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18.3 PSF Subtraction

Accurate PSF subtraction is a prime concern for an observer wishing to study faint features around bright objects. Typical situations are: host galaxy harboring a bright quasar; circumstellar nebulosity around a bright star; faint companions of a bright star, etc. The PSF subtraction technique has proved to be very successful in this kind of studies for HST data obtained with WFPC2. But imaging with NICMOS cameras 1 and 2 can probably easily exceed the success of WFPC2 thanks to a few important advantages:

Cameras 1 and 2 provide images which are diffraction limited, well sampled, and better resolved (see Figure 18.1 and Figure 18.2).
The MULTIACCUM mode automatically provides sub-exposures at different times as a way to look at increasingly faint features at different distances from the bright central source.
For saturated bright central point source, there is no blooming along the detector array columns. The best way to get a high quality PSF for PSF subtraction is to measure an isolated bright unsaturated star in the same image or to construct a composite PSF using good stars in the image. This can be accomplished using the IRAF digiphot package as described in A User's Guide to Stellar CCD photometry with IRAF¹. A PSF obtained from the same image, along with a number of obvious advantages, also takes care of the breathing effect (variations of focus position due to thermally induced mechanical displacements in the HST optical path). If no suitable star can be found in the image, one can resort to a synthetic PSF computed using TinyTim.

Figure 18.2: NIC1 Image with PSF Radial Profile

Figure 18.3: NIC2 Image with Radial Profile

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¹ This and other relevant IRAF documents can be obtained from the IRAF Web site at: http://iraf.noao.edu/.

stevens@stsci.edu

Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 11/13/97 17:28:21