Spitzer Documentation & Tools
IRS Instrument Handbook

2.8.3             IRS Detector Characteristics

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


2V / 6.2 K

1.6 V / 4.4 K

2 V / 6.2 K

1.8 V / 4.4 K


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.