The flat field is an image captured through the telescope of an evenly illuminated field, used to remove imperfections in the subject images caused by the optical train itself. Flat fields should be taken during each imaging session.

This flat field is actually a composite of 14 images taken during evening twilight, about ten minutes after sunset, with the telescope pointed at the southeastern sky. I usually capture the individual flat field frames in aperture-priority autoexposure mode, and exposure times are typically less than one second per frame. The composite bias frame was subtracted from each of the 14 flat field images first, and then, after averaging the 14 flat field images, the result was converted to grayscale using the "grey_flat" command in Iris software. Note that opening the flat field images in Photoshop or other generic imaging processing software, which first converts the raw DSLR images to RGB, and then converting the RGB images to grayscale, will not achieve the same result! It is important to use software designed for astronomical image processing to handle the flat field processing and the entire image calibration process.

You can see in this flat field image the effect called "vignetting", caused by uneven field illumination characteristic of most telescopes and readily apparent when a relatively large sensor such as a DSLR is used. You can also see several "dust motes", doughnut shaped shadows caused by small particles on the camera's digital sensor. All subject images taken through the telescope will display the same imperfections, and the flat field image is mathematically divided into the subject images in order to negate those imperfections. As long as the camera remains attached to the telescope in exactly the same orientation, and no "dust motes" are added or removed, the application of this flat field image will correct these imperfections. Correct use of flat frames is essential for DSLR astrophotography.

The contrast of this demonstration flat field image was stretched slightly to make the imperfections more visible.