UIST: Read Modes and Noise

UIST: Read Modes and Noise

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.


In imaging mode, readout areas of 1024×1024, 512×512 or 256×256 are available, each with either the 0.12 arcsec or 0.06 arcsec pixel scale. Subarrays are centered on the centre of the full array (unlike UFTI, there is no speed advantage when using an off-centre quadrant).

In spectroscopy mode, UIST is always used with the full array readout and the 0.12 arcsec pixel scale, giving a long-slit spectroscopy slit length of ~2 arcminutes. Subarrays are – strictly speaking – not available in spectroscopy mode.

NDSTARE (NDR) and Digital Averages

NDSTARE is UIST’s non-destructive readout mode. It is the default mode of operation for all spectroscopy, all IFU, and all non-thermal imaging. To minimise noise, as many reads as possible (one per second) will be fit into each NDSTARE exposure. In other words, the array is reset then read N times where N is at least 2 (for a 1sec exposure). The minimum exposure time (full array) with NDSTARE is 1.0 seconds.

To reduce extraneous noise each pixel is also sampled multiple times on readout and the result is “digitally averaged”. As a compromise between reducing noise and adding overheads, with NDSTARE the number of digital averages is always (automatically) set to 4.

With NDSTARE a reverse bias of 600 mV is used.

Correlated Double Sample (CDS) 

CDS, or Correlated Double Sampling, is available in imaging mode with each full and sub-array. In this mode the array is reset then read out just twice, at the beginning and end of the exposure (more detail is given here). 4 digital averages are taken per read.

CDS should only be used if a full-array readout with a very short (less than 1 second) exposure time is required, or if linearity is likely to be a problem. CDS might be useful on very bright targets where the full array is needed. The minimum exposure time (full array) with CDS is 0.623 seconds, which can of course be used with a number of coadds. 

With this mode a reverse bias of 600 mV is used.

Thermal Imaging 

Two readout modes are available for thermal imaging, THERMAL CDS and THERMAL ND. As the names suggest, CDS and ND are correlated double-sample and non-destructive readouts, respectively. Both modes utilise a higher reverse bias (900mV) which gives increased full well depth (saturation at a higher count level). However, these modes do no digital averaging (multi-samples = 1), facilitating faster readout.

Minimum exposure times (full array) are 0.172 sec and 0.20 sec for Thermal CDS and Thermal ND, respectively. Shorter exposure times are possible with sub-arrays.

Imaging Polarimetry 

Imaging polarimetry is unusual in that long and very short exposures are often required; the former for sky flat fields, and the latter for bright polarisation standards (which are often 7-9th mag). A read mode called IRPOL CDS is available for these circumstances. It is essentially the same as the THERMAL CDS read mode, except that the NULL read (see below) that precedes the first read of the CDS has a minimum exposure time (the dwell time on the NULL is set to 0.001 sec).

NOTE – IRPOL CDS is only available with the full 1024×1024 pixel array. 

The following is NOT required reading for observers

Noise Reduction with Digital Averaging 

Below we tabulate the noise measured in a single 1-second reset-read-read CDS exposure with different numbers of digital averages (data courtesy of David Atkinson, UKATC). These data were collected during ARC-controller commissioning, when UIST was on the UKIRT dome floor. Slightly higher values are encountered when UIST is on the telescope.

Data in the top two and bottom two quadrants of the array are presented separately. Increasing the number of averages clearly reduces the noise. However, more digital averaging leads to increased overheads. A value of 4 has therefore been adopted for non-thermal readout (NDSTARE and CDS).

Array tests, which use the MEASURE_READNOISE DR recipe, should be run at the beginning of each night of UIST observing. This will give a measure of the current readnoise on the array in a 1sec (4 digital averages) exposure. Past values are stored in a text file in /ukirt_sw/logs.

Noise with NDSTARE – Multiple Reads and Digital Averaging 

Read noise decreases with increasing exposure time, as shown in the plot below. In these data the digital averaging has again been set to 4; the number of NDSTARE reads increases with increasing exposure (one per second). The readnoise drops to around 8 electrons with long (greater than 60 sec) NDSTARE exposures; long exposures are clearly desirable with short-wavelength imaging and all IFU and long-slit spectroscopy of faint targets. (Data courtesy of David Atkinson, UKATC.)

Null Reads 

All UIST exposures are preceded by a NULL read, i.e. a read that is thrown away. The duration of this NULL read is the same as the total exposure time in CDS, or the read interval in NDSTARE (dT = 1 sec for full-array NDSTARE readout). Read intervals for the various sub-arrays and read modes are tabulated on the next page. (The only exception is the IRPOL CDS mode which has a fixed, very short NULL read – see above.)

As an example, in a 10 second NDSTARE exposure (full array), the controller returns 12 reads, the null read, plus 11 reads separated in time by 1 second. A 10 second CDS exposure would return only three frames, the NULL plus two further reads separated by 10 seconds.

The array is reset between the NULL read and the first read of the exposure.


With the ARC controller, the array is continuously RESET between exposures (as was the case with the Edict controller); the settling delay between resets is kept short at 0.01 sec. 

Idling is enabled as soon as an application (exposure with a given read mode) finishes executing. However, note that downloads to the controller do disable Idling – until the application has finished executing.

Periods of Idling can reduce the signal on the subsequent frame by 10-20 counts; at the telescope this may manifest itself as a dark with slightly negative counts. We did experiment with Idling that consisted of a RESET+READ. However, we found that when filter wheels were moved while idling (even when using a fast, 0.1 sec read), light was getting onto the array as an open or thermal filter passed over the array. This resulted in the first frame having high counts; in some cases this latent signal was hundreds of counts in a 60sec exposure.