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  1. #1
    nemesis256's Avatar
    nemesis256 is offline Bright Giants
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    Default Having problems with my Barlow

    So I have a Celestron 6SE, the 25 mm eyepiece that came with, a 15 mm Orion expanse eyepiece, and a 2X Orion Shorty Barlow. I find that (especially with the 15 mm) it's harder to focus using the Barlow because the focal plane is shallower. Is this caused by the Barlow alone, or is it simply because the focal length is increased? Am I running into the maximum magnification for my scope?

    Also, when I add the Barlow, the focus has to be readjusted quite a bit. Much more than when I switch from the 25 mm to the 15 mm. Would I run into this if I used a shorter eyepiece instead? In other words, would a 9 mm eyepiece have approximately the same focal point as the 15 mm and 25 mm?

  2. #2
    MitchAlsup's Avatar
    MitchAlsup is offline SUPER GIANT
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    The Barlow lens has to intercept the converging light cone far before it reaches focus inorder to spread out the image and deliver the arlow magnification effect. This is why they have long tubes with the lens at the bottom. It is not surprising at all that you have to refoce quite a bit to bring the Barlowed EP into focus.

    As to focus: There are two competing effects; a) the more slowy converging light cones, b) the ability of the eye to see detail. The physics of dealing with part a) indicates that focuse on a more slowly converging light cone has greater tollerances. F/10 is easier to focus on than F/5. However this is ONLY the case when the Barlow is NOT moving and the EP is being moved. When the Barlow is carrying the EP (99.993% of them) to criticality of focus is no different than when focusing with an EP of the same effective magnification.

    Which leads to part b) where the eye can see the image is OoFocus more easily at higher powers. And at some powers where diffraction is plainly visible, the eye may NEVER be completely happy with the focus. I used to get this a lot with my C11 observing planets; constantly tweeking the focus knob by the merest of amounts searching for a better focus.

  3. #3
    kencrowder's Avatar
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    Mitch answered most of your questions. But you asked about running into the limit of your scope. Take your aperture in mm. The diffraction limit begins when the magnification equals the aperture in mm. At twice that limit you are just magnifying the spread.



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