|HST Primer for Cycle 25|
The Hubble Space Telescope (HST) 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 conceptualized in the 1940s, designed and built in the 1970s and 80s. In April 1990, the Space Shuttle Discovery deployed HST in low Earth orbit (~600 kilometers). The initial complement of Scientific Instruments (SIs) was: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.During servicing mission 1 (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). It contained 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 and, in turn, degraded the telescope’s pointing performance.During servicing mission 2 (SM2) in February 1997, astronauts replaced the FOS and the GHRS with two new instruments:FGS1 was also replaced with an enhanced FGS, called FGS1R. It 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 exceeded that of the original FGS, resulting in FGS1R being designated the primary science FGS.NICMOS was unavailable for science operations between January 1999 and June 2002 (mid-Cycle 8 through Cycle 10); just after the SM2 launch, 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 was depleted by January 1999.During Servicing Mission 3B (SM3B) in March 2002, the installation of the NICMOS Cooling System (NCS), a mechanical cryo cooler, reactivated NICMOS operation, and restored infrared capability to HST. The temperature of the detectors using the mechanical cryo cooler were slightly warmer than before, so many NICMOS parameters were different during this second epoch of operation. Most notably the detector quantum efficiency (DQE) increased by ~30% to 50%.NICMOS continued to operate until September 2008, when the NICMOS Cooling System (NCS) failed to cool down the instrument, following a warm-up due to a planned safemode. The instrument was not successfully restarted after Servicing Mission 4.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.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. FGS2 was replaced by a refurbished fine guidance sensor, FGS2R, to enhance the performance of the pointing and control system (see Appendix 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, restoring the observatory’s functionality as planned.Servicing Mission 3B was carried out during the first ten days of March 2002. During this mission, astronauts replaced the FOC with a new instrument, the Advanced Camera for Surveys (ACS).Astronauts also 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 WFPC2, remained fully commissioned for science. STIS also operated nominally until it failed in August of 2004, but was restored to full functionality during SM4.B.1.5 Loss of STISThe Space Telescope Imaging Spectrograph (STIS) Side-2 electronics failed in August of 2004, during Cycle 13. The instrument was maintained in safe mode until its recovery during SM4.During Cycle 15, ACS WFC and HRC became unavailable for observations due to a failure in the electronics. The two cameras remained powered off until SM4.B.1.7 SM4Servicing Mission 4 (SM4) started with the Atlantis launch on May 11, 2009 and was completed, upon return of the Shuttle, on May 24, 2009. Space Shuttle astronauts installed two new instruments, the Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS), together with a new set of gyros and other flight hardware. WFC3 replaced the Wide Field Planetary Camera 2 (WFPC2) as the on-axis instrument; COS replaced COSTAR.The Space Telescope Imaging Spectrograph (STIS), that had suffered a serious electronics failure in August 2004, was fixed. The ACS repair successfully recovered the ACS/WFC that had ceased to be operational with the ACS/HRC in January 2007 due to an electronics failure. The Advanced Camera for Surveys Wide Field Camera (ACS/WFC) and the Solar Blind Channel (ACS/SBC) are now operating nominally. The High Resolution Channel (ACS/HRC) could not be recovered during SM4. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is still on board HST but is no longer in service.HST is expected to continue to operate in three-gyro mode for the foreseeable future.