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WFPC2 PSF FAQs

NOTE: Due to limited resources, these pages may not have been regularly updated. It is possible that the information provided below and/or in the links given may be outdated or inaccurate. If you come across conflicting information or are confused by the answers given, please contact the STScI helpdesk at: help@stsci.edu>.

Q: Where can I find observed WFPC2 PSFs?

A: One way to get observed PSF's is to extract them from the publicly available calibration fields, or calibration stars in STARVIEW. In particular, the targets to search for in STARVIEW are "Omega-Cen" and "NGC6752". Each of these fields has a wide variety of stars (saturated and unsaturated) covering most portions of each chip. Many of the filters are calibrated with a standard star instead of a standard field. The standard stars to search for in STARVIEW are "GRW+70D5824", "FEIGE110", "A+81D266", and, "BD+75D325". You may notice that for some of these images, STARVIEW's Quality Comment will say "Single Star Guidance". While this is unlikely to be a problem for short exposures, you should check longer exposures for spacecraft roll (do this by subtracting successive images to look for offsets in the stellar images). The task 'imcopy' in IRAF can be used to extract a single star for use as a PSF.

Another method is to use the WFPC2 PSF Library Search Tool.

Q: Where can I get theoretical WFPC2 PSFs?

A: The TinyTim software can be used to create a synthetic, accurate PSF.

Q: When searching for companions of bright targets, at what M(target)-M(QSO) does it become necessary to use PSF subtraction?

A: The table below gives the brightness of "object-like" features in the PSF, expressed as a delta-magnitude from the bright target. The third column gives the delta-magnitude detection limit, if one assumes "detection" means features which are three times brighter than the PSF features. At some radii, the OTA diffraction spikes are important. In those cases we give a range where fainter limits can be reached if the OTA diffraction spikes are avoided; this can be done by observing at several different spacecraft roll angles. These results are derived from TinyTim models of the WFPC2 PC PSF at F555W.

Observers with M(target)-M(QSO) larger than the limiting values below should be prepared to use PSF subtraction.

Radius       Brightness of PSF      Limiting M(target)-M(QSO)
from QSO     "features" expressed   magnitude (3 sigma)
on PC CCD    as delta magnitude
---------    ----------------       ------------------------
0.1"         3.2 mag                2.0 mag
0.3"         6.9                    5.7 
1"           8.5-10.1               7.3-8.9
3"           11.1-11.9              9.9-10.7

Q: How well can the PSF be subtracted? When searching for faint companions to bright targets, what is the limiting M(target)-M(QSO) magnitude in PSF subtractions?

A: Changes in the PSF due to OTA breathing limit the accuracy of PSF subtractions within the first few arcseconds. The table below gives the brightness of "object-like" features (meaning size ~2x2 pixels) in the difference between an in-focus PSF, and a PSF which is 5 microns out of focus. Five microns is the typical range of focus errors due to OTA breathing.

Observers with M(target)-M(QSO) larger than the limiting values below will have serious difficulties.

Radius       Brightness of       Limiting M(target)-M(QSO)
from QSO     "features" in       mag (3 sigma) in
on PC CCD    PSF subtraction     typical PSF subtraction
---------    ----------------     ------------------------
0.1"         4.7 mag             3.5 mag
0.3"         8.6-9.1             7.4-7.9
1"           11.4-11.9           10.2-10.7
3"           13.2-14.1           12.0-12.9

Q: When does saturation/blooming of the bright source become a problem when searching for faint companions of bright targets?

A: Saturation and blooming is usually not the limiting factor. As a rule of thumb, blooming will occur for (assuming typical broad band filters):

        WF              PC
V<      exp. time.      exp. time.
----    ----------      ---------- 
20      5000
19      2000            6000
18      800             2400
17      300             1000
16      120             360
15      50              150
14      20              60

Even when saturation occurs, it will tend to wipe-out pixels only in the vertical direction. If they are exposing more about 30 times longer than limit given in table, then they will wipe-out only 3 by 10 pixel ellipse. The direction of the saturation bloom can be controlled by rolling the spacecraft.