Re: [APML] Long vs. Stacked Short Exposures

["Alson Wong" <alsonwong@charter.net>]

I used the Lighten function in Photoshop to stack Leonid meteor images from the 2001 storm:
http://webpages.charter.net/alsonwongweb/20014.htm

For meteor photography, a fast film is desirable, to record as many faint meteors as possible. Conversely, a film with _high_ reciprocity failure will increase the amount of time that you can expose before reaching sky fog, allowing you to record more meteors on a single exposure. I used Ektachrome P1600, with five minute exposures at f/2.8; I easily could have exposed for ten minutes and maybe more without getting too much sky fog.

Alson

----- Original Message ----- 
From: "Ray Butler" <ray.butler@nuigalway.ie>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Wednesday, August 11, 2004 4:45 AM
Subject: Re: [APML] Long vs. Stacked Short Exposures


> 
> 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.
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> -- 
> 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
> 
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