RE: [APML] What's the Best Way to "Combine" Two Images of DifferingTimes?

[John Mirtle <spampit@shaw.ca>]

Wade,
	By a staggering coincidence, I just finished a stack of film images!
My images were stacked to bring out the faintest details, not to preserve
highlights. Actually, they were more of an experiment with PixInsight DBE
which I think I finally have the hang of. I aligned the images using
RegiStar as you did, but my version is old. I don't know if it can produce
stacks > 16 bit or not, my version can't. For stacking after registration I
use MaxIm DL as I have it for CCD work. I then worked levels and curves as
much as possible before returning to 16 bit space for my old Photoshop 6. I
would think that this would work with your images as well - adding them
together but not limiting max pixel values to 16 bit levels. PixInsight can
work with 32+ bit files as well instead of MaxIm. I am still learning this
software but I think that you can simply add them with pixel math. Season to
taste with curves and levels before reducing the stack to 16 bit. Would this
not help preserve highlights?

	Here are a couple of stacks from tech pan. In the 32 bit TIFF, stars
peaked out at a count of 130,000. Way more headroom than the 65,000 limit to
16 bit files! Other ways of doing this (which I am not familiar with)
involve layers in Photoshop. I believe that Rob Gendler has an article on
his web site about this technique. With these large dynamic range file
formats, applying CCD techniques to old film images is opening up a few more
doors!

http://www.astrofoto.ca/john/ic59.htm
http://www.astrofoto.ca/john/sh2-216.htm

John Mirtle
Calgary, Ab. Canada


-----Original Message-----
From: astro-photo-bounces@seds.org [mailto:astro-photo-bounces@seds.org] On
Behalf Of Thomas W. Earle
Sent: Monday, September 19, 2005 10:24 PM
To: astro-photo@seds.org
Subject: [APML] What's the Best Way to "Combine" Two Images of Differing
Times?

For bright objects, such as M42, it is a known fact that combining
several images at various times helps in getting the full dynamic
range of this nebula; however, I'm not quite sure what's the best
way to go about this.  I have Registar, but none of the Functions to
apply seem like the "perfect" choice.  I feel using them will either
destroy the shadows or the highlights.  I see that many recommend
using Ray Gralak's Sigma function; unfortunately, he recommends one
should have ten or more images to combine for "best" results.
There's no way I can get 10 shots since each film shot ranges from
30 minutes to 3 1/2 hours.  Anybody have a proven technique to get
the most out two or more images exposed differently?  For example,
let's say I take three images of M42 at F/5.0: one 30 minute, one 90
minute, and one 180 minute.  What's the best way to get the full
dynamic range of these images (i.e. visible structure from the
center of M42 to its faintest extensions?  Any suggestions or
comments would be greatly appreciated.

Thanks in advance,

Wade


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