Removing satellite trails in a long exposure?

[iceman (Mike)]

Hi guys

Is there any special stacking method that can reduce/eliminate a satellite trail when stacking?

Or should I clone the line out in the TIF first, before stacking?

Or do I just have to throw out that sub? It's a real nuisance losing a 5+ minute exposure to a plane or satellite!

[[1ponders] (Paul)]

Use median combine Mike. That should take care of most of it. Median combine won't be the best for noise reduction, but will help to deal with those nasty satellites, meteors and gamma ray strikes.

[RB (Andrew)]

Originally Posted by [1ponders]

Median combine won't be the best for noise reduction, but will help to deal with those nasty satellites, meteors and gamma ray strikes.

Cool, what then do you recommend as the best method for N/R Paul?
Average Stack or something else?

[[1ponders] (Paul)]

I'd need to check Andrew but from memory using the most common methods a direct add is the best for reducing noise, but you will quickly burn out your bright stars (track and accumulate is a common form that SBIG uses), then I believe averaging, but that will still leave the outlier pixel values (as will the add combine), and then median which is least effective on noise (still reduces but not as much) but deals with outliers.

when I get home I'll check for links and references for you to peruse.

[iceman (Mike)]

Thanks Paul, i'll give it a go.

So do you combine the median combine with an add or something?

[Geoff45 (Geoff)]

ImagesPlus has median combine as an option. Average(ie mean) combine gives the lowest noise, but will show outliers like satellite trails that you don't want. Median combine gets rid of unwanted stuff but has more noise. Pick your poison! Why not just toss out the offending frame? It's probably only 10 min or so and you'll spend more time than that cloning it out in Photoshop and making sure you don't introduce further artifacts.
Geoff

[Doug]

Mike in theory everything said is good so far.
Why not try a median run and compare it to an average stack run.
Sure you will still have your satelite trail in one, but that aside, how does the noise compare? I'd suspect that if you have enough sub frames you will have to really nit pic to see the difference in noise levels. If that is so then from a practicle stand point, median will prove to be your best overall option. But only you can decide.

Doug

[iceman (Mike)]

Thanks Geoff and Doug. I've got 10x 5min subs, so median combine might be good enough, or just eliminate one altogether.

[Dennis]

Median combine has always worked for me so far with the small chip of the ST7E. As the trail is only on one sub frame, the Median combine will reject the lone satellite interloper. Having made all that effort to collect those hard won 10 x 5 min subs, I’d be reluctant to throw one away unless it really degrades the batch!

Cheers

Dennis

[jase (Jason)]

Sigma X - but you need to know what you're doing otherwise you'll lose too much valuable data. I typically only use median combine for calibration frames unless I've got huge amounts of data to work with. Its a personal preference.

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Sigma Clip/Reject/Combine is a combination of Median and Average with other techniques to detect outlier pixels. The pixel values that deviate the most from the mean will be discarded providing the deviation is greater than the number of standard deviations configured by you. The standard deviation value is sometimes known as the Sigma Factor. Based on the Sigma Factor a new mean is calculated without the discarded value and is assigned to the pixel in the combined image

These are the main four algorithms. There are variations on these introduced by tools such as RC Console in MaximDL and CCDStack etc. For example Image Plus has a Sigma Average and Sigma Median. Ultimately they all follow similar principals.
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From post - http://www.iceinspace.com.au/forum/s...ad.php?t=24466