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  1. #11
    Brian Tung's Avatar
    Brian Tung Guest

    Default Proposal for an APO "standard:" TMBs 100mm f8



    John Savard wrote:

    That depends, as you well know, on the way in which that color correction
    is expressed. If you express it as fractional variation in focal length
    over a range of wavelengths, then it does not depend on the focal ratio.
    An achromat of whatever size, whatever focal ratio, will exhibit variation
    in focal length of about 1 part in 2,000 over the usual spectrum (C to f?).

    The reason why it looks less conspicuous at slow focal ratios is that the
    Airy disc has a larger linear size. When it becomes large enough, the
    different spot sizes for different colors all get obscured by the enlarged
    Airy disc.

    Brian Tung <brian@isi.edu>
    The Astronomy Corner at http://astro.isi.edu/
    Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
    The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
    My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

  2. #12
    William Hamblen's Avatar
    William Hamblen Guest

    Default Proposal for an APO "standard:" TMBs 100mm f8

    On Sat, 27 Nov 2004 19:57:16 GMT, jsavard@excxn.aNOSPAMb.cdn.invalid
    (John Savard) wrote:


    If you look up apochromat in the dictionary it says one is a lens
    corrected for chromatic aberration at three wavelengths and for
    spherical aberration at two. An achromat is corrected for chromatic
    aberration at two wavelengths and for spherical aberration at one. It
    is certainly possible to have worse color correction in an apochromat
    than in an achromat by those definitions.

    Say a visual achromat has a difference in focus of 1 part in 2000
    between the C and F lines and the D line and an apochromat has a
    difference in focus of 1 part in 8000 or a quarter the chromatic
    aberration. A semi-apo would have 1 part in 4000 or half that of an
    achromat, twice that of an apochromat.


  3. #13
    RichA's Avatar
    RichA Guest

    Default Proposal for an APO "standard:" TMBs 100mm f8

    On Sat, 27 Nov 2004 19:57:16 GMT, jsavard@excxn.aNOSPAMb.cdn.invalid
    (John Savard) wrote:


    That isn't true. A Newtonian reflector can be an apochromatic, for
    obvious reasons. A plain crown-flint doublet could be, if it's long
    enough, as you said.
    It doesn't matter what the scope is, if it achieves apochromatism,
    it's an apochromat. Here is Tom Bach's definition.

    "With the proliferation of apochromatic refractors that are available
    to the amateur astronomer, it is time to define the parameters of a
    true apochromatic objective lens. The modern definition of
    "apochromat" is the following: An objective in which the wave
    aberrations do not exceed 1/4 wave optical path difference (OPD) in
    the spectral range from C (6563A - red) to F (4861A - blue), while the
    g wavelength (4358A - violet) is 1/2 wave OPD or better, has three
    widely spaced zero color crossings and is corrected for coma."




  4. #14
    CLT's Avatar
    CLT Guest

    Default Proposal for an APO "standard:" TMBs 100mm f8

    "John Savard" <jsavard@excxn.aNOSPAMb.cdn.invalid> wrote in message
    news:41a8db30.613545@news.ecn.ab.ca...

    I'm not sure what you meant here --- refractor? Houghton?

    Clear Skies

    Chuck Taylor
    Do you observe the moon?
    Try http://groups.yahoo.com/group/lunar-observing/

    Are you interested in optics?
    Try http://groups.yahoo.com/group/ATM_Optics_Software/

    ************************************




  5. #15
    Brian Tung's Avatar
    Brian Tung Guest

    Default Proposal for an APO "standard:" TMBs 100mm f8

    Rich Anderson wrote:

    A long-focal-ratio plain crown-flint doublet does not, in general,
    satisfy the above definition.

    Brian Tung <brian@isi.edu>
    The Astronomy Corner at http://astro.isi.edu/
    Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
    The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
    My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

  6. #16
    Yuri's Avatar
    Yuri Guest

    Default Proposal for an APO "standard:" TMBs 100mm f8

    brian@isi.edu (Brian Tung) wrote in message news:<cob2kb$s1d$1@zot.isi.edu>...


    - Thanks Brian,
    That the is coolest thing I've read here! (-:
    Color spots obstracted by Airy disc! - But what is an Airy disc after all?

    Regards, Yuri

    "If you see spots in your scope - you are not in focus"

  7. #17
    Mike Jones's Avatar
    Mike Jones Guest

    Default Proposal for an APO "standard:" TMBs 100mm f8

    Chris1011 wrote:

    Thanks for the critique, and yes, I did notice R2 and R3 and the first
    airspace have very tight tolerances, which of course is not good for
    fabrication and with varying temperature. Good reason to keep airspaces
    thin and fill with optical fluids. Do you find the dn/dT of thin fill
    fluid layers is less troublesome over temperature than the effects of
    open airspaces you mention above? Are you simulating these thermal
    gradients by calculating discrete thermal gradients along the optical
    axis from front to back and radially from the edge in? ZEMAX has
    provisions to do this (separate temps for separate elements and
    airspaces) if you already know the gradients, but how are you
    calculating or estimating them in the first place? I would think FEM
    thermal propagation (SINDA/FLUINT, etc.) and NASTRAN runs on the lenses
    and barrels would be needed to get a realistic thermal gradient model,
    and this would be plowed into the overall optimization to come up with a
    thermally robust design. Not in any way a trival effort. What are the
    soak times and aberration performance changes on your lenses?

    Understand I'm just exploring and sharing apo designs as an after-hours
    hobby. I have no interest in taking on lens manufacturing for a living
    as you, Tom Back, etc. have done and done well. It's just spare time
    fun to try to come close to what you guys can do.
    Thanks,
    Mike

  8. #18
    John Savard's Avatar
    John Savard Guest

    Default Proposal for an APO "standard:" TMBs 100mm f8

    On Sat, 27 Nov 2004 23:29:15 +0000 (UTC), brian@isi.edu (Brian Tung)
    wrote, in part:


    I suppose then my point is that the threshold between 'achromat' and
    'apochromat', if it isn't going to be made on the basis of having three
    elements, should be based on color correction expressed in units
    independent of focal ratio, so that a telescope which only performs
    better because it has a longer focal length will not be called an
    apochromat.

    John Savard
    http://home.ecn.ab.ca/~jsavard/index.html

  9. #19
    John Savard's Avatar
    John Savard Guest

    Default Proposal for an APO "standard:" TMBs 100mm f8

    On Sat, 27 Nov 2004 18:41:01 -0800, "CLT" <not@thisaddress> wrote, in
    part:


    I did mean an achromatic refractor.

    At f/32, one scarcely needs to use an achromatic lens.

    John Savard
    http://home.ecn.ab.ca/~jsavard/index.html

  10. #20
    John Savard's Avatar
    John Savard Guest

    Default Proposal for an APO "standard:" TMBs 100mm f8

    On Sat, 27 Nov 2004 21:04:56 -0500, RichA <none@none.com> wrote, in
    part:



    Since, historically, a telescope with a two-element crown and flint
    doublet is an achromat, I would much rather have the term apochromat be
    consistent with that - and indicate a level of quality much higher than
    that provided by a crown and flint doublet *at the same focal ratio*.

    This would mean, of course, that with a high enough focal ratio, a
    telescope that is an apochromat would be an unnecessary luxury.

    John Savard
    http://home.ecn.ab.ca/~jsavard/index.html

 

 
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