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  1. #1
    Rich's Avatar
    Rich Guest

    Default Still one major hole in telescope offerings



    We've got more scopes out there than ever before. But one thing is
    missing.
    The fast inexpensive large aperture apochromat. Only it wouldn't be
    an apochromat, just better than an achromat. A 6" f4 ED scope for
    $2000 for the OTA. I'd buy it. If people can stand using 6" f5
    achros for casual observing, then an f4 using ED glass (a doublet)
    would be quite a bit better.

  2. #2
    chris1011@aol.com's Avatar
    chris1011@aol.com Guest

    Default Still one major hole in telescope offerings

    On Sep 23, 7:50 pm, Rich <rander3...@gmail.com> wrote:

    Achros can be made at F5 quite easily and the internal curves are not
    too steep. Thus it is not too difficult for the lens elements to stay
    in collimation with simple spacing and mounting techniques inside
    simple cell designs. Reason for this is that BK7 can be combined with
    SF1 or other high index high dispersion crown and achieve exactly the
    same color correction as any normal crown/flint combo, with the
    advantage that the internal curves are shallow and the individual
    element powers are low.

    ED apos, on the other hand, require very steep internal curves, so
    they become very sensitive to misalignment. If you try to combine an
    ED glass with a high index high dispersion flint in order to get
    shallow internal curves, the advantage of color correction goes away,
    and you begin to get closer and closer to achromat performance, but
    you have spent huge amount of money on the glass. To make a highly
    corrected doublet, you need to combine the ED glass with a short flint
    or even a crown glass. Now the internal curves are so steep that you
    cannot break the F8 barrier without also getting huge amounts of
    sphero-chromatism, which makes the images soft compared to a well
    designed achromat. James G. Baker once said that it is no advantage to
    produce an apochromat with reduced color error if the result is a
    white blur versus a colored blur of the achromat.

    Really fast apos require either a 3 element up front design, or a
    Petzval design, which is really just a long focus doublet with
    internal telecompressor. As such it will be 50% longer than an
    equivalent achromat. None of these designs will ever be cheap. Even a
    fast ED lens will inevitably be expensive because the glass itself
    will cost between 10 times to 20 times as much as ordinary crown
    material, and even that is not cheap. Right now the Chinese do not
    make any cheap ED that is suitable for fast doublets, even with steep
    internal curves. F8 to F9 is about the limit for the Chinese glass.
    The biggest problem is the internal quality, which is not guaranteed,
    and which has proven to be very poor up to now. The most suitable
    glass is made by Ohara, and this is very expensive - you could
    probably make an F7 doublet with reasonably good correction with this
    material, F4 would be a total stretch. They will NOT sell it cheap no
    matter how much you buy. Cheap ED scopes that claim to use Ohara glass
    are using Chinese materials, and there is no way for the consumer to
    know otherwise.

    Uncarollo

  3. #3
    Rich's Avatar
    Rich Guest

    Default Still one major hole in telescope offerings

    On Sep 23, 10:08 pm, chris1...@aol.com wrote:

    Scratch that then. I'd had enough of that in the late 1980s and early
    1990s watching "fast" f6-7 apos get trounced on the planets by "old"
    Unitron f15 achromats that did in fact have more colour than the apos
    but easily showed more planetary detail.

  4. #4
    VicXnews's Avatar
    VicXnews Guest

    Default Still one major hole in telescope offerings

    Rich <rander3127@gmail.com> wrote in news:ddc611bc-48c0-4e2e-802a-
    1248ea8fab86@79g2000hsk.googlegroups.com:


    I'm looking for a fast (f4-f5) small(66mm-72mm?)cheap(less than $400) apo
    with a 2" focuser...is that possilbe
    my dream scope is a A-P stowaway...but that's not in my future (wife's health
    problems)

  5. #5
    VicXnews's Avatar
    VicXnews Guest

    Default Still one major hole in telescope offerings

    chris1011@aol.com wrote in news:e02ada73-6801-4144-a847-
    dc0807b7a65b@c58g2000hsc.googlegroups.com:


    Right on que... that answers my question...I guess I will just have to keep
    catching photons in my dreams =8-(
    thanks Mr C...

  6. #6
    Quadibloc's Avatar
    Quadibloc Guest

    Default Still one major hole in telescope offerings

    On Sep 23, 6:50 pm, Rich <rander3...@gmail.com> wrote:

    Actually, I think that we're missing something even more important.
    The lightweight, convenient, portable telescope with 10" or more
    aperture.

    John Savard

  7. #7
    Rich's Avatar
    Rich Guest

    Default Still one major hole in telescope offerings

    On Sep 24, 7:27 am, Quadibloc <jsav...@ecn.ab.ca> wrote:

    With lightweight mirror castings I might be possible, but even a
    tapered SCT mirror isn't that heavy. The trick with such a scope is
    to make it rigid enough to hold collimation when assembled. Carbon
    fibre poles?

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

    Default Still one major hole in telescope offerings

    On Wed, 24 Sep 2008 04:27:39 -0700 (PDT), Quadibloc <jsavard@ecn.ab.ca>
    wrote:


    Are you talking about something with tracking, suitable for imaging?
    Because for simple visual use, there would seem to be quite a few
    lightweight, easily transported Newts based around Dobsonian designs.
    _________________________________________________

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

  9. #9
    Thomas Womack's Avatar
    Thomas Womack Guest

    Default Still one major hole in telescope offerings

    On a similar note, why is it impossible to get an f/1.4 camera lens
    longer than 85mm?

    The sequence 600/4, 400/2.8 and 300/2 all exist (though the 300/2 is a
    crazily exotic piece of Nikon glass produced briefly in the sixties);
    the sequence 600/5.6, 400/4, 300/2.8, 200/2 is in current production
    from both of the big camera companies, and those sequences lead you
    to expect 200/1.4 and 135/1.4 to be at least conceivable.

    I suppose there are few sports that take place on large enough dark
    enough courts for them to be useful in that context, and astronomical
    budgets pale in comparison to Sports Illustrated.

    Tom

  10. #10
    Tom Polakis's Avatar
    Tom Polakis Guest

    Default Still one major hole in telescope offerings

    On Sep 24, 4:27 am, Quadibloc <jsav...@ecn.ab.ca> wrote:



    John,

    Are you looking for a really fast 10" Dob then? It just occurred to
    me that very short focal ratios are becoming increasingly popular for
    reasons of portability and not climbing ladders, but a 10" f/3.8 is
    not available commercially. Maybe there is no market for it. My
    wife's 10" Meade Starfinder is f/4.5, and already the eyepiece is down
    in the dirt.

    Tom


 

 
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