Imaging: Optical Parameters

The 1024x1024 Raytheon (previously SBRC) InSb array has 27 micron pixels and the plate scales for the three camera modules are shown in the above table. The UIST display shows (approximately) East up and North to the left; the array columns are slightly offset from true East-West as shown in the above table (to get columns exactly E-W one would have to rotate an image counterclockwise by the angle given above).

You can rotate your reduced image in the Gaia display by clicking the button; this will show current and future images with North up and East to the left.

The position of a target on the array is set by the "instrument aperture", a number set by JAC staff that does not change from run to run (it only changes when the instrument or telescope tertiary mirror is taken off/put back on the telescope). A given telescope base position (e.g. target coordinates) should coincide with pixel [480,480] on the full-array, or [224,224] on the 512x512 sub-array. Inprecise telescope pointing or inaccurate guide star or target coordinates can move a target by a few arcseconds, and so your source may not coincide exactly with this pixel-coordinate on the array. When using a subarray with the smaller pixels it is advisable to use movie mode to check your positioning. When peaking up in imaging mode (as opposed to spectroscopic imaging acquisition) the command "left" moves the target up and "up" moves it to the right.

	Imaging: Current Filter Set

UIST has two 24-slot filter wheels which contain filters for both imaging and for blocking the spectroscopy grisms. The MKO consortium filter set is installed where possible (see the discussion of new JHK standards for example). We list the imaging filters in the table below, the actual population of the filter wheels is also available. Click on the filter name to see an image of the profile; text files are also available giving transmission profiles for some of the filters, contact Watson P. Varricatt. The 3.6nbL' filter appears to have some surface damage and produces trailed images at a low level.

	Imaging: Sensitivity Tables

The UIST 0.12"/pix and 0.06"/pix camera modules have similar sensitivities. These values assume background limited performance - at very short exposures signal-to-noise does not scale with square root of time for a given signal, as there is some extraneous noise seen in the instrument. See notes below on other filters and overheads.  

With narrow-band filters for continuum sources the source signal and background signal are both reduced by the reduced passband of the filter, i.e. by about a factor of 10. The Signal-to-Noise ratio for a continuum source will therefore decrease by the square-root of the ratio of filter bandpasses, sqrt{broad/narrow} ~ 3.2, and the magnitudes in the above table will decrease by 1.25. For an unresolved emission-line source, only the background signal is reduced by the reduced passband, so the Signal-to-Noise ratio for a line-emission source will increase by the square-root of the ratio of filter bandpasses, sqrt{broad/narrow} ~ 3.2, and the magnitudes in the above table will increase by 1.25.

Overheads for broadband thermal imaging can be significant. Either the 0.06 or 0.12 "/pixel cameras can be used; the smaller pixel scale is more efficient but of course gives a smaller field. The full 1024x1024 readout can be used with the 0.12"/pixel camera with the L' filter. Due to high background emission, the central region of the array goes to highly non-linear regime in M' with the 0.12"/pix camera when we do the 1024x1024 readout. Hence in M', one has to do 512x512 sub-array readouts. However, 1024x1024 readout is possible in the M' with the 0.06"/pix camera. Both 512x512 and 256x256 subarrays are available although the smallest subarray is very rarely used. Exposure times and overheads for L' and M' for each camera are given here. See also the separate page on thermal imaging.

	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.

Subarrays

In imaging mode, readout areas of 1024x1024, 512x512 or 256x256 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 1024x1024 pixel array.

The following is NOT required reading for observers

Noise Reduction with Digital Averaging

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.

Idling

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.

	Read Speed, Exposure Times and Efficiency

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.

Maximum Exposure Time

IMPORTANT: Currently, because of memory limitations on the acquisition machine, the MAXIMUM exposure time possible with UIST is 240 seconds.

Read Speeds and Minimum Exposure Times

The array can be addressed or "clocked" at various speeds. However, the faster readout rates lead to output coupling, where a fraction of the signal on one pixel is picked up on a pixel 8 columns away. Read speeds are therefore limited by a desire to minimise this effect.

With NDSTARE readout of the full array (used in non-thermal imaging and all spectroscopy modes), a read time of 622 millisecond has been adopted; with an additional dwell time of 378 secs, this means that the array is read out every second. With a 10 second NDSTARE exposure, the array is read 11 times (12 including the NULL read), with each read being sampled four times (the digital averaging). Faster readout is possible with sub-arrays, and of course the thermal readout modes (used only with thermal imaging), run with faster readout clocks leading to shorter read times, as shown below.

The READ, DWELL and TOTAL times in the above table represent the following:

• The READ time is time it takes to physically read out the whole array.

