Re: [APML] OT: M109, NGC 2403 and other collaboration images

["Greg Hartke" <ghartke@adelphia.net>]

Hi, Jason,

It turns out that the diameter of an imaged point target target like a star 
increases with the focal ratio. For that reason (within practical limits), 
you get the smallest stars with the smallest focal ratio. The ideal imaging 
platform is thus a large aperture with a small to medium focal ratio. 
Unfortunately we just can't escape the fact that greater aperture frequently 
makes for better images.

Regards,

Greg Hartke
Sykesville, MD USA

----- Original Message ----- 
From: "Lane, Jason R" <jason.lane@navy.mil>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Thursday, March 03, 2005 10:46 AM
Subject: RE: [APML] OT: M109, NGC 2403 and other collaboration images


Hi Jason,

I'm not clear on your definition of oversampling?  Are you talking about 
oversampling seeing due to the pixel count of the FPA or grain of the film, 
or...?  A longer focal length gives you a narrower field of view. At 32" and 
assuming f/8, that's a focal length of about 6500 mm.  I could get the same 
thing on my LX90 with a 3.5x Barlow.  Assuming I had a fancy enough camera 
to give reasonable enough exposure times at f/35 (!) and that seeing was 
such that the wider aperture isn't gaining anything, are you saying the 
images would be comparable?

That's really just an advantage of the focal length, but not directly due to 
the wider aperture.  People get different apertures simply because the 
aperture is a by-product of the different FOV's.  In visual use, it's the 
other way around.  They look for the biggest apertures they can afford.  But 
for photography, what you really want is an f/0.5 system.  <g>    I think 
the basic question is this:  Is aperture still just important for visual 
use, and is the speed of the system (a.k.a. f/#) still just important for 
photographic use?

Cheers,
-Jason

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