Spitzer Documentation & Tools
IRS Instrument Handbook

2.8.3             IRS Detector Characteristics

1. Photometric responseThe 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. GainThe 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.