B.2 UVIS Channel

Figure B.1 illustrates the shape of the UVIS channel field of view as projected onto the sky. As noted above, its rhomboidal shape is due primarily to the diagonal tilt of the CCD focal plane with respect to the chief ray (see the schematic mechanical drawing of the instrument in Figure 2.1). The angle between the x- and y-axes is ~86.1°. The field diagonals are tilted slightly from the V2 and V3 axes. There is a ~1.2 arcsec gap between the two CCD chips. The crosses in the diagram indicate where points in the image would be located without non-linear distortion, and the vectors, scaled up by a factor of 10, indicate the actual locations of the points on the sky, including the non-linear distortion components.

The corner displacements are about 30 pixels, corresponding to 1.3 arcsec. The principal effect is the diagonal variation of scale. At the center of UVIS1 (CCD Chip 1), the scale in the x-direction is 0.0396 arcsec/pixel, and 0.0393 arcsec/pixel in the y-direction. For UVIS2 (CCD Chip 2), these scales are 0.0400 arcsec/pixel, and 0.0398 arcsec/pixel, respectively. Between the corner of the UVIS image nearest to Amp A and the diagonally opposite corner near Amp D, the overall scale increases by 3.5%. UVIS1 forms a slightly distorted rectangle 162 × 81 arcsec in size, while UVIS2 subtends 164×81 arcsec.

The resulting variation of the projected pixel area on the sky requires corrections to photometry of point sources using images that have not been distortion-corrected. A contour plot of relative pixel size across the UVIS image, normalized at the photometric reference pixel, is shown in Figure B.2. The ratio of maximum to minimum pixel area over the detector is 1.074.

See http://www.stsci.edu/hst/instrumentation/wfc3/data-analysis/pixel-area-maps for a FITS file of the pixel area map and a discussion of its normalization and application for photometry.

Figure B.1: Linear components (crosses) and non-linear components (vectors, magnified by 10) of geometric distortion on the WFC3/UVIS detector.

Figure B.2: Variation of the effective pixel area with position on the UVIS detector. Darker shading indicates pixels with smaller area. Contours are drawn at 1% increments.