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Bad/Hot pixel map

The bad pixel map will prevent the auto-focus or slewing function locking on a hot pixel rather then on a star.

The bad pixel map will be only applied on preview images, focus or slewing images. It is not applied on captured images. Those images are not altered.

Use the menu File → Bad pixel map → Apply to current image If you want to apply the bad pixel map to an image you load in the program.
You can identify an image that was BPM processed with a comment at the end of the FITS header.


This menu show information about the current bad pixel map, the number of bad pixel and the size of the image it was taken from.

You have two options to create, using the camera directly or using a processed dark frame. The second option is preferred because the processed dark as less noise and you can use a lower sigma value to detect fainter deviant pixels.

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 an exposure time between 10 and 60 seconds.
  4. Cover the camera or telescope similar as taking a dark image. If your camera has a shutter it will remain closed during the exposure.
  5. Select menu File → Bad pixel map → Create from camera
  6. Click Continue. A dark will be taken and number of hot pixels will be reported in the log window.

Create from a dark file

Select menu File → Bad pixel map → Create from dark file

Select the dark file from the file dialog.

Clear the bad pixel map

To remove/clear the bad pixel map select the menu File → Bad pixel map → Clear bad pixel map

Check the bad pixel map

Take a preview image and check if the bright hot pixels are suppressed.

Or open a saved raw image and use the menu File → Bad pixel map → Apply to current image

If you still see some not removed hot pixel you can retry the procedure after setting a lower value for sigma.

Possible errors

Error: If you get an error message: “too many hot pixels”, increase the sigma threshold value in the menu Edit → Preferences → preview options. If successful the program log will report the number of hot pixels suppressed.

A typical threshold value is 5 for CCD and 8 for a modern CMOS sensors. Decrease the threshold value if you want suppress more hot pixels. Normally you should suppress maybe 5 to 30 hot pixels but you can go much higher.

With some camera it is not possible to get a good result here. In this case use a dark frame instead.

en/documentation/bad_pixel_map.txt · Last modified: 2021/09/08 15:35 by pch