Re: [ATM] Or over-correction?

["Scott Ewart" <scotte@frontiernet.net>]

Vlad,
    You bring up an interesting subject.  After finding a noticeable 
difference caused by a barlow while star testing an f/4.4 mirror in 
progress, I also noticed very slight differences between eyepieces. 
However, I did a little research and fact checking on this particular 
subject and here's what I found.
    It's wrong to assume spherical correction in eyepieces is over- or 
undercorrect, of even significant.  You conclusion that a fast mirror 
should be made overcorrect is dangerous based on your assumption about 
"fancy wide-fields with the negative Smyth lens".  I've been told that 
if you ray trace the published 13mm Nagler patent at f/5, you'll find 
that it's about 1/10 wave overcorrect, not undercorrect.
    I'd say that the only reason to figure your mirror to match any 
eyepiece is if you're SURE that that's the eyepiece you will always 
use with it.  And do the final figuring WITH THAT EYEPIECE. 
Otherwise, just figure the mirror as best you can to get the best test 
numbers and star tests with a wide variety of eyepieces.
    That f/4.4 Coulter mirror I was refiguring had already been 
refigured by Galaxy and was left about 2/3 wave undercorrect.  I got 
the same "thermal" excuse when I asked them about it that has been 
discussed in the recent thread.  I can tell you all that after 
CORRECTLY correcting my mirror myself, it ALWAYS performs much better 
than before.  How many Coulters have you seen that can split 
sub-arcsecond doubles?
    Just do it right and don't worry about theories.  Theories don't 
bend light.

Scott Ewart


----- Original Message ----- 
From: "vladimir sacek" <vla@toast.net>
To: <atm@atmlist.net>
Sent: Sunday, August 06, 2006 10:42 AM
Subject: [ATM] Or over-correction?


|A late reaction on the idea of deliberately making a mirror
| under-corrected. In the context of visual use, there is a
| reason to make it slightly (or more than slightly) over-corrected.
| The reason is the spherical aberration of the eyepiece, which
| increases rapidly as the F# goes bellow ~5. The eyepiece
| being a complex positive lens system, produces under-correction
| (this should include fancy wide-fields with the negative Smyth 
lens).
| The size of wavefront aberration is in proportion to the eyepiece
| focal length, and inversely proportional to the fourth power of the 
F#.
|
| While the eyepiece manufacturers still enjoy the privilege of not
| providing any specifications in regard to the level of eyepiece
| correction, the raytrace reveals that the correction level of a
| 5mm f.l. eyepiece at f/5 is likely to be between Lambda/11
| and Lambda/28 PV of under-correction. This doesn't sound
| as much, but if a mirror is, say, 1/7 wave under-corrected,
| adding, say, 1/15 wave from the eyepiece would result in
| 1/4.8 wave PV total error. Not quite negligible.
|
| At f/4, the likely range of a 5mm eyepiece under-correction is
| between Lambda/4.5 and Lambda/11.5 PV. Making a mirror
| ~Lambda/6.5 over-corrected would likely ensure best possible
| visual performance.
|
| Vlad
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|
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|

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