|PSF SUBTRACTION FOR WFPC2 IMAGES:|
A common analysis problem for WFPC2 observations is the subtraction of the PSF from a much fainter background object. Common examples are searches for fuzz around QSO's and the search for circumstellar disks and planets around stars. This memo is designed to provide some basic advice, both during the observation planning stage and the analysis stage. More details, along with pictures, can be found in the article by John Krist in Calibrating Hubble Space Telescope: Post Servicing Mission.
1. Take 2 or 3 images (e.g., a 1, 10, and 100 second exposure of a 10th magnitude star) 2. Use the TINY TIM SOFTWARE
1. Make PSF observations at the same location. 2. Use the TINY TIM SOFTWARE 3. Make observations near the center since most of the PSF's in the archives are there 4. Consider an interactive acquisition so the PSF and target can be put very near the same position.
1. Make PSF observations at the same location. 2. Make observations of your target at different roll angles so you can determine what is real.
1. Take 2 or more images with subpixel dithering 2. Use the TINY TIM SOFTWARE
1. Make PSF observations using the same filter, and using a star with a similar spectrum. 2. Use the TINY TIM SOFTWARE
1. For desorption, take PSF observations as close as possible in time to the target observations. 2. In principle, it may also be possible to take PSF's at various phases of the breathing and use the one most appropriate to your target observations. I don't believe anyone has tried this in practice.
Yes! WFPC2 PSF Library Search Tool.
It is not possible to give specific advice that is relevant for all observers, since the scientific demands of programs vary. However, here are the 4 basic approaches people use, roughly in order of how stringent the scientific requirements are.
A common question is what is the limiting magnitude of a point like object near a bright star. Here is a a table providing some rough estimates. Observers with M(target) -- M(bright star) larger than the limiting values in the center column should be prepared to use PSF subtraction (i.e., using PSF from Tiny Tim, the archives, or their own observations). The right column shows typical limits after PSF subtraction has been performed.
Distance from Limiting M(target)-M(bright) Limiting M(target)-M(bright) bright star magnitude (3 sigma) mag (3 sigma) in on PC CCD without PSF subtraction typical PSF subtraction ------------- ---------------------------- ---------------------------- 0.1 '' 2.0 mag 3.5 mag 0.3 5.7 7.9 1.0 8.9 10.7 3.0 10.7 12.9
The optimal wavelengths are around F555W. At shorter wavelengths, high frequency structure from the mirror zonal errors increases residuals. At longer wavelengths, the streaks increase in size and tend to move more with position changes.
Narrow band filters give very sharp Airy rings which are very hard to remove.
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