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    dmmd01's Avatar
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    Default Tweaking focus distance on Dob rebuild

    The saga continues! I've been rebuilding my 16" Meade starfinder and all seems to be going fairly well. I took it out today to test the focus and ran into a snafoo. I'm able to focus on objects (trees, flags, buildings) that are a few hundred feet away, but at around 1/2 of a mile, I am unable to rack the eyepiece in far enough to bring an object into focus. If I hold my 1.25in eyepieces inside the 2" tube, I can bring objects 1/2 of a mile away and farther into focus. If I am thinking correctly, this means that my truss poles are the wrong length. My question is, are my poles too short, or too long?

    As always, thank you!!!

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    If the problem lies in the length of the truss rods, having to rack the focuser in to its maximum travel implies the truss rods are too long (i.e. your true back focus distance lies at or inside the focuser's maximum inward travel).

    That implication is reinforced by the ability to obtain focus with a smaller diameter eyepiece, as you're able to move the focal plane closer to the true back focus distance.

    Have you ray-traced the instrument in its current configuration? This would confirm the truss rod issue.

    In an ideal world, your back focus distance and the imaging focal plane would like exactly half-way between the focuser's maximum inward and maximum outward travel distance. Alternatives to shortening the truss rods include moving the secondary and focuser exit hole slightly forward (toward the mirror) or using a focuser with greater maximum travel range (aperture and obstruction dependent).

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