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  1. #1
    justbeats's Avatar
    justbeats Guest

    Default The emperors new Naglers



    Part soapbox, part question...

    Having blown so much "initial telescope budget" on five Naglers, it's
    heartening to see them top every comparison or review I've read since.
    The most common "con" is cost (occasionally weight), but the "pro"
    column invariably bulges with clichés indicating top-notch,
    second-to-none optical performance.

    Take it as given that I'm absolutely chuffed to bits with my Naglers
    and wouldn't swap them for anything, ever! But, while I readily
    believe they knock the stuffing out of competing EPs (relative
    performance) I don't believe they really have the absolute performance
    attributed to them in most reviews. For example;

    "Absolutely flat field across the whole FOV" - or
    "No sign of any pincushion distortion"
    Crap! The image in the 31mm and 17mm (with a 10" F/10 SCT) shows
    negative pin-cushion distortion (the edges are "sucked in" rather than
    "bulged out"). This isn't that noticeable when observing the sky, but
    is obvious on terrestrial targets. Not a problem, but not what I'd
    describe as "No sign of…", that's for sure!

    "Perfect focus right up to the edge"
    Rubbish! There is a small but obvious difference in focus between the
    middle and edge of the wider FOV EPs. When stars in the centre are in
    focus the ones at the edge develop a very tiny flare or two. The
    converse is also true, perfect focus at the edge gives very slight
    defocus in the centre. It's not that bad, very good in fact, but it
    certainly isn't "perfect".

    "No internal reflections or ghost images"
    More rubbish. Get a bright enough source (e.g. Mars) and you'll get
    all of these things if you look for them, especially when the seeing
    is poor, but perhaps not as pronounced as in cheaper EPs. It doesn't
    hinder observations, but there ARE internal reflections and ghost
    images in certain circumstances…

    "Absolutely no detectable optical aberrations"
    My experience with microscopes is that "no aberrations" means exactly
    that, no aberrations. Not some, not a little, not nearly none – it
    means absolutely none (and I own microscopes that perform that well)!
    So given the aberrations noted above, this particular "review cliché"
    is just downright misleading.

    So, are the reviewer's right (meaning there must be something wrong
    with my SCT to cause aberrations when using my Naglers) or am I right,
    the reviewers habitually overstate the (absolute) performance I should
    expect from them?

    Cheers
    Beats

  2. #2
    Chris1011's Avatar
    Chris1011 Guest

    Default The emperors new Naglers

    >So, are the reviewer's right (meaning there must be something wrong

    The reviewers habitually overstate the performance.Nothing is ever perfect, but
    some things are good enough.

    RC

  3. #3
    Jon Isaacs's Avatar
    Jon Isaacs Guest

    Default The emperors new Naglers

    >So, are the reviewer's right (meaning there must be something wrong

    My suggestion:

    Stopping looking at the eyepieces and look through the eyepiece at the stars
    above. There are plenty of decent eyepieces out there and whatever the
    reviewers say about them, well, who cares?

    What matters is what you see with the eyepiece.

    jon

  4. #4
    Brian Tung's Avatar
    Brian Tung Guest

    Default The emperors new Naglers

    Jon Isaacs wrote:

    Although I certainly agree with that sentiment, I think Steve's got a
    point. Reviews should be just that--reviews, not advertisements. If
    you are the owner as well as the reviewer, then it is incumbent on you
    to bend over backward in an attempt to remain objective. Otherwise,
    the review will carry less weight, for all your conviction.

    For instance, at least some of the Naglers show unmistakable pincushion
    distortion, yet some reviewers claim it's not there. The Nagler is not
    at all an orthoscopic eyepiece. I think it's likely that price is
    translating into performance in the minds of some reviewers, rather than
    the other way around. Now, do they not see it because they're the
    owners of these fine eyepieces, or because they're comparing it to other,
    less expensive wide-field eyepieces? Probably a little of both.

    Reviewers ought to keep in their mind, at least in the back of it, the
    many ways that their conclusions might be in error.

