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

["vladimir" <svla@isp.com>]

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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