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  1. #1
    Danzup77's Avatar
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    Default 2x Barlow for a 2" system.



    I already have a post stating how I am in the process of converting my 8" Celestron SCT to a 2" from a 1.25" but this question is BARLOW specific.

    What are the best quality 2X barlows for 2" units? Also, I have heard good things about them as well as people saying that the barlows for the 2" diagonals do not fit sometimes. Something about them being too long? I dunno.....Want to get one but dont want to spend money just to run into issues.

    Thanks as always......I feel like Im wearing yall out lol......
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    Default

    Personally I recommend the Televue Big Barlow 2x, it does an excellent job with all of my 2" eyepieces and diagonals.

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    Danzup77's Avatar
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    Thanks BK! No issues huh...thats what Im looking for lol. Does it work well with a focal reducer as well (whick im just barely learning about so be gentle)? Televue seems to be a pretty popular name from what I hear so far.

    Also, does the big barlow you are talking about work well with SCT? They seem to be marketing it as Newtonian. I know newts and SCT are both reflectors but who knows.
    Last edited by Danzup77; 10-17-2011 at 12:49 PM. Reason: addition
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    I happen to like and use a 2X PowerMate (which is a telecentric Barlow).

    For an SCT, a 0.63 reducer takes the 2" FoV at the focal plane and converts it into a 1.25" FoV at a new focal plane. So, if you use a reducer you don't need 2" EPs, and if you have Big wide field 2" EPs you don't want a reducer in the light path.

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    Mitch can you please explain the difference between the Barlow and Power Mate? Any further info will be appreciated.
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    I chose a Powermate,I believe it has something about retaining the proper exit pupil(the cone of light entering you pupil).You can read all about it on Televue's websight.I am very pleased with my Powermate.

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    Default

    Quote Originally Posted by Danzup77 View Post
    Does it work well with a focal reducer as well
    I don't know much about focal reducers, but isn't it just the opposite of a barlow? So wouldn't a focal reducer + a barlow be the equivalent of an empty tube?

    Just seems like an odd thing to want to do.

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    Inexpensive 2" Barlow - GSO 2" 2x ED Barlow Lens. I have found that I don't use the Barlow as much as I thought I would.
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  11. #9
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    Quote Originally Posted by Danzup77 View Post
    Mitch can you please explain the difference between the Barlow and Power Mate? Any further info will be appreciated.
    An objective focuse an image on a focal plane by taking all the light passing through an aperture and delivering to a point. The light converges towards focus in the shape of a cone. The converging light cones have the geometric properrties of an F/X system (F/5 F/15, whatever).

    A Barlow intercepts the converging light cones before they arrive at focus. The Barlow makes the converging light cones converge at a slower rate. This slowing of the rate of convergence is what makes the F/5 system into an F/10 system after a 2X Barlow. While slowing the rate of convergence, the Barlow also causes the still congerging light cones to diverge from the optical axis (where they create a larger image at the new focal plane.) This divergence from the optical axis is what causes the exit pupil of an EP to be displaced (farther outward) and can be associated with vignetting if the EP lenses are insufficiently wide to carry the Barlowed image to the exit pupil.

    A PowerMate is a system where the converging light cones are made to converge more slowly (just like a Barlow) and produce a larger iamge (just like a Barlow), but the PowerMate has 2 groups of lenses. This first group acts much like the single lens group of the Barlow (i.e. diverging the converging light cones and creating a larger image) then the second group of lenses takes the diverging light cones and realigns them to the optical axis so that they are no longer diverging. So, you get the same bigger image, but no displacement of the exit pupil, nor do you suffer vignetting problems in EPs.

    {BTW the displacement of the exit pupil is mostly seem as beneficial rather than detrimental.}

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    I went with the 2X Televue Powermate as well and couldn't be happier. It can be used with 2" or 1.25" pieces and has several benefits over a regular barlow which they explain on their site.

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