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Thread: Perhaps Obvious...

  1. #1
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    Default Perhaps Obvious...



    This may be a colossally stupid question, but I'm asking anyway.

    If I use a bahtinov mask to achieve perfect focus on a star, will I be in focus on a planet/the moon as well?
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    Default Re: Perhaps Obvious...

    Quote Originally Posted by Fyrblade View Post
    This may be a colossally stupid question, but I'm asking anyway.

    If I use a bahtinov mask to achieve perfect focus on a star, will I be in focus on a planet/the moon as well?
    If you have a flat field, yes. Your F/3.9 astrograph may need a field flattener otherwise you might be focused on a star at the center while the focus is off towards the edges where the outline of the moon is.
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    Default Re: Perhaps Obvious...

    There is a slight bit of difference to me between the moon and the sun/stars. Anytime you change the object your going to image and slew to a different area of the sky you should always re-check your focus.
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    Default Re: Perhaps Obvious...

    The moon is a special target as it presents such a large and varied disk.
    You should shoot for focus somewhere in the middle of the radius...
    If you focus on the center, the edges will be fuzzy. Focus on the edges, and the center gets fuzzy.
    It's a sphere and it behaves like one. The distance from us to the center isn't the same as it is to the edge.
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  5. #5
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    Default Re: Perhaps Obvious...

    The distance to a star, a planet, or the Moon is so great that they are all effectively at infinity for the purposes of focusing.

    As Henk noted, your optics may not focus quite the same across the entire FOV, and as Leveye intimated aiming at different parts of the sky can result in focus differences.

    Also, remember that as the temperature of your optical system changes you will have your focus change as well.

    The net effect is that you should plan to check your focus when you change targets and you'll probably want to check it every 1/2 hour or so even when you are staying on the same target.

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    Default Re: Perhaps Obvious...

    Quote Originally Posted by jimminCT View Post
    The moon is a special target as it presents such a large and varied disk.
    You should shoot for focus somewhere in the middle of the radius...
    If you focus on the center, the edges will be fuzzy. Focus on the edges, and the center gets fuzzy.
    It's a sphere and it behaves like one. The distance from us to the center isn't the same as it is to the edge.
    Not really. If the distance to the moon is d=4e8 m and its radius is r=1700e3 m, then for a scope with focal length f=1 m the focal point at the edge is 1/(1/f-1/d) and at the center 1/(1/f-1/(d-r)). The difference is about 1e-11, 5 orders of magnitudes smaller than a micron. The curvature of the moon cannot be detected by the telescopes we use.
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    Default Re: Perhaps Obvious...

    Quote Originally Posted by CamelHat View Post
    Not really. If the distance to the moon is d=4e8 m and its radius is r=1700e3 m, then for a scope with focal length f=1 m the focal point at the edge is 1/(1/f-1/d) and at the center 1/(1/f-1/(d-r)). The difference is about 1e-11, 5 orders of magnitudes smaller than a micron. The curvature of the moon cannot be detected by the telescopes we use.
    I love it when Henk breaks out the math. Nicely done, Henk! (Sorry ET)

    Now to the OP, in theory the answer to your question is yes, BUT everyone has made very valid points on why a focus re-check is necessary. For me, I'm a "fiddler", I can't leave well enough alone. So I tend to "fiddle" with the focus between every planetary/lunar video I capture. Since my scope has a FeatherTouch focuser, I can make "micro" adjustments while looking at an onscreen image of the imaging object trying to "nail" the focus. Some would argue that my fiddling is unnecessary, I content that I do improve the focus and at worse make it no worse.

    Anyway, experiment and find out what works best for you.

    Cheers,
    JT
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    Default Re: Perhaps Obvious...

    All I know from personal experience is:
    I've never got a good focus on the moon by using a Bhat mask on a star. Not even close.
    I always seem to have to change focus from the edge of the limb, to the center.
    It's nearly impossible to get both in sharp focus simultaneously...

    So, math or atmosphere, I still stand by my statement...
    jaetea likes this.
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