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  1. #1
    jmelz28's Avatar
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    Default Bahtinov -Focus on star, then keep same focus on planet?



    So I made a bahtinov mask last night, and was impressed by how sharp it made my views.

    However, I have a question.

    Can I focus on say, sirius, get the diffraction spikes, use something like bahtinov grabber and once I've achieved perfect focus, slew over to mars and expect it to be in focus?

    Sometimes the atmosphere obviously plays with what I see, making me think it's not in focus, though it is just bad seeing. If I knew I could trust the bahtinov focus method for any object (other than say, the moon), I'd feel better before attempting to grab 2000 frames of something when it feels like it is out of focus.

    Your thoughts?
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  2. #2
    Andy in Spain's Avatar
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    Default

    I think you can! What I do is find a bright star, focus using the mask and then forget about the focusing until I change my eyepiece! Try using the mask to get focus on Venus, then slew round to see if the defraction spikes are still centered when you look at Sirus! If they are (which I'm sure they will be) then there is your answer! If I was going to take a few long exposures, I would find a bright star near to my objective and focus on that! That way there would be less chance of knocking the focus out when cranking your scope round to the target!

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  4. #3
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    Default

    The focus should be good for all sky objects ,, the atmosphere on the other hand will cause some fuzz

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    Default

    If you focus on a star near the planet, the planet will be in focus as they both would share the same atmospheric refraction. As a rule of thumb, the closer the star, the more accurate the focus.
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    Default

    Any object outside the Earth's atmosphere is far enough away to count as "infinity" as far as the optics are concerned. Distance is not a factor. You want to focus on as small an object as possible for maximum precision. That is why you use a star rather than an extended object. Once the star is in focus, lock the focuser, and it will be good for the entire session.

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    Default

    Quote Originally Posted by KeithBC View Post
    Any object outside the Earth's atmosphere is far enough away to count as "infinity" as far as the optics are concerned. Distance is not a factor. You want to focus on as small an object as possible for maximum precision. That is why you use a star rather than an extended object. Once the star is in focus, lock the focuser, and it will be good for the entire session.
    A clarification, I meant angular distance, not linear distance.
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