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

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

Hi, Jason,

Well... Let's think about it. Suppose you imaged the same extended object 
with the 2 scopes you proposed as examples. For simplicity consider the 
object to be of uniform brightness. Assuming no losses, the brightness 
density at the focus plane would be the same in the 2 cases because 
(obviously) the focal ratios are the same. However, the image scales are 
drastically different because the focal lengths are different by a factor of 
4. If the image from the smaller scope were expanded to have the same scale 
as that of the larger, the brightness density would decrease by a factor of 
4.

Now suppose the imaged object has interesting details we would like to 
resolve. When we matched the images scales from the 2 instruments, the 
imaged size of the Airy disk of a point target (which would be important to, 
say, the ability to capture detail in a spiral arm, etc.) from the smaller 
scope would also increase by a factor of 4. Aperture still wins. Bummer!

The old dictum from observational astronomy that "aperture rules" is still 
true for imaging but how vital it is it depends on what you're trying to do 
and how you present your results.

Regards,

Greg

----- Original Message ----- 
From: "Lane, Jason R" <jason.lane@navy.mil>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Thursday, March 03, 2005 11:27 AM
Subject: RE: [APML] OT: M109, NGC 2403 and other collaboration images


Hi Greg,

You're right, the Airy disc diameter increases as a function of f/#.  But 
that still doesn't explain the larger aperture.  A 32" f/2 scope and a 8" 
f/2 scope would have the same-sized star images assuming seeing is limiting 
the resolution of the images.  But the larger aperture scope has a longer 
focal length, so you get a narrower field of view.

I think this is why there are only a few locations on earth where large 
telescopes are used professionally...there really is no advantage to them 
(for imaging in particular) if seeing is the limiting factor.  I guess my 
thought is that you've gotta consider seeing as well as FOV, optical 
quality, and f/# when you're getting ready to buy an imaging setup.  Then 
you've got mechanical issues, mounts, gears, stability, etc.  That's a lot 
of factors....you're probably just as well off buying the scope that looks 
the coolest.  :)

-Jason 

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