Johan Samsing, Alex G. Kim
The accuracy in the photometry of a point source depends on the point-spread
function (PSF), detector pixelization, and observing strategy. The PSF and
pixel response describe the spatial blurring of the source, the pixel scale
describes the spatial sampling of a single exposure, and the observing strategy
determines the set of dithered exposures with pointing offsets from which the
source flux is inferred. In a wide-field imaging survey, sources of interest
are randomly distributed within the field of view and hence are centered
randomly within a pixel. A given hardware configuration and observing strategy
therefore have a distribution of photometric uncertainty for sources of fixed
flux that fall in the field. In this article we explore the ensemble behavior
of photometric and position accuracies for different PSFs, pixel scales, and
dithering patterns. We find that the average uncertainty in the flux
determination depends slightly on dither strategy, whereas the position
determination can be strongly dependent on the dithering. For cases with pixels
much larger than the PSF, the uncertainty distributions can be non-Gaussian,
with rms values that are particularly sensitive to the dither strategy. We also
find that for these configurations with large pixels, pointings dithered by a
fractional pixel amount do not always give minimal average uncertainties; this
is in contrast to image reconstruction for which fractional dithers are
optimal. When fractional pixel dithering is favored, a pointing accuracy of
better than $\sim 0.15$ pixel width is required to maintain half the advantage
over random dithers.
View original:
http://arxiv.org/abs/1202.0102
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