Hubble Space Telescope Primer for Cycle 17

B.1 Servicing Missions and Instrument Complements

The Hubble Space Telescope is a cooperative project of the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) to operate a long-lived space-based observatory for the benefit of the international astronomical community. HST was first dreamt of in the 1940s and designed and built in the 1970s and 80s. In April 1990 the Space Shuttle Discovery deployed it in low-Earth orbit (~ 600 kilometers). The initial complement of Scientific Instruments (SIs) was:

• The Fine Guidance Sensors (FGSs).
• The Faint Object Camera (FOC).
• The Faint Object Spectrograph (FOS).
• The Goddard High Resolution Spectrograph (GHRS).
• The High Speed Photometer (HSP).
• The Wide Field and Planetary Camera (WF/PC).

Soon after deployment, it was discovered that the primary mirror suffers from spherical aberration, which limited the quality of HST data obtained in the first few years of operation.

B.1.1 Servicing Mission 1 (SM1)

During servicing mission SM1 in December 1993, Space Shuttle astronauts successfully refurbished HST. They replaced the HSP with COSTAR, a corrective optics package. COSTAR's reflecting optics were deployed into the optical paths of the FOC, FOS, and GHRS, which removed the effects of the primary mirror's spherical aberration. The performance of the FGSs was unaffected by COSTAR. The WF/PC was replaced by a new instrument:

• The Wide Field and Planetary Camera 2 (WFPC2).

The WFPC2 contains its own internal optics to correct the spherical aberration of the primary mirror.

The astronauts also installed new solar arrays. This resolved the problem of thermal vibrations which affected the old arrays during day/night transitions which, in turn, degraded the telescope's pointing performance.

B.1.2 Servicing Mission 2 (SM2)

During servicing mission SM2 in February 1997, astronauts replaced the FOS and the GHRS with two new instruments:

• The Near Infrared Camera and Multi-Object Spectrometer
  (NICMOS).
• The Space Telescope Imaging Spectrograph (STIS).

Also, FGS-1 was replaced with an enhanced FGS, called FGS1R. FGS1R has an adjustable fold flat mirror which is commandable from the ground. This enables realignment in the FGS optical path to lessen the effects of the primary mirror's spherical aberration. As a result, the astrometric performance of FGS1R significantly exceeds that of the original FGS. FGS 1-R then became the primary science FGS.

B.1.3 Loss and Recovery of NICMOS

Just after launch during SM2, a thermal short in the NICMOS dewar caused the early exhaustion of its solid nitrogen cryogen, reducing the lifetime of the instrument to only about 2 years. The cryogen depleted in January 1999. During SM3B, the installation of the NICMOS Cooling System (NCS), a mechanical cryo cooler, re-enabled NICMOS operation, and restored infrared capability to HST. NICMOS was therefor unavailable for science operation between January 1999 and June 2002, from mid-Cycle 8 through Cycle 10, after which the NCS was activated and reached expected operating temperatures. The detector temperature has since been maintained at a slightly warmer temperature than before, so many NICMOS parameters are different. Most notably the detector quantum efficiency (DQE) increased by ~30-50%.

B.1.4 Servicing Missions 3A (SM3A) and 3B (SM3B)

HST has six rate-sensing gyroscopes on board. In the years after SM2, gyroscopes failed at a higher-than-expected rate, ultimately leading to a halt of HST observing in November 1999. In anticipation of this event servicing mission SM3, which had been in planning for several years, was split into two separate missions: SM3A and SM3B.

SM3A

In December 1999 Space Shuttle astronauts lifted off for servicing mission SM3A. Six new gyroscopes were successfully installed, which allowed HST to resume normal operations.

Along with the gyro replacements, the HST Project used this "unplanned" mission to make other planned upgrades and refurbishments:

1. Voltage/temperature Improvement Kits (VIKs) were installed to help regulate battery recharge voltages and temperatures.
2. The original DF224 spacecraft computer was replaced by a 486 upgrade, which provides a significant improvement in onboard computing power.
3. The FGS2 was replaced by a refurbished fine guidance sensor FGS2R to enhance the performance of the pointing and control system (see Section B.1.2). However, FGS1R remains the best choice for science observations due to its superior angular resolution.
4. The second tape recorder was replaced by a second Solid State Recorder (SSR), and a new transmitter was installed to replace one that had failed.

All of the upgrades underwent successful in-orbit verification and calibration and the observatory's functionality was completely restored according to plan.

SM3B

Servicing Mission 3B was carried out the first ten days of March 2002. During this mission, astronauts replaced the FOC with a new instrument:

• The Advanced Camera for Surveys (ACS).

Also, the astronauts installed the NICMOS Cooling System (NCS) to allow further use of NICMOS, which had exhausted its cryogen in January 1999. Installation of new solar arrays, electrical upgrades to the spacecraft's power control unit, along with various other engineering upgrades including an orbit reboost, were performed. After the servicing mission, the ACS and NICMOS instruments, as well as STIS and WFPC2, remained fully commissioned for science until the loss of STIS in August of 2004.

B.1.5 Loss of STIS

The Space Telescope Imaging Spectrograph (STIS) Side-2 electronics failed in August of 2004, during Cycle 13. The instrument has been maintained in safe mode since that time. STIS will again become available for scheduling in Cycle 17, after a best-effort attempt to return it to service during SM4. Data from earlier cycles also continue to be available for Archival Research (see Appendix Section C.1).

B.1.6 Loss of ACS Wide Field Camera and High Resolution Camera

During Cycle 15, ACS WFC and HRC became unavailable for observations due to a failure in the electronics. The two cameras remain powered off until SM4. Assuming the successful repair of the ACS during the servicing mission, they will again become available for observations in Cycle 17.

B.1.7 Servicing Mission 4 (SM4)

NASA plans to execute Servicing Missionþ4 in August of 2008. Since the exact date remains uncertain, Cycle 17 will begin at the completion of SM4. Cycle 17 will therefore have the NICMOS and FGS instruments plus two new instruments, COS and WFC3, along with the repaired ACS and STIS instruments. The WFPC2 will be retired at that time and 3-gyro operations will resume.