> -----Original Message-----
> Sent: Tuesday, December 14, 1999 3:31 PM
> Subject: [M]: Do different focus distances matter?
>
>
> Hi,
>
> I have been told that the distance of the EP/CCD from the
> visual back will
> affect the optical performance of a LX200 10". Is this true?

It certainly does. There's only one optimal primary to secondary mirror
spacing and therefore, only one backfocus setting that results from it. One
of the benefits of SCT operation is that you can change the focus so
dramatically but it does come with a significant price in optimal
performance when you stray away from this point. As a prime example, I have
a number of photos using a Lumicon Cassegrain Easy Guider. Problem is, the
CEG puts the film plane far enough back from this point that you can see the
effects in the final photo.


> If it is how
> does one determine the best setup.


Leastways for an SCT, I've found in my particular case that the optimal
setup was where the focal plane of the eyepiece was located about where the
focal plane of a camera attached to the standard Meade off-axis guider would
be located. This at least yielded an optical performance where there was no
detectable spherical aberration although it didn't speak to whether overall
color performance was necessarily optimized. Keep in mind that the
commercial SCT is a significant optical compromise to start with.

With that said, I'd use Suiter's test for spherical aberration as a test of
the most optimal primary/secondary distance and use an eyepiece mounted in a
barlow tube that has had the lens element removed as a means of conveniently
moving the eyepiece in and out with respect to the visual back.


> The dialog went something like: The distance between the primary and
> secondary will affect the amount of 'correction' in the
> optical path and
> the optimal placement of the EP will vary from scope to scope.

Bingo.

> Any comments or insight is welcome,
> Jim

Mike
http://www.concentric.net/~richmann/

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