Re: [APML] Testing Lens Aberrations

["Bobby Middleton" <bobm@koyote.com>]

Hi Thomas.

---- Original Message ----- 
From: "Thomas W. Earle" <twade@bmi.net>


> Would
> anyone happen to know the maximum exposure I can expect before light
> pollution becomes a problem (i.e., disappearing "brighter" stars).

Light pollution won't hurt "brighter' stars. It really won't hurt the
fainter ones either. Light pollution will affect recording power of faint
nebulosity because it will be as bright as the fainter nebulosity is. For
moderate light pollution at f/4 I'd limit my exposures to 10-15 minutes
since recording nebulosity isn't a goal. Double that time at each f-stop
increase.


> I'm hoping I can go long enough so the brighter stars near the edge
> start showing signs of aberrations.  I don't really care if the
> nebulas disappear in the sky glow.

You will be able to accomplish your goals of recording stars for lens
testing even in a light polluted sky. More important will be focus and
guiding. If you have errors with these two elements it might make it hard to
see the abberations that the lens may produce.

> Presently, I'm thinking about
> centering on Orion.  I only have access to the western sky so if you
> can think of a better area than Orion, please let me know.

Orion will do as well as any but it's sky position will limit your time to 2
hours at most of exposures; and most of that will be with Orion near the
horizion. For lens testing any good star field will do. Leo is near zenith
early in the night so that might be an alternate target.

> Basically, I'll be testing the lenses wide open (e.g., f4), and then
> closing one stop (e.g., f5.6).
>

That's a good testing regiment. If the lens is distortion free wide open it
will be good at all f ratios. The widest apertures are the lens'es most
susceptible settings.
Bobby Middleton


> Thanks in advance,
>
> Wade
>
>
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