Re: [APML] A learning Place for Astro Work

["Hap Griffin" <lgriffin@sc.rr.com>]

----- Original Message ----- 
From: "Ray Rochelle" <r.rochelle@sbcglobal.net>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Sunday, December 26, 2004 11:05 PM
Subject: [APML] A learning Place for Astro Work


>Hap,
>Is there a place you could recommend to us newbie to learn what the terms
like (aligned and stacked)mean including how and what software does these
things?
>Thanks for your help
>Ray Rochelle

In a nutshell, the name of the game with astrophotography is signal to noise
ratio.  We are taking pictures of generally VERY faint objects (i.e.
galaxies and nebulae).  With film, you need the signal (the information you
are trying to capture) to rise above the "noise", such as the film grain and
random cosmic ray hits, etc.  In electronic imaging such as with dedicated
CCD cameras or the new CMOS imagers in digital SLR's, it's electrical noise
of several types that you must get the signal to rise above.  With film,
longer exposures will give a higher S/N ratio until you reach the sky fog
limit where the ambient sky brightness begins to fog the film.  In
electronic imaging, longer exposures are the key as well but are limited to
about 5 to 10 minutes in the un-cooled DSLR's again because of accumulating
electrical noise, which is also temperature dependant.

So, a simple way to further increase the S/N ratio beyond what can be
achived with a single exposure is to take multiple exposures and "stack"
them.  The term "stack" refers to the old darkroom practice of stacking
several negatives while printing to get higher contrast out of faint images.
Obviously, the images have to be aligned so that the stars in the images
overlay each other perfectly.  Nowdays, all this is done in software.

Stacking the images, depending on the mathematical algorithm you choose to
use to combine the images, generally causes the noise to be reduced and the
signal (the subject of the picture) to increase in brightness.  The noise is
reduced because it is random in nature and averages, or smooths out, while
the subject matter, the "signal" which is not random, adds.  In film work,
generally only a few images are stacked because the S/N is fairly high to
begin with because of the longer exposures possible, while in electronic
imaging, it is not uncommon to stack dozens of frames.  My Horsehead region
photo at http://www.machunter.org/hap_alnitak_close.html is a stack of 48
five-minute exposures with a Canon 300D digital SLR, for a total exposure of
four hours.

I use ImagesPlus by Mike Unsold (www.mlunsold.com) for main processing,
falling back to the old tried and true Photoshop for final tweaking,
sometimes finishing with NeatImage (www.neatimage.com) for final noise
reduction.

One of the best online resources is Rob Wodaski's "The New CCD Astronomy"
which is avialable as an online book at
http://www.newastro.com/newastro/default_login.asp .  Other than that, there
are many Yahoo groups that specialize in astrophotography, both film and
electronic.  I hang out in Digital_Astro, Canon_Digital_Astro, and the
ImagesPlus Yahoo groups, among others.

Hope this helps,

Hap Griffin
www.machunter.org/hapspics.html
www.hapg.org/astrocables.htm

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