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Thread: Powermate 4x on large dobsonian.

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    Default Powermate 4x on large dobsonian.



    Hello, I recently upgraded my scope from 8” to 14”(fl=1650mm)(xx14g) and still somewhat experimenting eyepieces.
    So far the two times I observed the 82* of ES 30mm, 18mm, 11mm and meade 24mm it works great!
    I am looking into getting more higher magnification and came up with the idea of using a 2” powermate 4x on my eyepieces. This would allow me to use all my 2” and 1.25” eyepieces with different magnifications.
    I understand observing over 400x is rare and probably not so practical unless really clear conditions, but I read reviews that larger dobsonians can handle higher magnification?
    Please let me know if this would be a good idea or if there are other potential issues I might be facing.
    Thank you in advance!

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    Default Re: Powermate 4x on large dobsonian.

    Whether this will work well or not depends on two factors for the optics.

    1) How accurate are the curves on the mirrors?
    2) How smooth are the curves?

    Many larger dobs that are mass produced have surfaces that are not accurate and so suffer from spherical aberration. Some are intended to be used as rich field scopes only and are only corrected to 1/3 wave, not exactly diffraction limited.

    Others may have accurate figures but rough surfaces. They won't show spherical aberration but the image will be "dirty" due to scattered light.

    I used to have an xx14i and the 4x Powermate was not useful. Of course short focal length eyepieces weren't useful either. On my other Newtonian scopes not a problem.
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    Default Re: Powermate 4x on large dobsonian.

    If you want clear then I would rather use a shorter eyepiece instead of a barlow the less glass the better, 400x is quite optimistic with any scope, but I did manage it once with a 8 inch newt in perfect conditions was a skywatcher 8 inch on a EQ5 on Jupiter but only once was amazing but rather luck as never got more that 250x after that , I dont have that scope any more, I also observed at 198x with a 90 Mak but only 2 times , when the conditions are great , we had regional power cuts and it was winter during those times and the milky way was easily seen
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    Default Re: Powermate 4x on large dobsonian.

    If there is spherical aberration, is that more obvious on higher power? Because up to 150x I haven’t experienced much effect as far as I recall.
    If it is more prevalent on higher magnifications, I guess it wouldn’t matter if I use a high power eyepiece or powermate 4x, it wouldn’t be useful either way?

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    Default Re: Powermate 4x on large dobsonian.

    If the object is in the center then there is no SA but as it drifts you will notice out of focus , the higher the magnification the faster it will move , at 400x its like a racing car
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    Default Re: Powermate 4x on large dobsonian.

    Quote Originally Posted by 12richardk View Post
    If the object is in the center then there is no SA but as it drifts you will notice out of focus , the higher the magnification the faster it will move , at 400x its like a racing car
    Even with the tracking? xx14g is a goto tracking. However, I haven't used it for higher magnifications yet.

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    Default Re: Powermate 4x on large dobsonian.

    Quote Originally Posted by Reinverse View Post
    If there is spherical aberration, is that more obvious on higher power? Because up to 150x I haven’t experienced much effect as far as I recall.
    If it is more prevalent on higher magnifications, I guess it wouldn’t matter if I use a high power eyepiece or powermate 4x, it wouldn’t be useful either way?
    On your scope 150x isn't very high power so that's not a test. Right, it wouldn't matter either way.

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    Default Re: Powermate 4x on large dobsonian.

    Quote Originally Posted by 12richardk View Post
    If the object is in the center then there is no SA but as it drifts you will notice out of focus , the higher the magnification the faster it will move , at 400x its like a racing car
    This is incorrect. Spherical aberration is present at the center of the field of view. You're describing coma which is different from spherical aberration.

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    Default Re: Powermate 4x on large dobsonian.

    I am Not sure but it is an alt/Az mount so it will drift out , but should be ok for some time ,try it and let us know
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    Default Re: Powermate 4x on large dobsonian.

    Here is a page that shows what spherical aberration does to an image of Jupiter

    Comparative Images

    It would also fuzz out details of nebulosities as well.

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