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Re: [ATM] F5 Achromat Design Help
Thomas Janstrom wrote:
>Richard etal,
>
>Still not that good, but getting better, I really think that if those
>are the only glass types he has on hand then a slightly slower system
>would be a much better option. How about we look at F6 and F7 just for
>comparision purposes, after all who are we to say what he should do with
>his glass?
>
>A Strehl of 0.61ish isn't all that good, if we usually strive for 0.9ish
>with our mirrors then something close would be nice for this type of
>instrument too. I just don't think he'll get it without going to three
>elements with one of them being a more exotic glass......
>
>
At f/5.2 speed, perhaps the only visual consideration is the
probability of detection. I don't think it would take much to
convince me that is directly related to Strehl ratio. In that
case, there's less than half a magnitude (duck'n time)
difference between a Strehl of 0.61 and a Strehl of 0.95.
The other half of the equation would come when you get out the
2.5x Barlow and 4.8 mm EP... There are those who would demand
that kind of performance. I wonder if this is a point where a
subaperture corrector would be apropos. If one prefers not to
buy (perhaps from a vociferous Ukrainian;-), maybe something
like (at 180 mm f/8; corrector diameter 70 mm):
Surf Radius Thickness Glass
STO Infinity -5.13501
2 791.271 26 N-BK7
3 -537.009 10.72
4 -526.753 13.5 F2
5 -2572.06 960
6 -2068.21 5 N-KZFS4
7 105.811 16 S-FPL53
8 -110.034 5 N-KZFS4
9 84.8091 14.5 N-SF5
10 -238.687 432.908
IMA -4387.976
BTW... This is about as fast as I could push it. Does someone
have a good "simple" subaperture corrector prescription
for this speed? ("simple" requires glass with no trace
unobtainium oxide).
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