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
Return to Previous Section
Procede to Next Section
Return to Outline