicon detectors with different materials serve as detectors. The first model, called D1, uses a cesium iodide photocathode behind a lithium fluoride window, while the D2 detector uses a cesium fluoride cathode behind a magnesium fluoride window. This results in a measuring range of 110–180 nm (D1) and 170–320 nm (D2). The electrons generated behind the windows are then accelerated and mapped electron-optically onto a CCD array with 500 measuring diodes; another 12 diodes are used for calibration.

Five optical gratings and two echelle gratings are available for the formation of spectra. The former have a bandwidth of 800 to 1300 nm and achieve a resolution of 15,000 to 38,000. Although the echelle grids cover a larger bandwidth (up to 1500 nm) with higher resolution (up to 80,000), the signal strength is very low, so only very bright objects can be effectively observed or very long exposure times are necessary. With the help of the four focus diodes at the edge of the digicons, images can also be created rudimentarily. Although these are high-resolution with 0.103″/pixel, the field of view is extremely small with 1.74″ × 1.74″, which limits the scientific benefit to very special investigations and target objects.

Tasks and results of the Hubble Space Telescope

The operation of a telescope outside the Earth’s atmosphere has great advantages, since its filtering effect on certain wavelengths in the electromagnetic spectrum, for example in the ultraviolet and infrared range, is eliminated. There are also no disturbances caused by air movements (scintillation), which can only be compensated for with great effort with terrestrial telescopes.

With its complex instrumentation, the Hubble Space Telescope was designed for a variety of tasks. Particular attention was paid to a program to determine the exact distance of these galaxies by observing Cepheids in nearby galaxies (up to a distance of about 20 Mpc). By comparing it with the radial velocity of galaxies, it should be possible to calculate the Hubble constant, which determines the extent of the universe, and thus also the age of the universe. After overcoming the initial difficulties, the HST was successful in this and other areas. Particularly well-known results are:

• highly sensitive images to study the evolution of galaxies, such as the Hubble Deep Field, Hubble Ultra Deep Field and Hubble Extreme Deep Field.
• Calibration of the cosmic distance scale by observing Cepheids in nearby galaxies
• Investigation of accelerating cosmic expansion by observing distant supernovae (see cosmological constant or dark energy)
• Detection of black holes in the core regions of many nearby galaxies.

Hubble has transmitted more than 1.5 million images to Earth by 2022. During the same period, scientists have reported on Hubble’s discoveries in more than 19,000 peer-reviewed papers.

The Hubble telescope in the media

• Use of measurement results:
  • Some of the images taken by the Hubble telescope were made available to the science fiction series Star Trek: Voyager and served as wallpapers of space. Thus, many of the nebulae shown there were not created on the computer, but originate from reality.
  • The program Google Sky uses the images of the Hubble telescope.
• Use as a dramaturgical element:
  • In the episode When Aliens Attack of the series Futurama, the Hubble telescope is mistaken for an enemy spaceship and destroyed.
  • In the movie Mystery Science Theater 3000, the Hubble telescope burns up after being rammed by a space station.
  • In the movie Armageddon, the Hubble telescope is used to capture the first images of an asteroid.
  • In the film Gravity, a space shuttle crew is hit by a hail of space debris during repair work on the Hubble telescope and, among other things, the telescope is destroyed.

Visibility from Earth

Like other large Earth satellites, the Hubble Space Telescope is visible to the naked eye from Earth as a star-like object moving from west to east. Due to the low inclination of the orbit and the moderate orbital altitude, however, this is only possible in areas that are no more than about 45 degrees north or south of the equator. Thus, it is not visible in Germany, Austria and Switzerland, for example, because it does not rise above the horizon. The Hubble Space Telescope can reach a maximum brightness of 2 mag.

References (sources)

Hubble’s Solar Panels, ESA, 2012; Lynn Jenner: Operations Underway to Restore Payload Computer on NASA’s Hubble, 2021; Article Hubble Space Telescope, Wikipedia; HST, N2YO, 2017.