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Session 100 - Computational Techniques.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

[100.04] Astrometry and Photometry with Marginally Sampled CCD Data

M. B. Hoffman (Northern Arizona U. and Yale U.), B. Koehn, E. L. G. Bowell (Lowell Obs.)

We will describe a procedure for determining the point spread function (PSF) and the positions and magnitudes of stars and untrailed asteroids using nearly undersampled data. The procedure to be described was designed for the Lowell Observatory Near-Earth-Object Search (LONEOS), which makes use of a scanning 58-cm, f/1.91 Schmidt telescope equipped with a CCD camera containing two Loral 2048x2048 chips (eventually two 2048x4096 chips). The instantaneous field of view will be 10.1 square degrees, and the pixels will be 2.8 arcseconds on a side. Typical stellar images will occupy 9, and no more than 16, pixels. Hence, determining the PSF will be a delicate procedure. Our method begins by co- adding many (10^5) bright, unsaturated stellar images each centered on the brightest pixel in the image. Assuming that stars are randomly centered within that brightest pixel, we model the PSF with a sum of Gaussians centered in regularly spaced subpixels within the central pixel. This model is then compared with the sum of stellar images and adjusted accordingly. The model is then adjusted further via empirical fitting. The estimated PSF is then recorded in maps, each map with the PSF centered in a different part of the aforementioned subpixel grid. These maps are then compared to the images and best fits obtained for position. Then, the best-fit map from position is scaled for varying magnitudes and a best fit for magnitude is obtained for each object. Results from test data indicate that these measurements are significantly improved over measurement by simple moment analysis. This research was supported by the REU program at Northern Arizona University.

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