The STIS was built with two redundant sets of electronics. On 2001-May-16, the primary (Side 1) set of STIS electronics failed, and operations had to be resumed on the backup (Side 2) set. Although most operations on Side 2 are identical to those on Side 1, there are two important differences, both in regard to the CCD. First, the effective read noise through each of the four available CCD amplifiers has increased by approximately 1 e
-/pix in the
GAIN=1 setting (e.g., from ~4.
5 e
-/pix to
5.
5 e
-/pix for the default D amplifier, which has the lowest read noise). This increase in noise is spatially correlated (i.e., pattern noise), although the pattern can sometimes be mitigated through filtering. See
ISR STIS 2001-05 for full details on the CCD pattern noise and filtering techniques. The second ramification of the switch to Side 2 was that some of the thermal control of the CCD has been lost, causing variations in the CCD dark rate.
On Side 1, a temperature sensor mounted on the CCD carrier provided closed-loop control of the current provided to the thermoelectric cooler (TEC), thus ensuring a 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 on Side 2 is 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 often runs cooler on Side 2 than it did on Side 1. The result is that the median dark rate varies from 4 to
5 e
- per 1000 s on Side 2, as opposed to 4.6 to
5 e
- per 1000 s on Side 1. Details of the temperature dependence of the STIS CCD dark rate are somewhat complicated, however, and they are fully explained in
ISR STIS 2001-03.
Although no sensor is available to measure the temperature of the CCD itself, there is a sensor for the CCD housing temperature. 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 (
ISR STIS 2001-03). 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. Starting in January 2002, the CCD housing temperature for each Side 2 observation was included in the science header (keyword
OCCDHTAV) and applied to the dark reference file by the OTFR pipeline. Since January 2005, this information has instead been taken from _epc.fits files delivered by OTFR.
