Tech Note

Recovering an Image having an Irregular Background

Problem

Images with an irregular background can result from incomplete correction of vignetting or background color variations. A flat field correction is the first step to correcting both effects. Still, a large scale irregularity in the background may exist after the flat field corrections are applied. Furthermore, good flat field calibration images may not exist for a given instrumental setup, or the "flats" may not provide a complete correction. In such cases, how do we improve or, in the worst case, salvage such an image? In this Tech Note we show how to do this using Mira.

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 with background parameters 5 and 5. 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