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Thread: f ratio?

  1. #21
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    By now we must have Gappa thoroughly confused. A little bird was telling me I was making a mistake by bringing photography into the thread.

    Going back to the original topic of the thread .... Gappa, it looks like most folks are saying for visual observation, focal ratio is not that important; it's just one of many aspects of your kit (telescope, eyepieces, etc.) to consider.

    Now when you get into photography ... as you can tell it is a different horse so to speak. At this time you probably should not be worrying about photography, just learning astronomy and acquiring viewing skills so forget about focal ratio.

    Focal ratio for photography is not so black and white as I made it out to be but I just didn't want to really get into the technicalities for a couple of reasons ... the foremost being the scope of the originator of the thread's question and the second being I was afraid that the thread would be hijacked as it was. I should have listened to my inner voice about letting sleeping dogs sleep.

    If you guys want, we can make this excellent discussion about focal ratio and photography an independent thread.
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  3. #22
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    Rich,
    Oh, I see what you are saying. Aperture rules. Disregard my question -- I now see the answer.

  4. #23
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    well I'm looking at refractors right now since I know a little more about them and yes they are quite pricey, but for almost the same view you are saying a SCG will deliver for less money?

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    A 6"-8" SCT is an alternative that should deliver much larger aperture than a small refractor for the same price or less.
    Last edited by TCA; 11-07-2010 at 06:57 AM.

  6. #25
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    That refractor GLASS gets really expensive FAST! That's why we recommend a reflector to beginners --- the best bang for the buck in the Astronomy hobby.

    Generally there are three focal lengths--

    F5 or faster usually on reflector scopes

    F10- F12 usually on SCT scopes

    F15-18 usually on long tube refractors, or Maksutovs.

    Sometimes APO short tube refractors hit F7- or F8 which is a sort of sweet spot for astro-photographers. Beyond this the F number really has no consequence for amateur astronomers, except slower scopes can use cheaper eyepieces, than scopes F5 or faster.

    Slower scopes are better for planetary and double stars -- while faster scopes show brighter images on dim fuzzy objects------- generally.
    Last edited by Joe Lalumia; 11-07-2010 at 03:17 PM.
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  8. #26
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    can you still use the SCG or reflecting units for terrestrial viewing as well?

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    can you still use the SCG or reflecting units for terrestrial viewing as well?
    i assume you mean sct?
    reflectors return an image in the ep upside down and back to front.
    i have no experience with sct's but i THINK they are the same.
    you would need to get a corrector or similar to use for
    terrestrial viewing.
    clear skies,
    andy

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  10. #28
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    Yes, but you need an "erecting diagonal" for terrestrial viewing.

    Like this:

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  11. #29
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    yes I did mean sct or why wouldn't SCG work for Schmidt-Casse-Grain?

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    Quote Originally Posted by Gappa View Post
    yes I did mean sct or why wouldn't SCG work for Schmidt-Casse-Grain?
    LOL ..It makes sense ..But it is still SCT .. Another big debate with SCT's is the light blocking with the secondary mirror ..An 8in SCT might not bring in as much light as a 8in refractor ...

    Also , a correct image diagnol uses a prizem to flip the image which also loses light ...
    16in Night sky Dobsonian F4.5 , 90mm refractor 500mm FL , 7in LX200 classic Big-mac , Meade eye pieces with a few odd balls thrown in ...
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