2.2 STIS Performance Changes Pre- and Post-SM4

In May 2001, the Space Telescope Imaging Spectrograph (STIS) switched over from using Side-1 to Side-2 electronics when a short circuit blew a fuse, knocking out the Side-1 electronics. STIS operated on Side-2 electronics until it ceased science operations on 03 August 2004, due to the failure of a power supply within the Side-2 electronics.

In May 2009, during the fourth spacewalk of the HST Servicing Mission 4, (SM4), the STIS Side-2 circuit board containing the failed low-voltage power supply was replaced. This repair fully restored the STIS instrument to operation with capabilities similar to those it had before the Side-2 failure. In the appropriate sections of this Handbook, the performance changes since 2004 in detector dark currents, throughputs, and charge transfer efficiency (CTE) will be discussed.

Many figures and tables were originally updated for the Cycle 17 (first post-SM4) version of this handbook, using our best estimates of instrument performance for Cycle 17. In many cases, those figures and tables have not been revised to take into account the modest changes expected between those projected values and the estimated Cycle 33 performance. For more up-to-date performance values, users should consult the Performance section of the STIS web pages and the STIS ETCs.

The sensitivities and other instrument parameters adopted for use with the Cycle 33 ETCs have been updated with our best estimates for April 2026.

STIS will be among the instruments available for inclusion in Cycle 33 Phase I proposals. Please consult the Cycle 33 Call for Proposals and the Cycle 33 Proposal Announcement webpages (available through the Proposing Overview web page) for any updates on instrument status.

2.2.1 Summary of STIS Performance Changes Since 2004

The performance of STIS after its repair during SM4 is similar in most respects to its performance prior to the 2004 failure, but there are some important differences. While most of the changes in performance represent the continuation of previous trends (and were anticipated), some of the changes (e.g., in the behavior of the NUV-MAMA dark current) were not expected. All of these changes need to be taken into account when planning new STIS observations.  Detailed descriptions of the differences are given in the appropriate sections of this Handbook, but for the convenience of users familiar with the previous performance of STIS we list the most important of the changes below, linked to the appropriate sections of the Handbook:

• Changes in CCD Read Noise (7.2.3)
• CCD Dark Current (7.3.1)
• Charge Transfer Efficiency (7.3.7)
• Figure 7.1: Relative Sensitivity vs. Time for First-Order CCD L-Modes G230LB, G430L, and G750L (7.2.5)
  
• NUV-MAMA Dark Current after SM4 (7.5.2; see Figure 7.20)

For the most up-to-date values of some of the more frequently monitored quantities, users should consult the Performance section of the STIS web pages. The STIS ETCs are regularly updated to reflect the current/anticipated detector characteristics.