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    vanislandmike's Avatar
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    Default Open truss and light pollution?



    Hi all,

    In another post I've detailed my disappointment at waiting a month for my refractor only to find out it's now back-ordered for three months.

    So I'm reconsidering my choice of scope.

    Could one of you kindly tell me if a collapsible open-truss newt is more affected by light pollution?

    It would seem to me at first thought that more light would enter than in a closed tube design, but perhaps the angle of the mirror makes this entirely inaccurate thinking.

    Do open-truss dobs fare worse for a given amount of residual light pollution than closed-tube dobs?

    Many thanks,
    Vim
    Last edited by vanislandmike; 03-16-2011 at 02:55 AM. Reason: spelling

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    roverich's Avatar
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    Default

    Stick with your original choice and go with a refractor ...Much better for travel ...But for your initial question , Open tubes will allow more stray light in ...Also , more contaminants like dew , sneezes , pollen and salt spray if you view by the ocean ...
    Last edited by roverich; 03-16-2011 at 03:04 AM.
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  4. #3
    Art Bianconi's Avatar
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    Default Refractors VA Newtonians

    Light pollution and it's affect on viewing will be the same be it a refractor or a Newtonian. The loss of contrast that occurs is in the air not in your scope.

    Stray light that enters a scope from a light source causes loss of contrast as well. Where you can't do much about light pollution in the sky, you can baffle and shroud an OTA so as to eliminate the loss of contrast from ground sources.

    Diffraction losses in a Newtonian are also caused by spiders associated with secondary mirrors. While different spider geometry will reduce the effects of diffraction, it is still going to occur to some degree.

    Refractors do not have this problem. Instead they have the problem of requiring color correction. Light slows down as function of how much material it's passing through. All things being equal (which they rarely are!) you get more light gathering power for the dollar with a mirrored scope than with a scope using lenses.

    Light passing through a glass of a given thickness will get to the other side slower than the same beam through a narrower section of the same material. This difference causes a shift in focus created by a separation of the basic colors.

    For example, poorly made refractors will often exhibit a distinct color band around the edge of the moon, often deep blue. Color correction is an expensive solution in refractors requiring compound lens construction, different types of glass and special coatings.

    The biggest problem with refractors however, is that they are aperture limited. The edge of a lens is the thinnest part and it must bear the weight of the entire glass lens without bending or fracturing. This is one reason why you don't see very large aperture refractors.

    Each type of system has it's advantages. Ultimately, the type of viewing you do and the size of your budget will be the driving factor. If you are content with relatively slow optics, refractors are fine. If however, you wish to capture DSO's who faint light has traveled for millions of years, then it's probable that you will wind up with a mirrored telescope because a Newtonian can have the field of view and the light gathering ability of very large apertures.

    In the final analysis, magnification is the ultimate game and the more light you have going in, the the higher the magnfication you can go to before the image starts to deteriorate. As for compactness and ease of traveling I ride a big V twin and carry both a refractor or a wide field Newt in the saddle bag. There is no apparent difference in how each deals with the ride. However, having said that I can and have designed and built an open truss sciope that fits in an attache case and has more light gathering power than an refractor on the market. And at a lot less cost

    Hope this helps some

    Art
    Last edited by Art Bianconi; 03-16-2011 at 04:28 AM.

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