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Re: [APML] NGC 6992 uploaded
Guys,
I also tried a 'sum' in RS and I wasn't impressed. I think that I may
end up working on only the single negative expose. The stars are
sharper and the information is much nicer than what I got from the E200
exposure. That may have been enough to drag me down.
By all means continue this converstaion, I can only learn from it.
Alan "I would die and go to hell if only I could understand all this
scanning and processing business" Voetsch
(hope I did that as well as Ray, and Stuart do)
--- Ray Butler <ray.butler@nuigalway.ie> wrote:
> Hi Stuart,
>
> This statement of yours caught my eye:
>
> >Third, averaging the two scans of each negative will reduce the
> noise so why
> >not ADD the two resultant images vs. median combine - this way you
> get the
> >benefit of twice as much signal.
> >
> >
> It depends on how "median combine" is implemented. Strictly speaking
> you
> need a minimum of 3 samples (images in this case) to get a median,
> since
> it is defined as the mid-point of the range of values. What is the
> mid-point of 2 pixel values? It can't be the higher value and it
> can't
> be the lower value, since they are both extreme values - neither is
> in
> the middle. So the median is undefined. How this is dealt with
> depends
> on the software; it may default to a straight average of the two
> values,
> it may pick either the higher or lower values by default, or it may
> spit
> the 2 images back at you with some nasty error and question your
> intelligence! The most logical approach would be to take the straight
>
> average of the 2, since this is how the median should be computed
> when
> the number of samples is large (>3) but even in number: there is no
> single middle value, so the _two middle_ values would be averaged.
>
> What I'm saying is that the user needs to find out what their
> software
> actually does when asked to do a median combination of only two
> images.
> It is possible that it does actually default to an average, in which
> case all the signal is being used after all.
>
> Ray "who vaguely remembers an old scientists joke along the lines of
> '1
> and 2 are fine, but once you get to 3 you have to do statistics' "
> Butler
>
> --
> Dr. Ray Butler
> Lecturer, Physics Department & Computational Astrophysics Laboratory,
> National University of Ireland - Galway,
> University Road, Galway, Ireland.
> Web: www.nuigalway.ie/physics/ Email: ray.butler-AT-nuigalway.ie
> Tel: +353-91-493788 FAX: +353-91-494584
>
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SCT Astrophotography: http://www.pbase.com/avoetsch/astrophotography
FS-102 G-11/Gemini: http://www.pbase.com/avoetsch12952/tak_fs102
& http://www.pbase.com/avoetsch12952/fs102
& http://www.pbase.com/avoetsch12952/takpf
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