1.3 STIS Operations on Side-2

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 STIS ISR 2001-05 for full details on the CCD pattern noise and filtering techniques. The current readout noise of amplifier D in the GAIN=1 setting as of Cycle 25 is 6.2 e¯/s. 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 to the thermoelectric cooler (TEC), ensuring a stable detector temperature at the commanded set point (-83°C). 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°C 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 initially varied 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. The dark rate as of Cycle 25 on the detector is closer to 19 e¯ per 1000 s. Details of the temperature dependence of the STIS CCD dark rate are somewhat complicated, however, and they are fully explained in STIS ISRs 2001-03 and 2018-05.

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 (STIS ISR 2001-03). Note that the CCD housing is far hotter than the detector itself: the housing temperature is approximately 18°C during normal operations, while the detector runs at approximately -83°C. 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 pipeline. Since January 2005, this information has instead been taken from _epc.fits files.