The performance of most detectors can be improved dramatically by subtracting a dark frame; that is, an image taken under the same conditions but with the shutter closed. This removes dark current, which is sensitive to both temperature and exposure time. In most cases this has a dramatic effect on image quality.
Most CCD cameras have a built-in shutter and can perform this function automatically. The Camera Control window provides "auto-dark" capability, and remembers recent dark frames if you change the settings, so it is easy to flip back and forth. If your camera does not have a shutter, but you are using a filter wheel, you can arrange for one slot of the wheel to be used as the shutter. If your camera has no shutter, you will be prompted to manually cover the camera during dark frame exposures.
The auto-dark feature is intended primarily for setup and focusing. When taking high quality images you should perform a full calibration, including bias, dark, and flat-field frames. In addition you should average multiple calibration frames for the best noise performance. Using a single dark frame increases your overall read noise level by 41%; stacking 25 frames reduces this extra noise to just 8%.
Although it is essential for good results, accurate calibration can be a bit complicated. Considerations include sensor temperature, exposure length, zero point offsets, normalization, and stacking. MaxIm DL includes built-in tools to automate the correct application of calibration frames. For more information please see Image Calibration.