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Re: [APML] median vs. average vs. sigma
Hi Tony:
I hope one of the processing gurus can chime in, but I thought that median
combine worked on a pixel-by-pixel basis. In other words, whole frames are
not rejected but individual pixels within frames are rejected if they are
not similar. So, for example, a frame with an airplane trail would still be
averaged into the final result where the airplane was not contributing, but
rejected in the area of the trail.
Anybody else have information on this?
Thanks,
Bert
----- Original Message -----
From: "Tony Hallas" <tonyhallas@foothill.net>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Friday, October 24, 2003 7:42 AM
Subject: Re: [APML] median vs. average vs. sigma
Brian,
I'll make this quick since this is still a film site (anyone not wanting
to read this please hit "delete" now) ...
Median combining has a dirty little secret about it ... if all your
frames are not EXACTLY similar, median combining will actually reject some
of the frames ... after all, it's looking for anything that is not exactly
similar ... if you have some frames that aren't, they get the knife. The bad
thing about this ... you loose the "averaging" effect when you ditch frames
and your signal to noise is worse because of it. Signal to noise is directly
related to the square root of the number of frames that have been averaged,
so if you start loosing frames, the noise in your image will increase ...
sometimes dramatically.
Averaging, on the other hand, is locked in ... EVERY frame is accounted
for and all of them are combined so you end up with the smoothest image
possible and the highest S/N ... this would be all you needed except for one
thing ... those pesky cosmic ray hits that appear in a RANDOM pattern all
over your frames ... averaging does nothing to get rid of those. You have
the smoothest background, but it is littered with the cosmic ray debris.
Sigma processing is the best of both worlds ... although the exact math
is over my head, it essentially does a true average of the frames AND at the
same time rejects any "outliers" ... it is vital to set up the parameters
correctly when doing a sigma combine ... Ray Graylak's site has a good
explanation of it (I use Ray's Sigma 10 for my combining). I have run test
after test and the sigma combine beat the median combine every time ... if
you want the maximum S/N, it's the only way to fly.
Tony
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