Re: ATM Loveday Secondary

[Michael Peck <mpeck1@ix.netcom.com>]

On Tue, 5 Jan 1999 01:25:40 -0800 Anthony Stillman <atmer@flash.net> wrote:

>This design, as expected, is limited by coma and stig.  However, as noted,
>the large effective f/ and small field keep these abberations in check.  In
>  <...>

>Lastly, though given the large Airy disk this point is rather moot, the
>field is not flat.  In fact it is quite curved.  My best guess of field
>radius for this design is about 570mm, convex toward the secondary.  This
>

There's no reason to guess since OSLO will happily tell you the Petzval
radius if you ask. Click on "Show...Paraxial Setup", for example.

I did a quick check of a sample configuration (not identical to Dave's),
with the secondary located 225mm from the primary focus and a rc of 450mm.
This results in about an f/26.7 system (primary is 250mm diameter with rc
2450mm - sorry, I don't feel like dividing these by 25.4).

This particular configuration has a Petzval radius of 355mm. I'm not sure
what Anthony means by "limited by", but for a flat detector field curvature
is by far the dominant aberration and would ultimately limit system
performance if you could somehow produce a sufficiently large field. Coma
and astigmatism cancel out in the first two reflections, but there's a
small contribution from the 3rd reflection (off the center of the primary).

As Roger Ceragioli suggested a couple editions of the digest ago you can
indeed make a spherical secondary version with a prolate ellipsoid primary
(about 81% of a paraboloid in this example). Coma and astigmatism both
increase substantially in this variation, but you'd still be diffraction
limited over any plausible field (up to 10' or thereabouts).

Frank is probably wondering when the rest of us are going to get a life.

Mike Peck

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Michael Peck
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Wildlife photography page http://pw1.netcom.com/~mpeck1/index.html