SharpCap 4.0.8709 introduces a new feature to allow automatic satellite trail removal from deep sky images. Given the massive increase in the number of satellite trail problems recently, this feature should be very useful, particularly when live stacking.
How does it work?
There are three modes for satellite trail removal :
* Off - no satellite trail detection or removal
* Test - SharpCap will attempt to detect satellite trails in the image and if found will highlight them - this can be used to help adjust the trail detection settings, but *DO NOT* use this mode when capturing or live stacking.
* Remove - SharpCap will detect satellite trails, and if detected will replace the trail in the image with pixel values taken from nearby in a non-affected part of the image. This is rather similar to the way that scratch marks are removed from old photo images. Note that Trail Removal is a SharpCap Pro feature
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Satellite Trail Settings
There are three additional controls that let you adjust the detection of satellite trails:
* Trail detection sensitivity - this behaves as you would expect - higher values will help detect fainter trails, lower values can be used if the test mode reveals that false trails are being picked up
* Minimum Trail Length - this is sets (approximately) the minimum length of line that will be picked up as a satellite trail, in pixels. It's not a perfect switch from not detecting below the limit to always detecting lines longer than the limit - the chance of detection goes up from close to zero at or below the limit to reach full detection at about 1.5 times the limiting length
* Trail Width - This affects both the detection (set larger values to detect wider satellite trails) and the width of the region that will be replaced when wiping out the satellite trail in remove mode (or highlighting it in test mode).
How to get started
Ideally, find some previously captured frame images that have satellite trails in them to allow you to test and configure the detection settings. You can load these images into 'Test Camera 1 (Deep Sky)' using the Image browse control. (Note: Test Camera 1 will now automatically set the colour space based on the file you choose - for instance RAW16 if you select a 16 bit RAW FITS file).
Here's a frame that was sent to me to help develop this feature:
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Once you have selected a frame with a satellite trail in, turn the 'Remove Satellite Trails' control to 'Test' mode. This will draw bright lines onto the image over the satellite trails that have been detected, like this
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If your satellite trails in the image are not detected, try turning up the sensitivity, or reducing the minimum length.
If other parts of the image are wrongly detected as satellite trails, try turning down the sensitivity or turning up the minimum length.
If the highlighted area does not seem to cover the entire width of the trail, try turning up the 'Trail Width' setting.
If you cannot find a balance where your satellite trails are detected without detecting other non-trail parts of the image, please share the problem images with me for further testing
Turning Trail Removal On
Now that you have the trail removal settings largely correct, you can turn on 'Remove' mode - at that point you should see the trail removed from the image, like this:
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In the example above, the background brightness of the image is about 210 ADU (out of 65000). The satellite trail is about 600-700 ADU (so about 1% of maximum image brightness). After removal, the area is about 225 ADU, so about 97% of the brightness of the trail has been removed.
Things to Note
* The minimum length setting means that trails that only just show on the edge of the image may not be detected and removed.
* Like all other options in the 'Preprocessing' section, the changes to the image made by the satellite trail removal end up in the saved image, except in the case of saving RAW frames from live stacking, which are saved with no processing applied.
* The calculations needed to detect the satellite trails are quite extensive - approximately 1s to process a 15 megapixel frame on a high power PC. If you are using a high resolution camera with a low spec PC then the processing time may be many seconds.
* The extra settings (sensitivity, minimum length, trail width) only show when the satellite trail removal is turned to either 'Test' or 'Remove' - they are hidden when it is set to 'Off'. This is new with these settings, but I think it is a good idea to minimize clutter in the control list
Want to try it out
If you don't have any satellite trail images to test with, Test Camera 1 now has an option to add artificial ones to each frame - just turn the new 'Musk's Curse' setting to On...
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Robin
