If this image looks completely black to you, don't adjust your monitor. The bias frame is a very short exposure made intentionally with no light reaching the camera's sensor. It does, however, contain signal introduced by the camera's electronics, often consisting of vertical bands in DSLR images, and that signal would be amplified until it is very noticeable when many images of the subject are aligned and stacked. The purpose of bias frames in astrophotography image processing is to remove that signal from the subject images before the images are aligned and stacked.

This bias frame is actually a composite of 50 frames made at ISO 100 for 1/4000th of a second, with the camera lens removed and the camera body capped. Sometimes called an offset frame, the bias frame is mathematically subtracted from all other frames used to produce a final astrophotography image.

Bias frames are easy to take and don't need to be repeated frequently (once or twice a year is enough) since the bias signal won't change much over the life of a digital camera. There are alternate image processing procedures that eliminate the need for bias frames, but it is very important to use bias frames unless you are using one of those alternate procedures that specifically eliminates the need for them.

When using a DSLR for astronomical imaging, the bias frames and all other images should be acquired in RAW mode, and the calibration process described in this gallery should be completed before the images are converted to full color.