WFCAM Detector Channels

Document number VDF-TRE-IOA-00008-0004

Jim Lewis

WFCAM detectors are divided into 32 equal sized readout channels, each covering 128 x 1024 pixels. Knowledge of the orientation and readout timings of each of these channels is vital in order to correct things like linearity and crosstalk properly. This note is intended to be a repository of 'wisdom' on this subject and more information will be added when it becomes available. The next three figures and the included text are relevant to detector 1 in WFCAM. How we can generalise this for all four detectors is discussed afterwards. The ordering of the channels is shown in Figure 1. Channel number 1 is the black channel. The numbers increase from black to blue, from blue to red, from red to yellow and finally from yellow to white.

Figure 1

Because the readout is along columns or rows depending upon which quadrant we're discussing, I'll refer to these as 'lines'. Figure 2 shows how the pixels in each line are read out. The fast direction of readout is always along the short axis. Once a line is address, then the readout procedes along the line where the dark pixels in the figure are the pixels read first and the white ones are read last.

Figure 2

Having established which direction the lines are read we need to know what order the lines are read in. This is the final piece of information which allows us to form an exposure map (reset timing notwithstanding -- more on this later). Figure 3 below shows something like an exposure map. The black pixels show the lines that are read first and the white pixels are the lines that are read last.

Figure 3

The readout for each detector will be done such that the images will all be rotated with respect to each other (i.e. North will be a different direction on each detector). To fix this problem the data arrays will be rotated in memory before being written to disc. This means that the arrangement of the readout channels will also be different for each chip. The rule of thumb is that for each detector the readout channels will be rotated 90 degrees clockwise with respect to the channels on the previous detector. For example, the arrangement of the channels for detector 2 will look just like figure 1 (for detector 1), except it will be rotated by 90 degrees clockwise (see figure 4).

Figure 4

Last modified: Tue Jul 13 13:21:38 2004