IV. Flat-Field Correction

The goal of flat-field correction (or "flat-fielding") is to remove artifacts from 2-D images of astronomical sources that are caused by variations in the pixel-to-pixel sensitivity of the detector and by distortions in the optical path between the sky and the detector. When one is doing direct imaging, one can often correct for both sorts of problems with one set of calibration exposures ("flat-field frames"). This is typically impossible when one is taking 2-D spectra. In this case, one must treat the two issues (pixel-to-pixel sensitivity variations and distortions imposed by the optical system) separately. The standard strategy for doing so is to obtain a series of exposures of an internal continuum source ("lamp-flats") to determine the pixel-to-pixel sensitivity variations, and also to obtain a series of exposures of the twilight sky ("sky-flats") to correct for the distortions in the optical system. This last is often referred to as an illumination correction.

    Internal Lamp-Flats
        Lamp-Flat Test Procedure
        Reduction of Lamp-Flat Frames
        Analysis of the Lamp-Flat Tests
        Strategy for Obtaining Lamp-Flat Frames
    Illumination Correction
        Sky-Flat Test Procedure
        Reduction of Sky-Flat Frames
        Analysis of the Sky-Flat Tests
        Strategy for Obtaining Sky-Flat Frames
    Preparing a Combined Flat-Field Frame

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Updated: 2006 July 20 [pbe]