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Dark frame

With some camera it is difficult to use a bad pixel map to cleanup the auto-focus frames. In this case you can define here a dark frame that will be subtracted to remove the hot pixels.
Beware this processing require the dark frame is acquire with the same exposure time and at the same temperature than the auto-focus frames. This also use more resources than the bad pixel map processing. So the bad pixel map must be used in preference if applicable.

This same dark frame is also used with the image stacking option.

Use the menu File → Dark frame → Apply to current image if you want to subtract the dark from the currently displayed image.
You can identify an image that was dark subtracted with a comment at the end of the FITS header.


This menu show information about the current dark frame, the exposure time and the size of the image, you can also see the dark file FITS header for more details.

You have two options to define the dark frame, using the camera directly or using a processed dark frame. The second option is preferred because the processed dark as less noise.

Create from the camera


  1. Camera should be connected and cooling on if available.
  2. Set the required binning in Preview tool. Use the same binning as for the focus and slewing
  3. Select the same exposure time as used for the auto-focus.
  4. Cover the camera or telescope. If your camera has a shutter it will remain closed during the exposure.
  5. Select menu File → Dark frame → Create from camera
  6. Click Continue. A dark will be taken and saved for future use.

Load a dark file

Select menu File → Dark frame → Load dark file

Select the dark file from the file dialog.

The file must be taken with the same binning, exposure time, temperature than used for auto-focus. This file must use a pixel format of 16bit.

Clear Dark frame

To remove/clear the dark frame select the menu File → Dark frame → Clear Dark frame

Check the dark frame

Take a preview image, select the menu File → Dark frame → Apply to current image
Verify the hot pixels are removed and the processing do not create unwanted artifact.

en/documentation/dark_frame.txt · Last modified: 2024/03/17 11:06 by pch