Tech Note

Removing Background Irregularities from a CCD Image

Problem

The flat field correction removes two types of artifacts in CCD images: vignetting and other effects introduced into the image by the optical path, and local effects such as sensitivity variations as a function of position and wavelength of light. Since these two categories of artifact affect the image in mathematically distinct ways, the total flat field correction is best done in two stages. However, flat field corrections are usually made in one pass, and the result can be a residual non-flatness to the image because of incompletely corrected local effects. Uncorrected sensitivity variations usually reveal themselves as a patchy background as shown in the image below. The artifact we will correct is quite subtle. It is important to emphasize just how subtle this artifact really is. The contrast in the first 2 images below has been stretched to a very harsh level to show the effect we are trying to correct, as well as its correction. The bottom image is displayed normally after the background artifact has been removed.

Original image with greatly enhanced contrast to show residual background irregularities.

Solution

To correct the background patchiness, the Correct Background command in Mira version 6 was used. A high order polynomial fit was selected. The sandpaper appearance of the background is caused by high photon shot noise resulting from imaging through a bright sky background. The image contrast is also enhanced to illustrate the background irregularities.

Corrected image, with contrast greatly enhanced to illustrate the flatness of the background.

To take the flattened image one step further, the Mira Noise Reduction filter was used to remove the gritty appearance of the bright background. The image below is displayed with less contrast and a power law transfer function to show faint details in the galaxy against a dark gray background.

Background corrected Image, nicely displayed.