Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aas...19915703m&link_type=abstract
American Astronomical Society, 199th AAS Meeting, #157.03; Bulletin of the American Astronomical Society, Vol. 34, p.565
Astronomy and Astrophysics
Astrophysics
Scientific paper
Most CCD stellar photometric reduction packages use analytical functions to represent the stellar Point Spread Function (PSF). These PSF-fitting programs generally compute all the major partial derivatives of the observational model by differentiating the volume integral of the PSF analytical function over a CCD pixel. Deviations of the real-world PSF from the analytical PSF representation are then generally stored in a residual matrix. Diffraction rings and spikes can provide a great deal of information about the position of a star, yet information about such common observational effects generally resides only in the residual matrix. Such useful information is generally not used in the PSF-fitting process except for the final step involving the determination of the chi-square goodness-of-fit between the CCD observation and the model where the intensity-scaled residual matrix is added to the mathematical model of the observation just before the goodness-of-fit is computed. I describe and demonstrate some of the key features of my MATPHOT algorithm for digital PSF-fitting CCD stellar photometry where the PSF is represented by a matrix of numbers (e.g., a FITS image file) and the position partial derivatives are determined using numerical differentiation techniques. I show how critically-sampled or under-sampled data can be accurately photometered using over-sampled digital PSFs. I discuss the advantages and disadvantages of MATPHOT with respect to traditional PSF-fitting algorithms based on analytical representations of the PSF. A detailed MATPHOT analysis of simulated Next Generation Space Telescope CCD observations is presented which shows that millipixel relative astrometry with millimag photometric errors is feasible with digital PSFs. The C source code and documentation for MATPHOT is currently being distributed as part of my MXTOOLS package for IRAF (http://www.noao.edu/staff/mighell/mxtools). This work is supported by a grant from NASA, Order No. S-67046-F, which was awarded by the Long-Term Space Astrophysics (LTSA) program.
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