2.1 Hubble Space Telescope Operations

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.

2.1.1 Servicing Mission 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.

2.1.2 Servicing Mission 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.

2.1.3 Servicing Missions SM3A and 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.

2.1.4 Servicing Mission 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 2.1.2).
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.

2.1.5 Servicing Mission 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. Since the mission, the ACS and the NICMOS instruments, as well as STIS and WFPC2, have been fully commissioned for science.

2.1.6 Loss of STIS

The Space Telescope Imaging Spectrograph (STIS) failed in August of 2004. STIS will not be available for scheduling in Cycle 16. Astronomers are encouraged to use STIS data for Archival Research (see Section B.1 ).

2.1.7 Two Gyro Operations

In August of 2005, HST was switched to two gyro operations in order to conserve gyros and extend the lifetime of the telescope. There is some loss of scheduling flexibility and efficiency in two gyro mode, but this is compensated by the extended overall lifetime. On-orbit tests have shown that HST and instrument performance in two gyro mode is nearly indistinguishable from the performance in three gyro mode. See the HST Two-Gyro Handbook for more detailed information. In Cycle 16, all GO observations will be taken in two gyro mode.

2.1.8 Servicing Mission SM4

NASA is in the early stages of preparing for Servicing Missionþ4, subject to final authorization by the NASA Administrator. SM4 is currently scheduled for no earlier than December 2007, near the mid-point of a normal cycle. Since the exact date remains uncertain, and may slide to early 2008, Cycle 16 will be truncated at SM4. Cycle 16 will therefore have the same instruments as Cycle 15, and all observations will be made in Two-Gyro mode. Assuming SM4 is successful, the new instruments, COS and WFC3, will be available in Cycle 17, as well as, possibly, a repaired STIS instrument; the WFPC2 will be retired at that time.