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Thread: Why longer focal length on primary?

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    Default Why longer focal length on primary?



    So I'm already thinking ahead to my next telescope upgrade. I had thought I would do an 8 or 10 inch deb which have focal lengths of 1200mm. However, I was also taking a look at the Celestron NexStar (8 inch apo with 2000mm focal length). Ignoring any optics differences between a reflector and a Schmidt-Cassegrain for a minute, what is the advantage of the longer focal length. It is my understanding that max magnification is limited by aperture. So it seems to me that the NexStar wouldn't be able to magnify any more than the dob. I suppose you could get to that magnification with higher focal length eye pieces, but that doesn't seem like much of an advantage to me. I also thought the NexStar would be "slower" which seems like a DISadvantage for photography.

    I know I must be missing something. What are the advantages that longer focal lengths bring?
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    Default Re: Why longer focal length on primary?

    There is no particular advantage to a longer focal length for visual observing. As you said, magnification range is determined by aperture, not be focal length. The advantage for the manufacturer of SCT telescopes is that it is easier to manufacture the longer focal length optics.

    The 8SE, like all SCTs, is "slow", but the focal length is a benefit for planetary photography. On the other hand, the SE mount makes it unsuitable for long-exposure DSO photography, which is where you would want a fast scope anyway.
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    Default Re: Why longer focal length on primary?

    Although magnification is ultimately limited by the aperture, a longer focal length will give you more magnification with mid size eyepieces, so a 2000mm fl scope with a 20mm EP will give you 100x magnification and good eye relief.
    To get the same with a 750mm fl scope would need a 7.5 mm EP, which would have to be pretty good to be as easy to use as the 20mm one. And if you have one of those, you could get 266x magnification on the longer fl scope.
    But magnification isn't everything.
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    Default Re: Why longer focal length on primary?

    Quote Originally Posted by pnwoc View Post
    ... I was also taking a look at the Celestron NexStar (8 inch apo with 2000mm focal length).
    What Keith said.. also, the NexStar is not an apo.
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    Default Re: Why longer focal length on primary?

    What does longer focal length do for you? It sort of depends on what you are trying to do and how you achieve that longer focal length.

    For Newtonian reflectors and for refractors it is really quite simple. The inherent aberrations become much less problematic with longer focal lengths.

    Some of the problem can be how you get there.
    Let's say that I decided to make an F/4 Newtonian instead of an F/9 Newtonian? The diagonal mirror for the F/4 would need to be much bigger than would be the case if I had figured the mirror at F/9. This means more diffraction and more light loss.

    There are other issues such as being able to utilize eyepieces with better eye relief. Depending on your budget and choices that may not be an issue.

    For other optical designs it can be a bit more complex.

    Let's say we are trying to make an SCT? The primary mirror generally has a focal ratio of around 2. If you were to figure it for more like F/3 you might be able to use a smaller secondary mirror for less diffraction and light-loss. Other things can also intrude. Meade made an F/6 SCT for a while and most of us really don't want them because the secondary was rather large and the optics just weren't considered to be all that good - we'd rather get a regular F/10 scope and use a focal reducer for better optics and more light.

    Longer focal length can also improve some ergonomics for those of us who are taller (or ruin the ergonomics for those of us who are not so tall).

    There are few unalloyed blessings, but on average I consider longer focal lengths to be optically advantageous although they can also result in scopes which are unusably long, heavy, etc.

    Just a little illustration? That 40" refractor at Yerkes Observatory has a focal ratio of 19 in an attempt to control the aberrations. That results in a focal length of 62 feet!

    The larger refractor made for the Paris exhibition was totally impractical at 49+" and was set up to have a focal length of 187 feet!

    Small and highly engineered (and relatively expensive) refractors can control the aberrations without the slow optics. You can get a fast Newt and correct much of the aberration by adding more optics.

    There are always trade-offs. No way to tell which is really best, it is always a matter of priorities and options.

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    Default Re: Why longer focal length on primary?

    Quote Originally Posted by KeithBC View Post
    There is no particular advantage to a longer focal length for visual observing. As you said, magnification range is determined by aperture, not be focal length. The advantage for the manufacturer of SCT telescopes is that it is easier to manufacture the longer focal length optics.
    On paper the potential magnification is determined by aperture. But during actual usage I always seemed to think it was seeing conditions that was the final factor. Maybe some nights only 110x is possible regardless of aperture. What aperture gives you is greater detail compared to a smaller aperture.

    Or on a given night, will larger aperture give higher magnification than smaller scopes?
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    Default Re: Why longer focal length on primary?

    Quote Originally Posted by gcisko View Post
    On paper the potential magnification is determined by aperture. But during actual usage I always seemed to think it was seeing conditions that was the final factor. Maybe some nights only 110x is possible regardless of aperture. What aperture gives you is greater detail compared to a smaller aperture.

    Or on a given night, will larger aperture give higher magnification than smaller scopes?
    An upper limit is indeed imposed by the seeing conditions. But a larger aperture will be better able to take advantage of good seeing, and will probably perform better in average seeing than a smaller scope.

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    Default Re: Why longer focal length on primary?

    Quote Originally Posted by KeithBC View Post
    An upper limit is indeed imposed by the seeing conditions. But a larger aperture will be better able to take advantage of good seeing, and will probably perform better in average seeing than a smaller scope.
    I admit to not being the expert on this, but I generally agree with you.

    I am still intrigued, however, by the argument which I understand led to the aperture of the 3.5" Questar. The idea is that there are a sort of cells of turbulence within the atmosphere which tend to be a bit over 3.5 inches. Thus if you keep the aperture less than the size of the atmospheric cell, then at times you will be looking through a single cell - and the seeing will be more stable within that single cell.

    My personal experience has been that keeping the aperture small does not improve the seeing.
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    Default Re: Why longer focal length on primary?

    Just speaking about Magnification....

    I own several telescopes BUT only use 4 on a regular basis and no matter which one I use (for non lunar viewing) it is very rare for me to use any more then 200x with any one of them NO MATTER WHAT THE SEEING CONDITIONS ARE... The larger aperture to me means I may be able to see "detail" because I am collecting more light ..

    If you want magnification..then go out and buy a cheap department store scope which states plainly ... in big lettering ...it will give you 600x lol

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