Re: [APML] stacking & adding film images

["Carlos Milovic F." <cmilovic@yahoo.es>]

>      When you say "add" ... are you refering to
> actually adding the images
> together, as in a true stack, or are you averaging
> all the images together,
> as in a true average? 

Hi Tony

Adding and averaging are stadistically the same thing
in terms of SNR. Averaging just rescales the result
and thus the differences in signal or noise values are
only a constant.


> Adding all the images together
> might have a problem
> with all the stars becoming completely saturated ...

That is a problem with the "space" we are working.
Imagine that we have real numbers instead of integers
in the 0-255 or 0-2^16 range. Of course, when working
with "real" images (normal images) we are limited to
the bit deph, and thus averaging is the correct method
to combine the images if the sum of the images gives a
value grater than the upper limit.

BTW, when you average a set of images in some bit
deph, you get a rounding error, and thus working in
the higher bit deph avalayble is always a good choise
to preserve the data.If you have no choise (for
example in a Photoshop earlier than CS), there are
some ways to avoid a little this problem and is just
to distribute the data in the whole dinamical range in
a better way before stacking. I think that Chuck has a
method that takes that into account (see Jerry's
page).



=====
Regards,

Carlos Milovic F.
-------------------------
Astro & Photo - CMF
http://www.astrophoto.vze.com
-------------------------
Visita "AstroFoto", el foro de astrofotografía en español
http://espanol.groups.yahoo.com/group/astrofoto

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