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Statement of the Problem
Images with an irregular background can result from incomplete correction of optical vignetting and from scattered light that enters the optical system during the exposure. This Tech Note shows how to flatten the image background when no flat field calibration frame exists for this purpose.
A flat field correction is usually applied to correct for non-uniform illumination of the image. However, even when a flat field calibration exists, the corrected background still may show irregularities caused by scattered light that had a different character in the flat field calibration image. Alternatively, good flat field calibration images may not exist for a given instrumental setup, or it just may not provide a complete correction. In both cases, the calibrated background will not be "flat", meaning that it will not have a statistically constant value across the image. Mira provides a powerful background flattening tool that can correct for these background anomalies.
Both luminance images and color images usually require a flat field correction to correct the large scale vignetting. Despite such effort, a background irregularity may remain and, further, it is usually different in the different color planes of an RGB image. The image below shows a severe case of vignetting for which no flat field calibration has been applied. This Tech Note shows how to correct this image to obtain a background that is quite flat. The same technique is equally well applied to color images to remove background color irregularities.
Original image showing uncorrected optical vignetting.
Solution
The original image above shows background irregularity attributable to optical vignetting. The image below shows flattened background using the Correct Background command in Mira Pro and MX. In the command dialog, set both the column and row direction coefficients to 5 (often a lower value, such as 3 or 4 terms may be used) The bright core of the galaxy was excluded from the computation of the correction. Various artifacts such as out-of-focus dust shadows ("donuts") and patchy sensitivity variations on the CCD remain in the image because of the lack of a true flat field correction. However, the vignetting appears to be completely corrected. The image is intentionally displayed with the background as a medium gray to show the flatness of the corrected image. Both images are displayed in Mira using the same settings for the transfer function and palette.
Image showing flattened background, although patchy dust shadows remain uncorrectable.