STIS Update

Van Dixon,

Parallel and Serial CTI of the STIS CCD

After 13 years on orbit, the Space Telescope Imaging Spectrograph (STIS) Charge-Coupled Device (CCD) detector shows the effects of accumulated radiation damage. In particular, its charge transfer efficiency (CTE)—a measure of the ability of the CCD to move charge from one pixel to the next during readout—continues to degrade. CTE is expressed as the fraction of charge successfully transferred. In practice, one usually refers to the charge transfer inefficiency (CTI), where CTI = 1 – CTE. The CTI in both the parallel and serial directions on the CCD is measured twice per year. The most recent measurements, plotted in Figure 1, were derived using the extended pixel edge response test of Goudfrooij et al. 2009 (STIS ISR 2009-02).

An interesting pattern is apparent in Figure 1. While the CTI in the parallel direction (upper panel) has remained roughly constant since the last servicing mission (SM4), the CTI in the serial direction (lower panel) has risen steeply. The scatter in the upper plot illustrates the temperature dependence of the parallel CTI. The serial CTI, however, appears to be independent of temperature. The parallel CTI remains an order of magnitude larger than the serial CTI. We will continue to monitor both effects.

Recent Trending of the STIS NUV MAMA Dark Rate

Both the near- and far-ultraviolet (NUV and FUV) Multi-Anode Microchannel Array (MAMA) detectors were disabled in mid-September 2010 while a change to the flight software was developed. The purpose of the change was to protect the detectors in the event of an electronic upset in the science-instrument command and data-handling unit. The software update was installed on 15 November 2010 and appears to be working properly.  The dark rate in the NUV MAMA detector has fallen steadily since the repair of STIS during SM4, reaching a mean of about 0.0035 counts/sec/pixel (at a temperature of 38° C) in August 2010.  The dark rate rose during the two-month shutdown of the MAMA detectors, but it has since fallen to about 0.004 counts/sec/pixel (in early December 2010) and continues to decline.