satellite and hot pixel track suppression in software

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hha Contributing Member • Posts: 641
satellite and hot pixel track suppression in software

Airplane, satellite tracks and hot pixels are problem in long time exposure astrophotography. Most cameras have a dark frame subtraction option to eliminate hot pixels, but at the expense of loosing 50% of the exposure time. Most image processing software include an option to suppress artifacts in a long sequence of images. Some have the reputation of being “star eaters”. Here is how I solved this problem with an example using the “Accumulation 3.5” option in SEQUATOR.

I took a 2 hour exposure (120 shots at 60 sec each) using a f=135mm lens at f/4 of the Leo constellation. My Nikon Z6 was mounted on a SkyTracker to eliminate star trails. Rather than waiting for a satellite track, I intentionally misaligned the pole axis by 0.5 degrees. This produces 0.26 degree long star trail in 2 hours. The result of a simple star registration of the 120 images is shown on the left below. This is 1x1 degree cropped from the 10 x 15 degree field, zoomed in at 400% to make the pixels visible.

The stars are perfectly registered. The most prominent star on the lower left is +6.7 (HD97937) in Leo. The galaxy in the upper right is NGC3593 (+11.8). The faintest stars are +15. The bluish trace is produced by a hot pixel. It shows the worm period and a 3 pixel peak-to-peak worm error. Since each pixel of the camera with the 135 mm lens subtends 9”, the peak-to-peak worm error is 27 arcsec. This is a well known limitation of the SkyTracker, but it has almost no effect on the image quality with 60 second exposures. The hot pixel track is not straight and uniform, indicating that the mount flexed during the 2 hours.

On the right above is the same 120 images, but this time registered with the 3.5 sigma accumulation option in Sequator. Most of the effect of the hot pixel has disappeared, and there is no indication of the faintest star being effected. If the polar alignment had been near perfect, the hot pixel would have looked like a faint star in an unexpected location. The lesson from this (for me) is that a little polar misalignment can be helpful in eliminating hot pixels and avoiding wasted time to identify "new" supernovae.

There are various algorithms to accomplish this. The algorithm used by Sequator increases the data processing time from 5 minutes to 11 minutes for the 120 images. If anybody knows the details of the Sequator algorithm, please share.

There may be better and faster algorithms for this purpose. Please share your experience with 3 sigma editing with other star registration software.



 hha's gear list:hha's gear list
Olympus TG-630 iHS Nikon D3200 Nikon D3300 Nikon D7200 Nikon AF Nikkor 24mm f/2.8D +1 more
Nikon Z6
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