Re: [APML]: New Topic

["Tom Krajci" <krajcit@det5wg57.barksdale.af.mil>]

> From:          Chuck Vaughn <aa6g@aa6g.org>
 
> >Rule 1.  For extended (non-point) objects f/6 is f/6 no matter what 
> >the focal length.  All f/6 systems will give the same negative 
> >density of extended objects if they use the same exposure time.  
 
> This rule is true to a point. If an object has a low enough surface
> brightness it'll be beyond the limiting magnitude of a small instrument
> and won't be recorded. If this wasn't true any size instrument could 
> record any extended object no matter how faint - just at different image
> scales. 

Good point.  I bet this relates closely to earlier discussions about 
the modulation transfer function and how smaller and smaller objects 
(even if the object has very high contrast) are represented on film 
with lower and lower contrast as you approach the film/optics 
resolution limit.

And I bet this relates to our eye/brain and how we observe 
faint fuzzies.  It's easier to see a large low contrast feature than 
it is to see a small version of this same scene - even if it has 
identical brightness.  That's been demonstrated over and over by many 
observerers at the eyepiece.

Come to think of it, as you use shorter and shorter focal lengths a 
given extended object (based on film grain size, etc.) starts to 
appear to the film like a point source instead of an extended source. 
. .and then we have to look at rule 2 that I mentioned before.

There's an interesing interplay between f/ratio, focal length, 
and aperture on the one hand and time to sky fog limit, time to 
record nebulosity, and time to record stars on the other.

Tom Krajci

Capt Tom Krajci
B-52 Intelligence Officer
"There are only two possible outcomes in any war.
Operational success or intelligence failure!"
http://spur.barksdale.af.mil