Re: [ATM] Long Focus Refractor

[Michael Lindner <mikell@optonline.net>]

On Saturday 27 March 2004 07:31 pm, Ken Hunter wrote:
> Richard wrote...
>>
>> Actually, with a refractor, it is not as bad as that.
...
> This may be true for the difference in FOCUS POSITION
> but an AXIAL ray diverted one degree will be displaced
> 1.745 feet laterally on a 100 foot radius.

No, actually Richard is right, I was wrong. I'm thinking in reflective terms, 
not refractive. The ray is only displaced while travelling through the glass, 
and the angular error is thus very small. I modeled this in Oslo to confirm 
this, and the image displacement I got with a 4" diameter BK-7 biconvex lens 
(1/2" center thickness both faces' ROC 1283.4") tilted 1 degree is about 
0.006". Even with a 40mm eyepiece, the apparent image shift would be 2 
degrees...quite acceptable.

Of course, the focus shift is about 0.5", but as another poster noted, the 
depth of field of a scope like this is quite forgiving, and the 
near-diffraction-limited view is pretty much unaffected, even without 
refocusing!

In fact, I'm surprised at the performance one can get out of a singlet of this 
extreme focal ratio! The image is actually *better* at the edges of a 117" 
diameter focal plane than at the center!

-- 
Michael Lindner
http://www.starastronomy.org *** http://home.att.net/~mikel
http://www.atmsite.org *** http://www.atmlist.net

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