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Thread: f ratio?

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    Quote Originally Posted by roverich View Post
    Not necessarily true on the cassigrain thing ..The schmidt cassigrain was invented in the 40's by James Gilbert Baker ..As a modification of Bernard Schmidt' 1931 schmidt camera ..It uses a spherical mirror and a schmidt corrector plate ..It is a cassigrain configuration ...If you would like to read further , Look here ... Schmidt?Cassegrain telescope - Wikipedia, the free encyclopedia
    It's a Cassegrain telescope with a Schmidt corrector plate, but both of those are proper names. In this case, it's named after Laurent Cassegrain.
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    APO refractors if manufactured right are also some of the finest instruments available. The Meade UHTC units look very nice tho. I guess it's all a matter of the materials used..glass lenses or mirror and the coatings they use.

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    Coatings alone do not a great mirror make.

    You are correct, the Meade and Orion, SW, Celestron and others are mass produced (sort of) SCt /MCt designs - you can certainly spend a lot more on a high end hand crafted one. But the differences in view in any high quality hand made optic will always cost significantly more. Very significantly more. The mass produced ones are 'diffraction limited' whatever they choose that to mean, not like you can order one at 1/8 wave or 1/12 wave. Complicating this is there are two mirrors (Cat, Newt, SN) and each has it's criteria.

    Refractors on the the other hand have a generally accepted Strehl ratio / rating that is really a great overall judge of the optics.

    In any case you can always send your optics off to a testing lab if you 'jus gotta know'.

    You can try and order very high spec mirrors from folks like the Zambuto company (and others) for an amazing performing Newtonian.

    You can try your hand and patience at making your own lenses or mirrors and having someone coat them (if desired to coat them).

    My only point here is that there is optics, and often quality gets confused with cost. 'Good enough', and so on... subjective assesments. There are actual ratings and measures to be thankful for - otherwise it would be entirly up to the marketeers and they'd have us all thinking we should be seeing everything in high resolution color, at least up to the point they have your $$...

    This why my occasional comment that all apertures are not created equal. Absolutley very many of the mass produced optics are 'comparable by type'. I know it sounds silly but if possible try to divorce the $ part from the optical physics/human physiology part of it.


    I've been perfectly happy with less than great optics, but never did I fool myself that it was 'as good as it gets' because it was big and bright. It was what I could afford and I was very happy - and really that is all that is important.

    I'm sure the quality vs. quantity debate and musings will rage on, but understanding focal ratio (and if a visual only user, eyepiece exit pupil and local seeing) and the different designs (how they are impacted) will always be of fundamental importance - in my opinion always more important than aperture alone.
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    Very informative thread.

    Jim (klaatu2u), great information. Have you considered writing a book? I'm being completely serious, I would buy it! I've read allot of your posts here and there and I for one like to understand all of the technical reasoning behind things and you know your stuff and do a good job of laying it all out.
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    Default Re: f ratio?

    Quote Originally Posted by Nemrod View Post
    Actually, f numbers are only valuable reference for photography, visual performance will not be notably effected. For photography, lower numbers mean generally a wider field of view (for a given aperture) and less exposure time.

    JR
    I'd say visual performance will be greatly affected by the eyepiece. For viewing in a "fast" scope, you will need high quality eyepieces (ya wants ta go fast, yas gotsta pay fer it); slower scopes (f7 and up) are more forgiving of eyepieces (but the scope is more expensive). As an example, the Orion Expanse 20mm works great in an f8 scope, but put it in a fast f4.7 dob, and it becomes scarey bad. On the plus side, premium eyepieces only improve a slow scope.

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    Default Re: f ratio?

    Why is this a sticky?

    It's an absolute mess of a thread for anyone curious about the nature of f-ratio. Hats off to jenniferchristine for initially stating the obvious.

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