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  1. #1
    Space Traveler's Avatar
    Space Traveler Guest

    Default Building a Refractor?



    Hello All:

    I've built a few Dobs scopes over the past several years. Now
    William-Bell has a new book coming out titled, "Building a Refractor
    Telescope". I'd love to try this! Does anyone have any experience
    with a project of this type? If so please post. What does one do
    about coating the lens?

    Thank you,

    Kurtis


  2. #2
    Alan French's Avatar
    Alan French Guest

    Default Building a Refractor?

    "Space Traveler" <kallen37@gmail.com> wrote in message
    news:1167141294.275142.165540@79g2000cws.googlegro ups.com...

    Kurtis,

    I am looking forward to the book too, and hope it addresses getting
    coatings. In reality, coatings are not going to make a huge difference for
    most designs, especially if you are doing lunar and planetary work, but it
    would be nice to have the option.

    I would think standard MgF coatings would be available and not be too
    expensive. The multicoatings are, I understand, considerably more
    expensive.

    Sending a lens out for coatings is always a bit risky. Most coating
    application involves heating the lens, and I would think you'd want to take
    care to carefully finish all edges and bevels to be free of chips to reduce
    the chance of breakage.

    I think the Schupmann book will be a bit more interesting. Now there is a
    real "ATM" telescope.

    Clear skies, Alan


  3. #3
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default Building a Refractor?

    On Tue, 26 Dec 2006 10:17:13 -0500, "Alan French"
    <adfrenchremoveallthis@nycap.rr.com> wrote:


    I don't follow. This is precisely when you _will_ need very good
    coatings. It doesn't matter much for DSOs, but when you look at bright
    objects with uncoated optics you see myriad ghost reflections. Uncoated
    achromats are largely useless for lunar and planetary viewing. They will
    work okay with dimmer objects, although ghost images from stars may
    prove irritating.



    These days, coating processes don't necessarily involve significant
    heating of the optic. Sputtering techniques are cold- I've had plastic
    components multicoated this way. The last time was nearly 10 years ago,
    and the cost was about $500 for a run, which worked out to about $5 per
    element. The problem with sending a single item (or just a few) is that
    unless there are other components in the run, you pay a lot by absorbing
    all the setup costs.

    _________________________________________________

    Chris L Peterson
    Cloudbait Observatory
    http://www.cloudbait.com

  4. #4
    Alan French's Avatar
    Alan French Guest

    Default Building a Refractor?

    "Chris L Peterson" <clp@alumni.caltech.edu> wrote in message
    news:18g2p2ta8he26q73q4nil1ls36rvtrpmma@4ax.com...
    for
    it

    It depends on the design. If ghost reflections are a problem, then it
    should be coated. I had an old Lohmann Brothers 5" f/15 achromat from
    around 1910 that was uncoated, and there was no problem with ghost
    reflections.

    take
    reduce

    It's good to know there are alternatives.

    Clear skies, Alan


  5. #5
    Joe Bergeron's Avatar
    Joe Bergeron Guest

    Default Building a Refractor?

    In article <18g2p2ta8he26q73q4nil1ls36rvtrpmma@4ax.com>, Chris L
    Peterson <clp@alumni.caltech.edu> wrote:


    So would you say all those old Clark refractors are largely useless on
    the moon and planets?

    --
    Joe Bergeron

    http://www.joebergeron.com

  6. #6
    Rich's Avatar
    Rich Guest

    Default Building a Refractor?


    Alan French wrote:

    Do ghosts show up with elements that are not equi-convex? Wasn't there
    a way to apply a rudamentary single-layer coating on a lens using
    chemicals at home? Didn't "straw coating" originate this way?


  7. #7
    decaf's Avatar
    decaf Guest

    Default Building a Refractor?



    On Dec 26, 7:53 am, Chris L Peterson <c...@alumni.caltech.edu> wrote:

    Chris,
    How many types of uncoated achromats have you looked through?
    My homemade uncoated 6.6" f/19 air spaces fraunhofer exhibits NONE of
    the
    problems you cite and is as far from useless as you can imagine for L/P
    work-
    with stark blackish backgrounds to boot. The Baker achromatic design is
    also
    free from detectable ghost reflections in an uncoated state. Even the
    unoiled Littrow
    and equal R1, R2-R3 radii, which does indeed show a ghost image, is
    hardly useless for L/P work.


    Dan Chaffee


  8. #8
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default Building a Refractor?

    On Wed, 27 Dec 2006 03:08:00 -0500, Joe Bergeron
    <jabergeron@earthlink.nettled> wrote:


    Ok, not useless. But they aren't very good by modern standards (at
    least, not the one I've seen).

    _________________________________________________

    Chris L Peterson
    Cloudbait Observatory
    http://www.cloudbait.com

  9. #9
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default Building a Refractor?

    On 27 Dec 2006 02:13:51 -0800, "decaf" <dchaffee@blitz-it.net> wrote:


    Hi Dan-

    I regret using the term "useless"- that was far too strong. But in
    general, most achromat designs will show ghosting effects. I guess many
    of these older designs were specifically optimized to minimize ghosting
    simply because AR coatings hadn't been developed. However, given the
    wide availability of inexpensive and effective coatings today, the
    optical designer has much more flexibility in his design. Are you aware
    of any fast achromat designs that would be suitable for uncoated use?

    _________________________________________________

    Chris L Peterson
    Cloudbait Observatory
    http://www.cloudbait.com

  10. #10
    Larry G.'s Avatar
    Larry G. Guest

    Default Building a Refractor?

    On Wed, 27 Dec 2006 08:36:32 -0600, Chris L Peterson
    <clp@alumni.caltech.edu> wrote:


    Chris,

    I was under the impression that an uncoated objective was too
    far removed from the image plane to seriously contribute to
    ghosting. If anything, only uncoated eyepieces, and those
    of specific designs (even when coated) were prone to this
    problem.

    It seems that the curves of the objective would have to be just
    right to form a reflected image anywhere near the direct image.
    Is this not the case?

    Thanks,
    Larry G.


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