Space Telescope Science Institute   5.6.5 PSFs at Red Wavelengths and the UV Chapter 6:  Polarimetry, Coronagraphy and Prism/Grism Spectroscopy

5.6.6 Residual Aberrations


Residual aberration levels at the center of the field in each camera are 1/30 wave (HRC) and 1/20 wave (WFC) rms at 5500Å (excluding defocus). Coma and astigmatism are minimized at the field center of each camera. The ACS PSF varies far less over the field of view than those of WFPC2 and STIS. WFPC2 especially suffers from a variable obscuration pattern that significantly alters the PSF structure depending on field position. Lacking the additional obscurations present in WFPC2, ACS PSF variations are instead due to changes in aberrations and charge diffusion.

At the extreme corners of the WFC field, increased astigmatism slightly elongates the PSF core. The axis of elongation rotates by 90º if the system passes through focus due to breathing. This may affect ellipticity measurements of small galaxies with bright cores at the field edges. Focus variations in the WFC, which alter the amount of light in the peak, are largely due to detector surface height irregularities and amount to the equivalent of 5 microns of breathing (1/18 wave rms). The largest focus offset is along the gap between the two CCDs. Variations in the width of the PSF core are dominated by changes in CCD charge diffusion, which is dependent on the thickness of the detector (12 to 17 microns for the WFC). The PSF FWHM in F550M, for example, can vary by 20% (0.10 to 0.13 arcseconds) over the field.

The PSFs in the HRC and SBC are reasonably constant over their fields. The HRC FWHM is 0.060 to 0.073 arcseconds in F550M. More details on ACS PSF field variations are provided in ACS ISR 2003-06. The Tiny Tim PSF simulator includes field dependent aberrations and charge diffusion and may be used to estimate the impact of these variations.


 5.6.5 PSFs at Red Wavelengths and the UV Chapter 6:  Polarimetry, Coronagraphy and Prism/Grism Spectroscopy
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