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  1. #1
    karateman's Avatar
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    Default Barlows.....????



    So, whats a short focus Barlow, and I rightly or wrongly assume that there is probably a long focus barlow,no?....

    Whats the difference and how would you decide which to opt for. How does a x2 short focus and a x2 long focus say affect performance.

    I thought all barlows just multiplied the mag of any eyepiece /objective combination, do barlows of the same size, eg x2, but different makes have different characteristics apart from glass type etc

    Also does it matter whether it is located say before the diagonal or after it

    Phil
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  2. #2
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    For all practical purposes -- today-- a well made shorty barlow will work just fine in all types of scopes.

    BUT-- hard headed old timers will tell you that a long tube barlow works better in a reflector--- than a short barlow (assuming that it does not stick out into the light path). Long tube barlows can be dangerous to use in a diagonal as you need to be careful they do not impact the mirror-- and depending on the scope (and diagonal) there might not be enough back-focus to come to focus.

    It's similar to the difference between long tube and short tube achromatic refractors--- some long tubes refractors have much better color correction and abberitions than short tube achromats. But the better quality short tube barlows use ED glass -- and lens designs that minimize any aberrations even in the short barlow (shorty) configuration.

    Hope that helped.

    Just buy a good shorty style and or Televue branded barlow and all will be good.
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  3. #3
    kencrowder's Avatar
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    Default

    For a more technical explanation, a 2x barlow changes the focus to produce a beam which is equivalent to double the base focal length. An f/5 looks like an f/10. The point where the lens intercepts the incoming beam determines where in space the focus will be.

    If there were an inch to go, the barlow turns it into 2 inches in this example. If there was half an inch to go, the barlow turns it into an inch. The typical barlow is like the first example and the shorty is like the second.

    There may be some difference if you are looking at very wide true FOVs.

  4. #4
    MitchAlsup's Avatar
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    Default

    Quote Originally Posted by karateman View Post
    So, whats a short focus Barlow,

    How does a x2 short focus and a x2 long focus say affect performance.
    A short focus Barlow has to create a larger divergence of the converging light cones in order to create the barlow effect in less distance than the more normal long focus barlow.

    The divergence of the barlow creates the magnification effect AND at the same time pussues the Exit Pupil of the EP outwards. Should the EP lenses alred be in use all the way to the body of the EP, vignetting will occur. Shorties Barlows display MORE of the deliterious effects than to long focus Barlows.

    Most of this diatribe is irrelevent if your EP collection consists of Abbes (Orthoscopics) and Plössls, only somewhat important if your collection contains Erfles, Königs, Wide Fields, Panoptics, but really important if your collectin is based on Naglers and other UWF EPs.

  5. #5
    pat c's Avatar
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    In my personal opinion I prefer a long barlow. Al Nagler stated that a long barlow is perferable to a short one. Pat

  6. #6
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    Default

    i have a budget end short x2 tal. im happy with it for £20

 

 

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