Re: [APML] Long vs. Stacked Short Exposures

[Ray Butler <ray.butler@nuigalway.ie>]

In addition to Stuart's reply, I'd like to add that whether the results 
are equivalent, better, or worse also depends on how you stack them.

If you use a plain average, you will dim and (with many images) 
eventually lose the meteor images; because the average of bright meteor 
in frame 1 + dark background in frame 2 = dimmer meteor. (Unless by 
sheer chance lightning does strike the same place twice, and you get one 
meteor trail overlaying the other!)

So I'd recommend that you use a max(N)-type stacking function, which at 
each point selects the brightest pixel of the stack of N pixels (N 
exposures). This would in theory make stacking a superior approach to a 
single long exposure, because the high contrast between the meteors and 
the shorter exposures' darker background can be maintained. This is 
analagous to the method of getting high-contrast star trails from 
digital cameras which was described in Sky & Telescope a few months ago.
I wonder if anyone has tried it yet with film?

The opposite approach, using min(N) or median(N), is used in CCD imaging 
and spectroscopy to _reject_ cosmic rays, meteors, and other randomly 
positioned transients from exposure stacks. Without this, HST images 
would be obliterated with the high flux of cosmic rays above the Earth's 
atmosphere. I've worked with HST images, and man are the raw frames 
seriously contaminated! But you'd never know it when you see the final 
result.

Ray "hate to admit it but CCD data is a lot easier to process reliably" 
Butler


RBissinger@aol.com wrote:

> I'm sure this has extensively been covered but I'm planning on doing some wide field shooting of the Milky Way this weekend, and can expect to have a few Perseid meteors in my shots.  Maybe the streaks will look good, but I was wondering if I'd be better off taking, say two 20 minute exposures rather than a single 40 minute exposure?  If I took those two 20 minute shots, scanned them, and stacked them, how close would I get to the results in a single 40 minute exposure?  I'll be shooting E200 with a 28mm lens at f/2.8, thanks.
> _______________________________________________
> Astro-Photo mailing list
> Astro-Photo@seds.org
> http://seds.org/mailman/listinfo/astro-photo

-- 
Dr. Ray Butler (ray.butler@nuigalway.ie || ray@physics.nuigalway.ie)
Lecturer, Dept. of Physics || Computational Astrophysics Laboratory
National University of Ireland, Galway, University Road, Galway, Ireland.
Tel: +353-91-524411 ext. 3788   FAX: +353-91-525700

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