Re: ATM Percent illumination question

["Frank Ward" <thewards@mindspring.com>]

Howard,

> Is there some geometric diagram I can see so that I can understand
> what you're saying?
>


I thought Newt.exe does a good job.

Frank
----- Original Message -----
From: Howard Lester <hlester@as.arizona.edu>
To: Mel Bartels <mbartels@efn.org>; Holmes, Michael H. <MHolmes@tigr.org>
Cc: <atm@shore.net>
Sent: Friday, April 13, 2001 2:13 AM
Subject: Re: ATM Percent illumination question


>
> Mel, I don't think I could be any more confused.  :)
> Thinking like a semi-normal observer, if I stick my
> eyeball further and further down the drawtube,
> I will see the primary surrounded by more and more
> darkness, thus the apparent allowance for a smaller
> diagonal. A tall focuser would seem to require a correspondingly?
> larger diagonal.
>
> Is there some geometric diagram I can see so that I can understand
> what you're saying?
>
> Howard Lester
>
> > Just keep in mind the extreme case where the diagonal is right in front
of
> > the primary. Even though you do not get 100% illumination, you will get
> > the same illumination across the entire field of view.  This is
equivalent
> > to sweeping the diagonal across the primary, where you can see that a
> > movement of 0.5 inches still illuminates all of the primary.  Moving the
> > diagonal right next to the focal plane will actually cause a total
cut-off
> > of light when moved 0.5 inches!
> >
> > Seems paradoxical, but this is yet another good argument for tall
> > focusers.
> >
> > Mel Bartels
> >
> >
> >
> >
>
>