|
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
|
_______________________________________________ Astro-Photo mailing list Astro-Photo@seds.org http://seds.org/mailman/listinfo/astro-photo