Re: [ATM] Need Design Suggestions for a 16" f/1.1 Spherical Mirror

["Scott Ewart" <scott.e@quicksurfer.com>]

    A friend showed me an interesting mirror that had been given to him. 
It's a 13.75" clear aperture spherical mirror with a focal length of 6.625". 
That's right, f/.5 (not even).  It's .75" thick with a 3" hole.  I was told 
it came from some kind of projector, and it had a secondary (if only he 
still had that).
    It sounds like it's way too fast for a Schmidt camera.  Can he make some 
kind of Cassegrain out of it for photography?  If so, what type and what 
figure on the secondary?  Gregorian, maybe?  Can we get the field flat?
    Or should he just put the dip in the hole and arrange veggies around it?

Scott


----- Original Message ----- 
From: "vladimir" <svla@isp.com>
To: <atm@atmlist.net>
Sent: Wednesday, September 28, 2005 5:34 PM
Subject: Re: [ATM] Need Design Suggestions for a 16" f/1.1 Spherical Mirror


| Been playing with large, very fast spheres and what - if any -
| optical purpose they can serve. The best I came up with is
| a 2-mirror 3-reflection system that is stopped down to 37% linear aperture 
of such sphere (although third reflection does use nearly its entire 
surface).
|
| The second mirror, placed after the prime focus, also spherical but 
convex, directs diverging light back to the
| primary, which then focuses behind the secondary, which is
| contained within the converging cone at its location.
| The field can be made essentially flat, with some remaining
| coma/astigmatism, but with overall field quality still good for
| imaging purposes. An example with the above mirror is this effective
| 150mm f/2.5 system:
|
| R1: -900mm
| primary-to-secondary: -674mm
| R2: -1000mm
|
| Minimum obstruction (by the secondary in the converging
| cone after third reflection) is 60%. The spot size increases
| from bellow the Airy disc diameter at the field center to about
| 0.025mm nearly 1 degree (6mm) off-axis, due to combined
| coma and astigmatism. The blur size decreases
| with slower primaries.
|
| The parameters are fairly constant: 37% stopping down,
| ~2.2x increase in primary's F# and 60% minimum c.o. System
| scales fairly well with either primary's F# or aperture.
|
| Baffling the system seems like lots of fun, but doable.
| All in all, who have a large fast sphere at hand, and some
| spare time, can consider making a "partial" use of it.
|
| As for more conventional systems, they would likely have
| very tight misalignment tolerances with this fast primary.
| Enough to make them impractical for the amateur arena.
|
| Vlad
|
|
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