Re[4]: [ATM] Chromatic effects visibility

[Richard <cnc@cncservo.co.uk>]

Hi vladimir,

Tuesday, February 1, 2005, 12:20:11 AM, you wrote:

vs> It doesn't surprise me that such a pair has
vs> near-perfect achromatism in a 
vs> converging cone. However, it is news for me that it can be that effective as
vs> a RC field corrector.
vs> Makes me wonder why isn't it even mentioned in the literature, while more
vs> complicated and not better correctors like aspheric plates and triplets are.

Wynne made reference to four ways of correcting an RC in :

Wynne, C.G., 1980. Mon.Not.R.astr.Soc V193, pp7-13
"Field correction of a Ritchey-Chrétien telescope at several
focal ratios."

He considered the four simplest methods were :

<quote>
(1) The addition of a single aspheric plate in the converging
beam behind the secondary mirror.
(2) A single aspheric plate as in (1), together with an
interchange of the secondary mirror of the telescope for one
with different asphericity.
(3) The addition of a doublet lens between secondary mirror and
focus.
(4) As (3), together with a change of secondary mirror
asphericity
</quote>

Although I started with (3) which had been mentioned in earlier
literature. I leter allowed the secondary conic constant as a
degree of freedom in the optimisation merit function, thus it
becomes (4). This gave good results so I fixed the secondary
conic constant. I then got some slightly better results. After a
few days of minor changes I forgot about the secondary conic
constant and again added it in to the optimisation and ended
(probably by luck) with an excellent configuration.

I agree later large telescopes have gone on to 3 or more element
correctors, but they are often correcting a much faster primary
than my example.

-- 
Best regards,
 Richard                            mailto:cnc@cncservo.co.uk

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