Temperature Dependence of the STIS CCD Dark Rate During Side-2 Operations (2001)

Due to a failure of the primary (Side-1) STIS electronics, the STIS has been operating on its redundant (Side-2) electronics since July 2001. Side 2 does not have a functioning temperature sensor for the CCD, and so a constant current is applied to the CCD thermoelectric cooler. Thus, Side-2 operations exhibit a variation in CCD temperature that was not present during Side-1 operations. Fortunately, these variations in temperature can be tracked by a temperature sensor on the CCD housing. In this report, I describe the temperature dependence of the STIS CCD dark rate. For pixels at a given dark rate, the change in rate with temperature is extremely linear over the observed range in CCD housing temperature (approximately 3 oC). However, the change in rate with temperature depends upon the rate itself. Pixels with very low dark rates ( 1 es-1) show a 6%/ oC rate variation. Introduction On 16 May 2001, the primary (Side-1) set of STIS electronics failed, and operations were subsequently resumed on the redundant (Side-2) set. Although most operations on Side 2 are identical to those on Side 1, some of the thermal control of the STIS CCD has been lost. On Side 1, a temperature sensor mounted on the CCD carrier provided closedloop control of the current provided to the thermoelectric cooler (TEC), thus ensuring a Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. Instrument Science Report STIS 2001-003 stable detector temperature at the commanded set point (-83 oC). Side 2 does not have a functioning temperature sensor, and so the TEC is run at a constant current. Thus, under Side-2 operations, the CCD temperature varies with that of the spacecraft environment, and these temperature changes are accompanied by changes in detector dark rate. Because the current to the TEC is now fixed at a higher value than the typical value required to hold the -83 oC set point on Side 1 (i.e., 3 A vs. 2.7 A), the detector is often cooler than it was during Side-1 operations. The result is that the median dark rate varies from 4 to 5 eper 1000 s on Side 2, as opposed to 4.6 to 5 eper 1000 s on Side 1. Using data from dark rate monitoring programs, I have characterized the dark rate dependence on the CCD housing temperature. Although no sensor is available to measure the temperature of the CCD itself, there is a sensor for the CCD housing temperature, associated with telemetry keyword OCCDHT. The hot side of the TEC is bonded to the CCD housing baseplate; hence with fixed TEC current, the CCD housing should track closely the detector temperature under Side-2 operations, and this can be seen by the excellent correlation between the dark rate and the housing temperature (reported in the following sections). Note that the CCD housing is far hotter than the detector itself: the housing temperature is approximately 18 oC during normal operations, while the detector runs at approximately -83 oC. Data Reduction The analysis focused on approximately 2 months of data from programs 8864 and 8901, spanning the time period from 13 July 2001 to 21 September 2001. I used the long exposures (1100 s each) from these programs, taken with a gain of 1 and no on-orbit binning. At the time of this writing, there have been three CCD anneals since the start of Side-2 operations. During each anneal, the TEC is turned off, and the rise in CCD temperature repairs some of the hot pixels that have accumulated in the detector. Thus, I grouped these data into 3 subgroups, representing each period between anneals. The raw frames were processed through the CALSTIS pipeline, which includes a bias subtraction from the CCD overscan region and the subtraction of a superbias. A mean temperature for each frame was calculated by averaging the CCD housing temperature, as a function of time, across the time period spanned by the exposure. The processed frames in each anneal period were then sorted by temperature. Groups of 3 frames, with each frame having nearly the same temperature, were combined with cosmic-ray rejection, using the OCCREJECT routine. Each resulting coadded frame was assigned a new temperature, from the mean of its three input frames.