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B. Illumination Correction

To determine the pattern resulting from imperfections in the optical system between the sky and the detector, one would ideally want to obtain a spectum of a spatially extended, uniform light source. That is, one wants to take spectra of the clear twilight sky. A cloudless sky is quite uniform on an angular scale of a few arcminutes (the length of the slit). And the twilight sky is bright enough to allow for the collection of high signal-to-noise spectra in a few minutes integration time with our system. The twilight sky does not, of course have a flat spectral energy distribution. Thus it is primarily the variation in intensity along the slit (in the y-direction of the resulting image) that we must attempt to characterize.

The result of an exposure of the twilight sky will be a spectrum dominated by the Solar spectrum and modified by telluric absorption features. Most of this spectral information must be filtered out to provide a map of the spatial struture only. As there is often some spectral variation to the slit illumination, some care must be taken to make a proper illumination-correction frame. Below, I discuss a series of tests conducted to determine the strategy for obtaining and reducing twilight sky-flats with our spectrograph.

        Sky-Flat Test Procedure
        Reduction of Sky-Flat Frames
        Analysis of the Sky-Flat Tests
        Strategy for Obtaining Sky-Flat Frames

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Updated: 2005 October 17 [pbe]