Space Telescope Science Institute   10.3 Distortion in the ACS  10.3.2 HRC

10.3.1 WFC


The rhombus shape of the WFC is evident in Figure 7.8. The angle between the x and y axes is 84.9º for WFC1 and 86.1º for WFC2. The WFC distortion is illustrated in Figure 10.127, a vector displacement diagram which shows the contribution of the non-linear part of a quadratic fit to the data. The vectors represent the displacements and have been scaled up by a factor of 5 for display. The corner displacements are about 82 pixels. The principal effect is the diagonal variation of scale.

At the center of chip WFC1 the scale in the x direction is 0.0493 arcseconds/pixel, and 0.0486 arcseconds/pixel in the y direction. For WFC2 these figures are 0.0498 arcseconds/pixel, and 0.0503 arcseconds/pixel. Between the corner of WFC nearest to the V1 axis and the diagonally opposite corner, the overall scale increases by 10%. WFC1 forms a slightly distorted rectangle 201 by 100 arcseconds in size while WFC2 is 203 by 103 arcseconds. There is a 2.5 arcsecond gap between the two chips.

Figure 10.127: Non-linear component of the WFC Distortion.

 
Note that this figure is rotated 180o with respect to the pipeline calibration products, where WFC2 is the lower half of the detector.
 

The resulting variation of the projected pixel area on the sky requires corrections to the photometry of point objects. A contour plot of relative pixel size across the WFC normalized to the central pixel, is shown in Figure 10.128. The range of area is from 0.89 to 1.08 times the central value.


 10.3 Distortion in the ACS  10.3.2 HRC
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