- amp-glow or starburst
- hot pixels
This is a dark frame from a ZWO ASI120MC captured at 60s exposure and gain 50. The frame has had a histogram stretch applied in Siril. The hot pixels can clearly be seen. This artefact would be present in all light frames unless removed.
These defects can be removed by collecting dark frames - cover the camera or sensor and capture frames with the exact settings used for the light frames. Ideally the temperature for both lights and darks should be the same or very close. A temperature mismatch can result in 'raining' across the image or an effect like running paint or a noisy background.
The dark frames can be captured in the same way as light frames. A master dark frame would be created and applied to the light frames during post processing via software such as Deep Sky Stacker, Siril, PIPP or Autostakkert. Alternatively dark frames can be captured and a master dark created automatically by SharpCap via Capture > Capture Dark . This is particularly useful for those using the Live Stack facility.
The use of dark frames, although not essential, will greatly enhance the appearance of an image.
Some cameras can mitigate the above problems by having in-built cooling systems where the user can control the sensor temperature (expect to pay ££££'s for this facility). With this type of camera, it becomes practical to build a dark frame library which can be re-used, thus removing the requirement to take darks frames during every imaging session.