Space Telescope Science Institute  Description  10.3.1 WFC

10.3 Distortion in the ACS


The ACS detectors exhibit more distortion than previous HST instruments. The principal reason for this is that the optics have been designed with a minimum number of components, consistent with correcting for the spherical aberration induced by the OTA, without introducing coma. The result is a high throughput, but focal surfaces far from normal to the principal rays. The WFC detector is tilted at 22º, giving an elongation of 8% while the HRC and SBC have a 25º tilt leading to an elongation of 12%. In each case, the scales in arcseconds per pixel are smaller along the radial direction of the OTA field of view than along the tangential direction.

The orientations of the ACS detector edges are approximately in line with the V2 and V3 coordinate axes of the telescope. Consequently, the eigenaxes of the scale transformation are along the diagonals for WFC, and the apertures and pixels appear non-rectangular in the sky projection. For the HRC and SBC the situation is even more irregular because the aperture diagonals do not lie along a radius of the HST field of view. Figure 7.8 shows the ACS apertures in the telescope's V2V3 reference frame. For a telescope roll angle of zero this would correspond to an on-sky view with the V3 axis aligned with North and the V2 with East.

If these were the only distortions they would not really present much difficulty. Their impact on photometry and mosaicing, or dithering could be simply computed. A more problematic effect is the variation of scale across each detector. For the WFC this amounts to a change of 10% from corner to corner. For the HRC and SBC this variation is only about 1% as they cover much smaller fields of view. The area on the sky covered by a WFC pixel varies by about 18% from corner to corner, allowance for which must be made in photometry of extended objects. Dithering and mosaicing are complicated by the fact that an integral pixel shift near the center of the detector will translate into a non-integral displacement for pixels near the edges. Even this is not a fundamental difficulty, but will imply some computational complexity in registering images and will depend on an accurate measurement of distortions.

The results presented here are derived from on-sky measurements. For WFC and HRC multiple pointings of 47 Tucanae were taken through the F475W filter, resulting in several thousand star location measurements. These were analyzed to express the distortion as quartic polynomials. The residuals of the fit to these polynomials are of order 0.2 pixels, much larger than the errors. Correction images describing these residuals are now supplied and used in calacs to bring the final errors down to about 0.01 pixels. Additionally, an area of open cluster NGC188, for which astrometric data are available, was used to establish the exact location and orientation of the aperture in telescope coordinates. At the same time, the scale factors were confirmed. For the SBC, the distortion measurement used the target NGC6681 and filter F125LP. The alignment was established by observing this same target with the HRC and SBC consecutively to establish the relative locations. The SBC position was thereby derived from the HRC position.


Description  10.3.1 WFC
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