UIST – Gain and Linearity

UIST – Gain and Linearity

Much of this is NOT required reading for observers; a script for linearity correction is currently provided to be used as an add-on to the ORAC-DR pipeline. This will soon be incorporated into the pipeline. 

The information below pertains to the new ARC (formerly SDSU) controller commissioned with UIST in December 2006. For numbers specific to the old Edict system, please contact the instrument scientist.

Bias Voltages and Saturation 

The detector reverse bias is automatically selected by the low-level software. With NDSTARE readout (with full and sub-array imaging and all spectroscopy modes) 600 mV is used; for thermal imaging (THERMAL ND and THERMAL CDS) 900 mV is used. The latter gives increased well depth and better linearity, which are important with the high background flux encountered at L and M.

The array goes into hard saturation at around 21,000 counts (~135,000 electrons) with NDSTARE; in the thermal saturation occurs at 33,200 counts (~210,000 electrons). See below for details…

Dark Current 

At 600 mV reverse bias the dark current is ~0.1 e-/pixel/second. At 900 mV a value of 0.4 e-/pixel/second has been measured.

Pixel Settling Time and Output Coupling

Performance in the thermal is improved by increasing the reverse bias, which increase the full well depth, but also by reducing the readout speed. In thermal readout modes video processing has been minimized so that the pixel processing time is dominated by the pixel settling time. 2.5 microseconds is the minimum pixel settling time (limited by the fibre optics speed). However, as the settling time is reduced, output coupling increases. That is, the fraction of signal on an output channel that remains when the channel is next sampled. This produces a ghost image 8 columns along the detector. The output coupling resulting from a 2.5 microsecond pixel settling time is shown here.

For non-thermal readout a pixel settling time of 7.1 microseconds has been adopted. This leads to an output coupling factor of only 0.44%. In the thermal, a settling time of 4.0 microseconds is used, which gives output coupling of 1.16%.

Gain and Linearity 

Tests with the ARC controller suggest that there is no single value for the system gain. Rather, this seems to vary with flux level, from around 5.75 electrons per data number (per count) at ~10% full-well, to ~6.65 electrons per DN at ~90% full-well with 600 mV reverse bias (non-thermal readout). Similar behaviour is seen with 900 mV reverse bias (the gain is slightly lower because of reduced detector capacitance).

Linearity curves for the 600 mV (non-thermal) and 900 mV (thermal) bias settings are shown below. The blue lines give the linearity in data numbers; the pink lines give linearity after correction by the flux-dependent system gain level noted above. This correction results in a linear plot. 

In the thermal, it is recommended that the same exposure time (and/or counts on the source and sky), is used with the target and the photometric standard star.

Linearity – 600 mV – Non-thermal readout
Linearity – 900 mV – Thermal readout
Residuals – 600 mV – Non-thermal readout
Residuals – 900 mV – Thermal readout

NOTE: use the right-hand (900 mV) plots ONLY with thermal imaging modes. If you’re unsure about your data, check the DET_MODE fits header: 900mV is used with modes that end in ‘T’, e.g. ND1T, CDS1T, etc.

Interpixel Capacitance 

Finally, there is very little interpixel capacitance coupling on the UIST array. Approximately 10% is transmitted to the surrounding 4 pixels, either side in a row and top and bottom in a column (with both 600 mV and 900 mV reverse bias).

Data and analysis courtesy of David Atkinson, UKATC (Winter 2007).