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    Question long focal length reflector



    Hi there,
    The fast telescopes seems to be the standard these days among dobsonians. What about long focal length newton telescopes, say f/8 or more with an aperture of 8 inches? Are images compromised if you intend to 1) have a great DSO scope but also 2) a telescope suited for moon and planets?
    In other words, can I achieve the same on deep sky with a long focal instrument?
    Borje

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  2. #2
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    I think that what has happened is that cassegrain scopes have pretty much taken over long focal length duties - and you can get them in apertures up to 20".

    Dobs, due to their primarily visual use, benefit from fast optics to provide a wide, bright FOV without needing to stand on a stepladder (for most common sizes).

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    Quote Originally Posted by borje hastbacka View Post
    What about long focal length newton telescopes, say f/8 or more with an aperture of 8 inches? Are images compromised if you intend to 1) have a great DSO scope...
    The quick answer to your question is no, images will not be compromised with a f/8 Newt. The reason for the fast focal ratio Dobs is simply aperture vs convenient OTA size.

    Quote Originally Posted by borje hastbacka View Post
    but also a telescope suited for moon and planets?
    In other words, can I achieve the same on deep sky with a long focal instrument?
    Yes, it is not unusual for a f/8 scope to perform well on both planetary as well as deep sky objects. You just need to make build your eyepiece collection around your scopes focal ratio. I'm in the process of replacing all my eyepieces over 20mm to the 2" version for my f/4.9 Dob for the exact reason. Well, that and because I've been stuck in my own little eyepiece world for the past several years

    And welcome to the forum.

  5. #4
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    Quote Originally Posted by borje hastbacka View Post
    Hi there,
    The fast telescopes seems to be the standard these days among dobsonians. What about long focal length newton telescopes, say f/8 or more with an aperture of 8 inches? Are images compromised if you intend to 1) have a great DSO scope but also 2) a telescope suited for moon and planets?
    In other words, can I achieve the same on deep sky with a long focal instrument?
    Borje
    You use the word "great" for scope. Well, a lot depends on what you mean by "great". The clarity of the image depends on many things, besides the "speed". But once you've decided the aperture -- 8 inches, for instance -- then clarity is going to depend on eyepieces. The shorter the focal length, the more you'll need top quality eyepieces. Astronomy Hacks suggests that you can rely on medium quality eyepieces for scopes f/6 and above. Anything faster, like a f/5, requires eyepieces with better optical precision.

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    Thanks so lot. My thought was that a long focal length makes images dimmer and therefore faint objects are harder to see in a comparison head to head with two telescopes of same aperture but different focal lengths. For sure,the field of view will be smaller in a f/8 than in a f/5, with the same eyepiece. But I reckon this is no big deal, if images are equally bright, since most DSO:s don´t fill the whole FOV.
    Börje

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    The whole "fast = brighter" argument I believe is more about photography than for visual use. I mean, it's true that there must be more photons arriving at the focal plane, or else CCDs wouldn't be imaging faster. But for visual use, I don't think that "slow" or "fast" optics really affect whether you can see a given DSO. Spotting DSOs is all about contrast, not so much about overall brightness (as far as I know).

    This actually piques my interest -- perhaps some more experienced and knowledgeable folks can comment on the net effects of fast vs slow for visual use...

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