Space Telescope Science Institute   7.7 ACS Apertures  7.7.2 Ramp Filter Apertures

7.7.1 WFC Apertures


The active image area of each WFC detector is 4096 by 2048. The mean scale is 0.049 arcseconds/pixel, and the combined detectors cover an approximately square area of 202 arcseconds on a side. In establishing reference pixel positions we have to consider the overscan pixel areas which extend 24 pixels beyond the edges in the long direction. So each CCD must be regarded as a 4144 by 2048 pixel area. The gap between the two CCDs is equivalent to 50 pixels. In Figure 7.3 the letters A, B, C, and D show the corner locations of the four readout amps.

We define apertures named WFC1 and WFC2 which represent the two CCDs, with their reference points near the geometric center of each chip. The positions have been moved about 50 pixels from the center line to avoid a discontinuity at the amplifier readout boundary. However, we keep two other apertures named WFC1-FIX and WFC2-FIX at the original central locations (2072,1024). For extended sources, choosing new positions may not be of any advantage and it may be more effective to use these fixed positions.

The aperture WFC encompasses both detectors, and has its reference point near the overall center but about 10 arcseconds away from the interchip gap. This has been chosen to be position (2124,200) on the WFC1 CCD. Again, this has been moved away from the center line, but the reference point for WFC-FIX remains at (2073,200). Selection of WFC1, WFC2 or WFC only changes the pixel where the target will be positioned. In all three cases data is normally delivered in a file containing two imsets, one for each detector. See the ACS Data Handbook for details of the ACS data format. Reading out a subarray, which consists of part of only one of the chips, is done only if requested.

Figure 7.3: WFC aperture definitions.

 

WFCENTER is similar to WFC, but is placed at the center of the combined WFC full field. The center is defined as the average of the four corners in the distortion corrected space. Because of the scale variation this does not appear at the center in pixel space, but rather is on WFC2 about 20 pixels from the edge. Selection of WFCENTER can be of use in obtaining observations with maximum overlap at unique orientations and for mosaics.

For sets of observations which take place over a substantial part of a year, the telescope roll limitations will require measurements to be taken over most of the angular range. On sky, the WFC aperture is roughly square, and it is natural to design observations in steps of 90º to consistently cover the same area. There will be some region at the edges not covered at all four orientations. However, a square area of side 194.8 arcseconds centered on WFCENTER, and with edges parallel to the V2 and V3 axes, is overlapped at all four positions. In designing a mosaic which combines observations at 90º steps, a translation of about 190 arcseconds between pointings would provide continuous coverage.


 7.7 ACS Apertures  7.7.2 Ramp Filter Apertures
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