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2.0 The WFPC2 PSF: Dynamic Range and Photometry

To be read in conjunction with Chapter 5 of the WFPC2 Instrument -Handbook, Version 4.0.
Here we supplement the Instrument Handbook with short discussions of the use of PSF subtraction to maximize image dynamic range and to obtain accurate stellar photometry. We also discuss a source of error in the published values of the HST aperture correction.

Dynamic Range

The WFPC2 PSF has structure on very small scales, with significant power on scales smaller than 1 PC pixel. Thus faint objects near bright objects can be difficult to detect and to distinguish from PSF artifacts. Model PSFs (for example those produced by the TinyTim software1) are quite good for many purposes, but can leave residuals as large as 10 to 20% of the peak.

Recent results indicate that PSF subtraction and detection of faint objects very close to bright objects can be improved by using a composite PSF from real data, especially dithered data. Table 1 indicates limits that may be obtained for well-exposed sources (nominal S/N > 10 for the faint object) where a dithered PSF image has been obtained.

Table 1: Limiting Magnitudes for PSF Subtraction Near Bright Objects

Separation in arcsec

(on PC)

Limiting m

(without PSF subtraction)

Limiting m

(with PSF subtraction)
















A technique that has been used with some success to search for nearby neighbors of bright stars is to image a source at two different roll angles, and use one observation as the model PSF for the other. In the difference image the secondary source will appear as a positive residual at one position and a negative residual at a position separated by the change in roll angles. PSF artifacts generally do not depend on roll angle, but rather are fixed with respect to the telescope. Thus small changes in the PSF between observations will not display the positive or negative signature of a true astrophysical object. Again, it is recommended that the observations at each roll angle be dithered.


PSF subtraction is also an effective means of accurate and repeatable photometry on HST. Papers presented at the 1997 HST Calibration Workshop by Remy et al. and Surdej et al. show that the subtraction of synthetic or scaled observed PSFs can be used to obtain 1-2% stellar photometry.

In spite of the ability to obtain photometry through PSF subtraction, the total fraction of the light of the PSF within a given radius is not known to better than a few percent due to the difficulty of measuring the light in the faint wings of stellar PSFs (remember that there are over 17,000 PC pixels inside a radius of 3\xfd , each contributing read noise to the observation!). This difficulty has contributed to a minor error in Table 6.7 of the WFPC2 Instrument Handbook, which gives the fraction of encircled energy in the F555W filter within a 1\xfd radius as 100%. This table is based upon the encircled energy figures from Table 2(a) of Holtzman et al. (PASP, vol. 107, p.156, 1995). Examination of several filters shows that about 10% of the light in the PSF is missing at 1\xfd in the PC. Observers estimating aperture corrections for their images should be wary of this effect and note that in a later paper (Holtzman et al., PASP, vol. 107, p.1065, 1995) the same group normalized the HST magnitude system to the light enclosed inside of a 0.\xfd 5 radius to minimize errors caused by the uncertain aperture correction at large radii.

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1 http://scivax.stsci.edu/~krist/tinytim.html

Copyright © 1998, Association of Universities for Research in Astronomy, Inc. All rights reserved.

Last updated: 06/16/98 10:38:00