Cloudy Night Maintenance

In astrophotography, darks (or dark frames) are calibration images taken with the same exposure settings as your light frames, but with the lens cap on or the telescope covered so no light enters the camera.

Purpose

Dark frames capture the thermal noise and hot pixels that your camera sensor produces during long exposures. This noise is temperature and exposure-time dependent, appearing as:

Random bright pixels scattered across the image
A general “glow” or pattern from the sensor heating up
Amplifier glow in some cameras

How They’re Used

During image processing (called calibration), dark frames are subtracted from your actual photos (light frames). This removes the sensor noise, leaving you with cleaner data of just the astronomical objects you photographed.

Best Practices

Take darks at the same temperature as your light frames (within a few degrees)
Use the same exposure time and ISO/gain settings

Take multiple darks (10-20+) to create a “master dark” by averaging them together

Some astrophotographers keep a library of dark frames for different exposure/temperature combinations
Dark frames are part of the standard calibration workflow along with bias frames (shortest possible exposures) and flat frames (even illumination images to correct for vignetting and dust)..