• The DWELL is effectively a pause after the READ:
  • The DWELL is fixed with NDSTARE - longer exposures are created by increasing the number of reads. For example, a 0.6sec NDSTARE 512 exposure would consist of a Null read, a 0.184sec read, a 0.016sec dwell, a 0.184sec read, a 0.016sec dwell, a 0.184sec read, a 0.016sec dwell, and finally a 0.184sec read.
  • The DWELL is variable with CDS - longer exposures are created by increasing the dwell time. For example, a 2sec CDS 512 would consist of a Null read, a 0.184sec read, a 1.816sec dwell, then the final 0.184sec read.

• The TOTAL time listed in the above table is READ + DWELL, and therefore represents the minimum exposure time with each readout mode: your exposure time can be (much) longer than this.

Readout Overheads and Efficiency

As described earlier, digital averaging is used to beat down the read noise in each exposure. The read noise also decreases with longer exposure times, reaching an optimum above about 60 seconds. Long exposures are also by far the most efficient. Only saturation on source or non-linearity, or the desire to collect data before the sky background changes appreciably, should limit the exposure time used.

If short exposure times must be used, then it may be desirable to combine these with a number of co-adds. Although this makes each individual observe less efficient, overall this may be more efficient when considering the time taken to nod the telescope between "source" and "sky". Increasing coadds also brings down the read noise (a little).

Optimum exposure times

The optimum exposure time is dependent on many factors, such as resolution, wavelength, object brightness and weather conditions. Consequently, although clicking on "Default" in the instrument configuration in an OT sequence will yield a reasonable exposure time for a specific source magnitude, you may need to fine-tune this value. Note, however, that generally the maximum possible exposure time is the optimum exposure time, as overheads are reduced to a minimum.

Spectroscopy tests during commissioning, where the same source was observed for the same total on-source period of time, but using 10sec, then 30sec, then 120sec and then 240sec exposures clearly showed that the better detection was obtained with a few long exposures as compared to many short exposures. Indeed, it seems that 200-240 sec exposure times are optimum for non-thermal spectroscopy of faint sources, or half of this if the OH sky lines aren't being subtracted off too well. In imaging mode, a five or nine-point jitter pattern should probably be acquired within 5-10 minutes, so that a suitable flat field frame can be created from the median average of the jittered target images.

	Gain and Linearity

Much of this is NOT required reading for observers; a linearity correction is currently being applied in the ORAC-DR 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 minimised 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.

Click for an expanded plot

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).

	Imaging: Saturation and Sky Counts

Note that saturation magnitudes (the brightest point source that can be observed at the shortest possible exposure time for each mode) are very seeing dependent.

A saturated frame has a typical pattern as shown below. An exposure consists of a single global reset followed by two or more reads of array where the four quadrants are read out in parallel, from the outer corner towards the center. The output image is the difference between the reads so that the interval between reads is constant across the array. The center of the array is always exposed longer in this read mode and hence in the case of a high background the center will saturate sooner than the outer regions; a saturated NDSTARE frame has a characteristic pattern with a strong gradient in counts across the frame, as shown in the image below.

	Notes on Thermal Imaging

Overheads

Overheads for broadband thermal imaging can be significant. Either the 0.06 or 0.12 "/pixel cameras can be used; the smaller pixel scale is more efficient but of course gives a smaller field. The counts from the night sky with the minimum exposure times with the full 1024 x 1024 array using the 0.12"/pix camera are given in the table below. Both 512x512 and 256x256 subarrays are available although it is expected that the smallest subarray will not be frequently used. Exposure times and overheads for L' and M' for each camera are given in the table below. These are preliminary values, which will be updated after more measurements.

^b We have found that the 512x512 readout and the 0.06 camera at L' is not as stable as the 1024x1024 readout - the background is not always cleanly subtracted. We strongly recommend using the 1024x1024 readout with the 0.06 camera at L' - efficiency is still quite high.

Photometric Behaviour and Telescope Offsets

We have found that the low-QE lower right quadrant (upper left now) and the central 100-pixel wide area of the array are not always well behaved, in that subtracted pairs of images can show some variation in these regions. Also photometry in these regions can give fainter values compared to positions elsewhere in the field. This is not fully understood, but the problem in the centre may be related to the fact that the array is read from the outer corners into the middle, so that the central region is exposed to sky for longer in a reset-read-read NDSTARE observation which may lead to non-linearity effects. The problem is especially bad at L' with the 0.06 camera and the 512x512 readout - we recommend not observing in this configuration but instead use the full 1024x1024 readout if using the 0.06 camera at L'. The 512x512 performs reasonably for M'.

To deal with the problem of the lower QE, the template and standard star libraries were set up to use offset patterns that avoid the lower right and centre as far as possible, where the low-QE region was located before flipping. For the nod-8 pattern and the 512x512 subarray this means that we use the left (North) side of the array only, as the telescope aperture places the target slightly left of centre. The array orientation was flipped (up-down) in the cryostat in March 07. To take care of this change in orientation, the offsets in the libraries and the templates will be modified soon.