    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

  5. #5
    Richard DeLuca's Avatar
    Richard DeLuca Guest

    Default The emperors new Naglers

    In article <8bdaa68.0311040844.54fb9356@posting.google.com> ,
    steve_beats@hotmail.com (justbeats) wrote:

    SNIP_
    SNIP_

    Just a minor point, and somkeone will undoubtably correct me if I'm
    wrong. Pincushion, by its very name, means 'sucked in.' When the edges
    are bowed out, that's known as barrel distortion.

    Starry Skies,
    Rich

  6. #6
    Brian Tung's Avatar
    Brian Tung Guest

    Default The emperors new Naglers

    Richard DeLuca wrote:

    That's as I've always understood it. So pincushion distortion means
    that magnification scales faster than linearly with off-axis angle,
    and barrel distortion means it scales slower than linearly.

    Also incidentally, I understand that distortion doesn't necessarily
    imply that the star images are aberrated (although they might be).
    It just means that they are formed further from the center (pincushion),
    or closer to the center (barrel), than they "should" be.

    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

  7. #7
    Chris1011's Avatar
    Chris1011 Guest

    Default The emperors new Naglers

    > The Nagler is not

    They don't see it because they don't know what all these aberrations really
    are. People confuse distortion with astigmatism, atsignamtism with coma, etc.
    In fact, the original poster complained about stars not being in focus at the
    edge of the field, little realizing that the field curvature of his SCT is
    really to blame.

    The problem with almost all reviews is that people lack measuring equipment.
    They cannot measure what they are reviewing.

    Roland Christen

  8. #8
    Edward's Avatar
    Edward Guest

    Default The emperors new Naglers


    "Chris1011" <chris1011@aol.com> wrote in message

    really
    etc.
    the

    How about a quick primer? Let me know if this is true:

    Field curvature - you can focus to correct the apperance of stars at the
    edge of field, but only at the cost of the focus (sharpness) of on-axis
    stars. Caused by?

    Coma - An abberation resulting from the use of a parabolic mirror in a
    newtonian telescope, worsens as the focal length gets shorter, if its a
    problem, get a paracorr.

    Astigmatism - resulting from the eyepiece, worse with faster scopes, live
    with it or get a new eyepiece (or slower scope).

    Ok, let me have it!

    Ed



  9. #9
    Jon Isaacs's Avatar
    Jon Isaacs Guest

    Default The emperors new Naglers

    >

    To be precise, I think one should replace "Focal Length" with "Focal Ratio" and
    probably "Shorter" with "Faster." But if the mirror diameter is kept
    constant, it doesn't matter...

    jon

    Jon

  10. #10
    Chris1011's Avatar
    Chris1011 Guest

    Default The emperors new Naglers

    >Field curvature - you can focus to correct the apperance of stars at the

    Can be caused by the telescope or the eyepiece. Not easy to tell which one
    causes it. Short refractors and commercial SCTs have significant field
    curvature. Newts and Mak-Newts have very low field curvature even at fast focal
    ratios. Petzval refractors typically have no detectable field curvature.


    Coma can be positive or negative. Can be found in all types of telescopes,
    depending on design. Coma is an open-ended or seagull shape.


    Almost every telescope will show astigmatism off-axis, including coma-free
    scopes like Petzvals and RCs. Even Newts will have a mix of astigmatism and
    coma off-axis. Astigmatism is a closed shape resembling a football or oval.
    However, most eyepieces have even more astigmatism off-axis than a typical
    telescope, so their aberration will dominate. This includes flat-field and
    orthoscopic ocular designs. The amount of astigmatism exibited will generally
    be higher with faster F-ratio light beams.

    Distortion is a change of magnification across the field. It will misplace the
    stars in the outer part of the field, but not change their individual shape.
    They can still be quite sharp.

    Orthoscopic oculars are free of distortion and thus can be used with reticles
    in applications where you are measuring tiny distances (measuring microscopes).
    Orthoscopics are not necessarily free of astigmatism and can show mishaped star
    images at the field edges. An eyepiece that has no distortion and is free of
    astigmatism and coma at the edges of the field is orthoscopic, anastigmatic and
    aplanic.The same would be true of a telescope that had all these
    characteristics. Usually, one or two of these is sacrificed to achieve the
    other(s).

    Roland Christen

 

 
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