K3's Astronomy - Deep Sky
"When I consider your heavens, the work of your fingers, the moon and the stars which You have set in place, what is Man that You are mindful of him?" -- Psalm 8:3,4

Fighting against hot pixel in long exposures

It was very bad weather this evening - very dense fog. So I decided to experiment with dark frames in my personal sky (it is always clear ;-)). So a little story about our personal sky:

My father thought out very nice present in Christmass 2000 for me - phosphorescent stars. He wanted to mount them on the roof in our bedroom, but my mother prevented him from it. She knew my sense for details and precission, so she supposed that I wish to do it myself. So we made our personal sky in our bedroom with my wife Zuzka.

In the next section, there is a description of procedure how to remove hot pixels from long exposure photographs

1. I took 10x5s (gain 60%) with my Philips Vesta 675 SC camera. Here is stacked result. Hot pixels are clearly visible:

2. I covered the lens and took another 10 frames with the same camera settings. Here is stacked Dark Frame (DF)image:

3, I stacked to frames in point 1, with subtraction of DF in K3CCDTools. As the hot pixels in DF were saturated, the black pixels appeared in result frame after DF subtraction (they are visible only in brighter pars, because the sky is black):

4, I found very useful filter in Corel PhotoPaint: menu Effects/Noise/Dust & Scratch. As stars are not perfectly pin-pointed, but the hot pixels are, it removes hot pixels or dark points after subtraction of DF very nice without deteriorating image quality very much. Here is the result:

5. You can use program PixelZap for eliminating hot pixels, too. Although it is very good software for removing hot pixels from common night scenes, I found it less useful for astrophotography, especially if many hot pixels are present in DF. I tried out all four options for elimination hot pixels. For detection of hot pixels I used DF frame (very similar results I obtained with auto detection setting, too). Here are results:

a, PixelZap, method Border 2 point:

b, PixelZap, method Border average:

c, PixelZap, method Neighbor median (it has best result with PixelZap, but some hot pixels remain):

d, PixelZap, method Sobel edge 2 point:

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Last Update: 17.10.2001