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PSF Calibrations |
In most cases for processing the data you will need an estimate of the point spread function achieved during the observation. For example, stellar photometry in a crowded field will require the PSF for PSF-fitting photometry or, at very minimum, knowledge of what is often a very large aperture correction. For classically-scheduled observations, all necessary calibration data are the responsibility of the observer.
By nature the point spread function (PSF) of an adaptive optics system is complex and varying with time and position in the field. The size and time scale of these variations depends on the conditions at the time of the observation. For information on the effect of this PSF variation on data reductions check out the Data Reduction page.
A PSF may be extracted from the observed fields if the field contains an isolated point source or if the field contains a number of known point sources which can be combined to give an average PSF. Potential problems are:
If the observed field does not contain suitable PSF sources, then the observer is forced to observe a separate PSF calibration field. In choosing fields for PSF calibrations take the following into consideration (in order of priority):
The usefulness of separate PSF calibration fields varies depending on the above considerations and on the stability of the atmosphere during the observations. We have observed large variations in the seeing (and hence performance of Hokupaa) on timescales of minutes but have also observed times of stability which last for hours.
For science targets where the guide star is an extended source, separate PSF calibration field should be treated with care. The correction obtained on a extended source depends strongly on the distribution of the flux. As such the PSF calibration field guide star will rarely mimic performance obtained on the science field. See the Data Reduction page for ideas on how to handle extended object PSFs.
Fields with single PSFs appropriate for PSF calibrations can be found by searching star
catalogs for nearby stars with similar magnitude and color to the science field wavefront
reference source. The USNO and Guide Star Catalog II may be useful for this purpose.
Potential pit falls here are: (1) When you observe this field, don't saturate the PSF
calibration source, (2) Make sure you have a list of 2-3 possible PSF calibration fields
per science field as these stars often turn out to be binaries/double stars! (3) The
short-exposure PSF (total integration time ~ a few sec) can be significantly different
than a longer exposure. Make sure that the total integration time on the PSF calibration
is at least a 2 minutes. You may also need to integrate to a particular SNR in the wings
of the PSF.
We are developing a library of PSF stars from previous observations. Check
these out on the Hokupa'a PSF Library.
Last update 11 February 2001; Mark Chun