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  1. #1
    jpcannavo@yahoo.com's Avatar
    jpcannavo@yahoo.com Guest

    Default Star test of Orion 100mm ED



    I have a question regarding the Orion 100ED. I recently purchased one
    and on artificial star testing, I find that outside focus revealed a
    symetrical pattern of concentric rings - ie no astigmatism, pinch,
    misallignment etc - but inside focus virtually no rings are visible on
    defocusing. Now I know this sounds stupid of me but so far I have been
    testing with the star diagnal in place. Interestingly a second 100ED -
    which the dealer allowed me to compare with my first - showed exactly
    the same pattern. The second scope, however, went up to 75 per inch
    with virtually no image breakdown on terestial targets. The original
    scope did not handle this much mag as well so I chose the second.
    My question is this: I have not yet star teseted w/o the diagonal. It
    seems strage that both scopes would come out with the exact same
    inside/outside focus disparity. Any thoughts... the star diagonal?
    Joe


  2. #2
    Brian Tung's Avatar
    Brian Tung Guest

    Default Star test of Orion 100mm ED

    Joe wrote:

    It's possible the star diagonal plays a role in this, but the first
    thing that comes to mind is that your artificial star is too close.
    That can introduce spherical aberration which might very well reveal
    itself the way that you've seen. Harold Suiter's star-testing book
    has a table in it that gives the minimum recommended distances for
    artificial stars to avoid this.

    If this is indeed the issue, then star testing on actual stars will
    not show it.

    But yes, go ahead and star-test without the diagonal, then do it with
    the diagonal. You should then be able to identify where the problem
    (if any) is.

    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

  3. #3
    Peter's Avatar
    Peter Guest

    Default Star test of Orion 100mm ED


  4. #4
    Jeff's Avatar
    Jeff Guest

    Default Star test of Orion 100mm ED

    Joe,

    This is exactly the type of star test pattern that my ED100 shows.
    Initially I wondered if there might be something wrong with the scope
    because of this star test pattern. I looked in Suiter's book for an
    explanation of this "problem" but did not find much help there.
    (Confession: I don't understand about half of what Suiter is talking
    about and usually just go back to observing rather than sorting it all
    out.)

    But there was some discussion on SAA a while back about this effect.
    Always be wary of such discussions but apparently this star test result
    is due to the particular optical design of the scope - something to do
    with the types of glass selected for the "APO" doublet. Not all APO
    designs show this pattern but apparently some do. I took this to mean
    that possibly my star test result was normal and not necessarily
    indicative of a defect. I am no expert. Perhaps a more knowledgeable
    person here on SAA can explain this again.

    The diagonal does not affect this result for my scope. I have an
    ordinary Orion brand diagonal.

    I will say that I am pleased with how well the scope works and am no
    longer the least concerned about the star test. The scope provides
    beautiful double star views - something I had hoped it would do. A
    scope like this makes me appreciate the beauty of clean star images -
    Airy disks and such. And it not only reaches Dawes limit but can show
    fairly difficult, uneven brightness doubles such as delta-Cygni. I've
    heard this is a good test of contrast performance. The amount of detail
    on Jupiter is more than I expected for a 100 mm scope. I have an 11"
    Zambuto reflector which, of course, shows an entirely other level of
    detail but this little refractor gives clean, crisp views of Jupiter
    with a beauty of their own. Again I am no expert on scope comparisons
    but my ED100 seems to work pretty well! I can't possibly see how there
    could be much wrong with it.

    Jeff


  5. #5
    RichA's Avatar
    RichA Guest

    Default Star test of Orion 100mm ED

    On 23 May 2005 10:22:00 -0700, "Jeff" <jeffrey.marsh4@verizon.net>
    wrote:


    There isn't. It's a MYTH that all so-called apos show identical
    diffraction patterns in an out of focus. Very few scopes do.
    Why? Because the scope has to be colour FREE, not just partly colour
    free to show patterns like that. An SCT won't, neither will most of
    the apos. But a Newtonian WILL as will the very best (most colour
    free) apos like the top APs. As it stands, it makes very little
    difference when viewing an object IN-focus. You'd be far better of
    paying attention to things that indicate spherical aberration or
    astigmatism, like the shape of the rings or their thickness as they
    radiate outward.
    -Rich

  6. #6
    William Hamblen's Avatar
    William Hamblen Guest

    Default Star test of Orion 100mm ED

    On 2005-05-23, jpcannavo@yahoo.com <jpcannavo@yahoo.com> wrote:

    You need to take out the diagonal to test the telescope. For one thing,
    the diagonal could be a little out of collimation. To test a refractor
    you need to use a deep green or yellow filter to reduce the effect of
    chromatic aberration. Don't go too far inside or outside of focus.
    You want the inside focus appearance to look like the outside focus
    appearance.

    You also should use a real star.

  7. #7
    Dan Chaffee's Avatar
    Dan Chaffee Guest

    Default Star test of Orion 100mm ED

    On 23 May 2005 06:34:25 -0700, jpcannavo@yahoo.com wrote:


    It is ultra-unlikely that the diagonal would cause anything but
    an astigmatic type of aberration. Since refractors show
    UNDERCORRECTION (as opposed to reflectors) when a
    point source is too close and this description indicates
    overcorrection, it's unlikely the point source is too close. As has
    been pointed out, some short focal ratio
    APOs or semi-APOs can show higher order aberrations that
    are misleadingly strong. I think the suggestion of using a
    yellow-green filter is really the next step. But really, if it shows
    sharp contrasty images images at 300x or so, it is a good scope
    and that's more important than the star test.

    Dan Chaffee.

  8. #8
    Brian Tung's Avatar
    Brian Tung Guest

    Default Star test of Orion 100mm ED

    Dan Chaffee wrote:

    Mmm yeah, assuming that "inside focus" is being used correctly. I've
    seen it used incorrectly (in person) about a quarter of the time. It's
    not anyone's fault--it's apparently just not an immediately obvious
    term for some people.

    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

  9. #9
    Joe's Avatar
    Joe Guest

    Default Star test of Orion 100mm ED

    Thanks for all the feedback. As per the issue of too close a target
    resulting in 1st order (?) spherical abberation, I selected targets at
    aprox 100 meters or more. Also, Dans point bears on this as well. As
    for usage, by "inside focus" I mean when the focuser moves the eyepiece
    from the point of focus towards the objective. "outside focus" I assume
    means moving the eyepiece from the point of focus in a direction away
    from the objective...correct usage? Perhaps the issue here is the
    extreme sensitivity of the star test revealing spherochromatism (?) in
    the absence of a filter. I am admittedly quite new to star testing. One
    thing that is clear, is that on terestial targets (at dusk when ground
    turbulence is less a problem ...and astronmical seeing is at its
    worst!) the scope takes my 3mm radian quite well, i.e. 75/inch. In case
    anyony is wondering if this is a comment on my deteriorated 45 yo eyes,
    I still have 20/20 vision for distance uncorrected. (It is an often
    overlooked fact that poor vision results in being able to take a scope
    up to higher power). In any event I look forward to some steady -
    however rare - east coast skies.
    I still wonder why I dont hear much about this scope on line ...
    thoughts?
    Clear and steady skies
    Joe


  10. #10
    Tim Killian's Avatar
    Tim Killian Guest

    Default Star test of Orion 100mm ED

    Try the test again with a green filter. APOs are color corrected, not
    color free. The wavelengths still focus at different positions and this
    can throw off star test results.

    jpcannavo@yahoo.com wrote:


 

 
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