1. Photometric response –The detector response stability was better than 1% over multiple cool-downs separated by months, and the responsivity of the detectors was unaffected by radiation. Overall repeatability depended upon the selected pointing (Peak-Up) accuracy but was as good as 2%.
2. Effect of overexposures – Collection of over 300,000 electrons/pixel before a reset caused a 1–2% reduction in responsivity in the affected pixels for a period of approximately one hour.
3. Read noise – The read noise was 30 electrons/pixel in SUR (“sample up the ramp”) mode with 16 samples per ramp (see Figure 2.13).
4. Dark current – At a bias of 1.6 volts, the dark current was less than 10 electrons/sec per pixel for the SL/SH arrays and less than 40 electrons/sec per pixel for the LL/LH arrays.
5. Gain – The gain was 4.6 electrons/DN (data number).
6. Pixel masks – The pixel badness criteria are (1) having a dark current greater than 40 electrons/sec in SL/SH or greater than 160 electrons/sec in LL/LH, or (2) a responsivity less than 50% (or greater than 150%) of the median responsivity of the array.
7. Cosmic ray transient effects – Pre-launch proton irradiation tests showed no cosmic ray transient effects after the usual boost/reset sequence. However, there may be transient effects within a given ramp for a small number of reads before the next boost/reset. This is corrected in the science data processing pipeline.
8. Bias – As part of the normal maintenance plan for the IRS, the bias levels were occasionally adjusted. see Table 2.2 for a summary of these changes. This is corrected in the science data processing pipeline.
Table 2.2 Summary of changes in bias and temperature for all IRS modules.
IRS Campaign
SH bias / temp
LH bias / temp
SL bias / temp
LL bias / temp
IOC* campaign K2 - 24
2 V / 6.2 K
2 V / 4.4 K
2 V / 6.2 K
1.8 V / 4.4 K
25-44
2V / 6.2 K
1.6 V / 4.4 K
2 V / 6.2 K
1.8 V / 4.4 K
45-61
2V / 6.2 K
1.6 V / 4.4 K
2V / 6.2 K
1.6 V / 4.1 K
*In Orbit Checkout
9. Droop – Droop is a slope proportional to the photocurrent summed over all pixels, which is added to every pixel of the array when it is non-destructively read in SUR mode. Noise measurements show that this slope is a multiplexer (MUX) artifact, not a true current. The droop coupling constant (droop/[total array photocurrent]) is typically 3x10-5 for all arrays, which is equivalent to a ratio of droop to average dark current of 0.48. Droop is present in all IRS arrays.
10. Well depth – Well depth, defined as the point at which the differential responsivity has fallen to 90% of its small-signal value, exceeds 2x105 electrons/pixel for Si:As and exceeds 105 electrons/pixel for Si:Sb. The full well depth is a few times these values and irrelevant for the user, because digital saturation (301,466 electrons=65535 DN) is reached before electronic saturation.
