The High Resolution Channel has its edges aligned approximately along the V2 and V3 axes. In this case, the center of the aperture lies on a line passing through the V2V3 origin and making an angle of 22º with the V3 axis. The diagonal of the aperture does not correspond to a radius of the HST field of view. So the distortion has no particular symmetry with respect to the detector axes. Again, because the focal plane, and therefore the detector plane is 25º away from the plane normal to the light path, the scales along the axes differ by 14%. However, since the HRC is less than 30 arcseconds across, the scale variation over the field is much less than for the WFC, being about 1%. At the center the x and y scales are 0.0284 and 0.0248 arcseconds/pixel respectively. The average scales across the middle of the detector are 0.02842 and 0.02485 arcseconds/pixel making the x and y widths 29.1 and 25.4 arcseconds. The slightly non-square projected aperture shape is evident in Figure 7.8. The angle between the x and y axes on the sky is 84.2º. A vector plot of the deviation from linearity is given in Figure 10.129 in which the deviations have been magnified by a factor of 10 for illustrative purposes. The largest deviation is 4.9 pixels in the top left corner and corresponds to about 0.1 arcseconds. The variation of pixel size across the HRC to be used for photometric correction of point sources is shown in Figure 10.130. The maximum deviation from the central value is just over 2%.
Figure 10.128: Variation of the WFC effective pixel area with position in detector coordinates.
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