Space Telescope Science Institute   5.6.3 Encircled Energy  5.6.5 PSFs at Red Wavelengths and the UV

5.6.4 Geometric Distortions


Geometric distortions produce a significant impact on the shape of the PSF in all three of the ACS channels, as can readily be seen in Figure 5.11 and Figure 5.12, which display WFC and HRC PSF images. The log stretch enhances the spider diffraction patterns, which the distortion renders non-perpendicular, and the outer Airy rings, which appear elliptical. The distortion owes primarily to the tilt of the focal surface to the chief ray at the large OTA field angles of the ACS apertures. The linear, field-independent approximation for the WFC produces a difference in plate scale of about 8% between the two diagonals of the field and, in the HRC and SBC, about a 16.5% difference in scale between orthogonal directions rotated about 20º from the aperture edges. Field-dependent distortions, measured as actual vs. predicted distances from field center, amount to about 2% peak in the WFC and about 1% in the HRC and SBC.

The distortions render the pixels, as projected on the sky, trapezoidal in shape and their area varies over the field by about 19% and 3.5% in the WFC and HRC/SBC, respectively. These variations have significant ramifications concerning appropriate techniques for flat-fielding and photometric calibration, especially when complicated by resampling in order to combine dithered image sets. Related issues are the manner in which the halation effects of the HRC and SBC detectors are removed and the treatment of spectra from the prisms and grism, which are not subject to the same distortion effects.

More details concerning geometric distortions in ACS can be found in ACS ISR 2002-02 and ACS ISR 2004-15. An introduction to calacs, and to multidrizzle which applies corrections for geometric distortion, is available on-line at http://www.stsci.edu/hst/acs/analysis/multidrizzle.

Figure 5.9: Encircled energy for the CCD channels.

 
Figure 5.10: Encircled energy for the SBC.

 
Figure 5.11: ACS WFC PSF - F625W.

 
Figure 5.12: ACS HRC PSF - F625W.

 

 5.6.3 Encircled Energy  5.6.5 PSFs at Red Wavelengths and the UV
Space Telescope Science Institute
http://www.stsci.edu
Voice: (410) 338-1082
help@stsci.edu