WFCAM science arrays
This page provides technical information about the detector arrays,
and knowledge of these details is not needed to prepare your
Detector Array characteristics
The WFCAM IR detector arrays are
Hawaii 2™ 2048×2048 pixel HgTeCd PACE devices. In
operation, the arrays are cooled to 75K and each array is read out in
its 8 channel per quadrant mode through a 32 channel SDSU controller.
Table 2.1, “Array Overview” summarises the
properties of the 4 WFCAM arrays.
These arrays have been found to show inter-pixel capacitance,
meaning that adjacent pixels are not independent. With about 20% of
the incident on any given pixel appearing in adjacnet pixels. This has
2 effects - slight degradation of the image quality, and correlated
noise. One effect of that latter is that a simple gain measurement
assuming poisson statistics overestimes the gain by about 20%. This
effect has been included in the numbers quoted below.
Cosmetically, the array for camera number 3 is the cleanest and
also shows good QE. Thus, if your observations target a single object,
we reccomend that you place it on camera 3. To do this, you should
specify a telescope pointing centre 795 arcseconds South and 795
arcseconds East of your target co-ordinates.
Table 2.1. Array Overview
||Position in Focal Plane
||Array ID Number
||Full Well [electrons]
||Full Well [ADU]
The WFCAM IR arrays are each divided into 4 electronically
isolated quadrants, each of which is subdivided into 8 segments. All
32 segments are readout simultaneously through the 32 parallel
channels of one SDSU array controller. There are 4 such controllers,
one for each array. The 4 contollers are synchronised to minimise pick
up of readout clock signals between the arrays.
Figure 2.1, “Channel and array layout”
shows the layout of
the channels on the array and the orientation of each of the science
arrays in the focal plane.
Figure 2.1. Channel and array layout
WFCAM offers a limited number of readout modes. STARE mode is
for use in engineering only, science readout modes are limited to two
types of NDSTARE modes: CDS and MNDR.
CDS - Correlated Double Sampling is the default readmode and
should be used for all broad-band observations. Basically, this is a
reset-read-read sequence; the array is reset, then read out as soon as
possible. As soon as the first read starts, the exposure timer
starts. After the specified exposure time, a second readout takes
place. The second readout is subtracted from the first readout to give
the data values for that exposure. By doing two readouts like this, we
optimally remove the array bias signal and reduce readnoise. Each
readout step takes approximately 0.61 seconds to cover the entire
array. Although all the pixels are not readout simultaneously, the
interval between the two reads of any given pixel is exactly equal to
the exposure time. Figure 2.2, “CDS reaout timing”
illustrates this. The overhead for an exposure is approximately 1.2
seconds (reset-time plus read-time).
Figure 2.2. CDS readout timing
MNDR - Multiple Non-Destructive Reads will probably be used for
the longer exposure time modes, ie the narrow band filters. It's
possible that we'll use it with the broad band filters too. This will
be determined during commissioning. MNDR is similar to CDS, except
that readouts occur more frequently right through the exposure. Once
the exposure has finished, a straight line is fit to all the readouts
of each pixel in turn. The gradient of the line represtents the flux
arriving at that pixel. This helps to further reduce the readnoise in
the exposure, though the increased readout activity could cause pickup
More recent measurements on this subject (20090630)
Overhead for a CDS exposure: 1.39 +/- 0.05
Overhead for an NDR exposure: 1.61 + 0.075 * exptime
(seconds). For example, 5 seconds: 1.97 seconds, 10 seconds: 2.36
seconds, 20 seconds: 3.14 seconds, 30 seconds: 3.85 seconds.
Delay before a new image is started: 0.8 +/- 0.15
Total overhead between two consecutive images if a new guide star needs to be acquired:
10 seconds if within the same 4-points tile
11 seconds if starting a new tile
Total overhead between two consecutive images if a filter change is involved:
The array readnoise is approximately 25 electrons for CDS
exposures. NDR exposures have somewhat lower readnoise, but not as low
as would be expected due to some systematic effects. Typical NDR
readnoise is 15 electrons.
The readout gain is approximately 4.6 electrons per data
number for cameras 1, 2 and 3 and 5.6 for camera 4. Because of
inter-pixel capacitance, adjacent pixels are
coupled by approximately 20%, meaning that a simple Poisson noise
analysis will over-estimate the gain by about a factor of 20%.
The fundamental system parameters for sensitivity calculations
are the telescope collecting area, the filter bandwidth and the total
system throughput. For the purposes of these calculations, the
telescope is considered to have a collecting area of
. The filter bandwidth is given in
Table 1.1, “WFCAM science filters”
. Table Table 3.2, “WFCAM total system throughput by filter”
gives the mean total system
throughput under photometric conditions. For the narrow band filters,
assume similar throughput to K band.
Table 3.2 WFCAM total system throughput by filter (from CASU, and private communication with S. Hodgkin)