Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 17

2.1 STIS Repair and Return to Operations

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The Space Telescope Imaging Spectrograph (STIS) ceased science operations on 2004-Aug-03, due to the failure of a power supply within the Side-2 electronics, which had powered the instrument since 2001-May-16, when a short circuit had blown a fuse, knocking out the Side-1 electronics.

The failed power converter on Side-2 had delivered power to all the mechanisms within STIS, including the aperture wheel, the Mode Select Mechanism (which selects gratings and mirrors for a given observing mode) and the CCD shutter. At the time of the failure, the STIS was in idle mode, in which the light path is completely blocked (a precaution to prevent over-illumination of the MAMA detectors). In the absence of a working power supply, the mechanisms cannot be moved from these default positions, and the instrument is inoperable.

The current plan for the next servicing mission, SM4, includes replacement of the STIS Side-2 circuit board that contains the failed low voltage power supply. It is expected that this repair should fully restore the STIS instrument to operation with capabilities very similar to those it had before the Side-2 failure. In the appropriate sections of this handbook, anticipated changes in detector dark currents, throughputs, and charge transfer efficiency (CTE) will be discussed.

The plan for SM4 is an ambitious one. In addition to the STIS repair, SM4 will include an ACS repair, installation of replacement batteries and gyroscopes for HST, two new instruments (COS and WFC3), a replacement Fine Guidance Sensor, and repairs to thermal insulation. Relative priorities of these various tasks have yet to be finalized, and on-orbit events might result in some planned items not being completed. So it will not be known until after SM4 whether STIS has been successfully restored to operation.

It is currently planned that STIS will be among the instruments available for inclusion in Cycle 17 Phase I proposals. It is expected that the panel reviews and TAC will be held, and the Cycle 17 proposal pool will be selected, prior to the occurrence of SM4. Please consult the Cycle 17 Call for Proposals and the Cycle 17 Proposal Announcement Web pages, available through the Proposing Overview Web page at:

http://www.stsci.edu/hst/proposing/docs/proposingOverview

for any updates on instrument status, and for policy statements regarding procedures for dealing with any differences between the expected and actual post-SM4 HST instrument complement.

Three-Gyro Guiding

We anticipate that all HST observations in Cycle 17 will be undertaken in the Three-Gyro Science Mode, providing greater scheduling flexibility and greater sky coverage (at any epoch) than in Cycles 14, 15 and 16. Proposers should plan their observations accordingly, using the Target Availability Table in the Primer (Table 6.1) and the visibility tools on the HST Phase I Proposal Roadmap Web site.