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  1. #1
    Z28500's Avatar
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    Default Verbal/written/discussed scope design for a fast apochromatic 155mm scope



    Hello everybody,
    The lenses are the multicoated 155mm apochromatic ED triple main lens assembly combined with the Lanthanum approx 85mm corrector doublet assembly from Owl Astronomy, I know they must be getting pretty well known by now.

    Here is part of an email I sent to a machine shop about making the Collimatable lens cells & tubes for them:

    On the 155mm & corrector.
    It would be tricky, each one would be in it's own tube , the 155mm at the end where lenses usually are, and the smaller corrector inside, in the middle of the other tube which would slide back and forth inside the one that holds the 155.
    Probably would have to extend, or slide/adjust the telescoping tubes where desired, lock them in place with a few screws on the exterior of the tube, then collimate the focuser and the lenses, look through the focuser, and ...I doubt one's ever been made like that. Maybe could just have a collimatable cell that slides inside a tube (corrector lens) and then lock it in place instead of 2 tubes. But I think the focal length would have to be adjustable for it to work though, because it would still leave the problem of the focuser. So it would have to have 2 tubes I think with the focuser attached to the corrector tube.
    Or, I could just determine a setting and permanently set the corrector in the tube, (still in a collimatable cell) but then I wouldn't be able to adjust to different size flat fields, and f/ratio's.
    I don't know much about it, no experience, only reading about it.
    Yup, it is pretty tricky, I have to build a rack 1st with a dark half tube over it maybe, to see if it will work.
    All the scope will be good for I think, since it is so fast, is deep sky and maybe lunar, but time will tell, I'm waiting on the lenses.

    Here are some numbers on those lenses:

    (Part of an email)

    The main, triplet is an ED lens. The rear element is a Lanthanum element for ED performance (extra-low dispersion) element.

    The focal length of the system varies with the separation of the corrector and main cell. The corrector is inside the focus of the main cell. I have tested various spacings from about 135 mm to 175 mm. This gives focal lengths from about 645 mm to 235 mm. About 160 mm separation gives about the largest flat field and f/2.
    TOM (OWL1)

    I think that the longer focal length would be best, even though the flat field would be smaller.
    There seems to be many people who have built scopes out of these lenses. (Zooms also)

    However, even if the lenses weren't used "as-is", the main "apo" assembly could be re-ground into something "slower", could they not?
    But no way! I'm building the fast scope! And probably with the sliding tube assembly.
    Thanks everybody,
    Z

  2. #2
    Z28500's Avatar
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    Default

    One more thing I forgot to mention and my 15 minute time limit expired for editing, so I have to "REPLY" instead.


    I think the slowest f/ratio would be in the f/3.5+ range with the longest focal length. Therefore, as fast as that scope will be, wouldn't it be best to have as long as focal length as possible?

    Thanks again,
    Z

  3. #3
    Z28500's Avatar
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    Here is an idea of a rough draft except the screws would be flush and/o r counter sunk
    Z

  4. #4
    Z28500's Avatar
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    I don't know why 4 pictures went on, and then covered up some writing, but oh well, it was the first time I uploaded to this site!
    Z

 

 

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