Re: [ATM] sizing secondary mirrors,and the size of the 100% illuminated field

[Chris Todd <christopher.todd@gmail.com>]

Bob,
 Whoa there, pardner, put those guns back in their holsters. :-) I certainly
didn't mean to kick a bee's nest.. I merely wanted to determine whether
there was an objective (rather than subjective) basis for the 10mm
statements I had seen, as none of the authors I referenced provided any
explanation for their statements.
 From your answer, it sounds like there are no hard, fast rules with respect
to visual observing, though obviously film and CCD imaging have different
requirements.
 To answer your final question, I am building a binocular newtonian with
12.5" primaries, so my scope will be used exclusively for visual observing,
mostly of deep-sky objects @ low/mid magnitude with wide fields of view and
1.25" e.p.s most of the time (though there are some models of 2" e.p.s I may
be able to use). While I would hope to occasionally use 150-200x for
planetary and lunar viewing, that will likely only work if I do an
exceptionally good job building the scope, since merging the images at high
power becomes difficult unless everything is perfect.
 Chris
 On 11/14/05, Bob May <bobmay@nethere.com> wrote:
>
> Prepare the guns for ACTION ! ! !
> Please note that a lot of this is all relative to the
> application. One person may be satisfied with a FOV of 10mm
> while another will want a 45mm FOV on his scope for a fully
> illuminated field. Don't bother to argue with either fo them
> because that is what they will find acceptable for their
> purposes.
> Another thing that I will note is that a 70^ illumintated zone is
> about 1/4 mag. drop in intensity from the 100% illumination and
> that isn't much in reality. You can see it on a photograph that
> shows the background illumination buy you probably won't notice
> it with a visual inspection of the view.
> I'll note that the R&VR value is more for 1.25" EPs as this is
> about the size of higher power EP FOVs. Lower powers will be
> seeing the vignetting to about that 1/4 to 1/2 of a mag and this
> isn't objectionable for visual use. For photograpy, larger
> secondaries are usually recommended to get the unvignetted FOV
> out to a larger size (providing that is the vignetting source!)
> to get the image flatter in brightness. Please also note that a
> 35mm film won't fit in a 2" focuser and you have to use something
> larger to get the corners of the film exposed without vignetting
> by hte focuser itself.
> Now, what do you want to do with your scope???
> Bob May
> bobmay at nethere.com <http://nethere.com>
> http: slash /nav.to slash bobmay
> http: slash /bobmay dot astronomy.net <http://astronomy.net>
> Replace the obvious words with the proper character.
>
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