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Thread: Correction within eyepieces

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    Default Correction within eyepieces



    Hi all
    I thought I had seen a comment about eyepieces with poor correction within the eye piece itself.
    I have a 16" F4.4 and use various eyepieces as one does for visual observing.
    Is there such a thing as poor correction within a eyepiece itself?
    If so what is it ?
    Rob
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  3. #2
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    Default Re: Correction within eyepieces

    In a defective/ inexpensive one I'd say yes? Those I would not use.
    My eyes
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    Default Re: Correction within eyepieces

    Eyepieces are designed to work at different focal ratios.
    Orthos and plossls while capable of good views down to f/4 work better at higher/ slower f ratios.
    Widefields are normally built to have the various artifacts corrected to the edge below f/6.
    rseasy1, helicon64, Leveye and 1 others like this.
    Refractors: Antares 105 f/15, Celestron 150 f/8, Stellarvue NHNG 80 f/6.9, TAL 100RS f/10, TS 102 f/11, UR 70 f/10, Vixen SD115s f/7.7
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    Default Re: Correction within eyepieces

    With Newtonian reflectors faster than f/5, the major optical defect you will see is astigmatism caused by the eyepiece and not coma. Even expensive eyepieces that are not designed to handle the steeply converging light cone found in "fast" optics, will produce astigmatism in the image. However, eyepieces like the TeleVue Nagler ... released in response to the large, fast optics being produced for the then "new" Dobsonian scopes ... are designed to work with fast optics and do a great job in taming astigmatism. The addition of a coma corrector to scopes with really fast optics will further correct the final image.

    A good example is my Celestron Comet Catcher which is 5.5" f/3.64 Schmidt-Newtonian telescope. The front corrector plate reduces coma down to that found in a f/7 reflector. When used for photography, this scope produces a wonderfully flat and sharp image corner to corner, with hardly a trace of coma and no eyepiece astigmatism (obviously). With a good quality eyepiece ... like a TeleVue Plossl .... the stars quickly begin to "bloat-up" from astigmatism. However, when using a 16mm Nagler ... which is designed to minimize astigmatism ... produces sharp stars across the entire field of view.

    So, to get the very best images from a "fast" reflector, you'll need not only a good eyepiece but also one that minimizes astigmatism in its design.
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    Default Re: Correction within eyepieces

    Human eye tends to concentrate on the center of the view and more forgiven to some peripheral imperfections. Also, most of modern EP design try to minimize these imperfections. In my personal experience I hardly see astigmatism even in budget EPs, so unless it is very obvious and distracts you from viewing I would not worry to much about it
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    Default Re: Correction within eyepieces

    Thanks guys
    I use my 18 mm E.S eyepiece the most and probably need to purchase a corrector for it but would be a nice addition
    to my collection of Astro gear it's something I think I need and can be used with my other eyepieces.
    The Televue line as dilapo mentioned sounds like would be the way to go but are a bit out of my price range.
    Time to shop!
    Thanks guys Rob
    10538 likes this.
    Too many hobbies ,Some for day time some for night.
    Orion 16" F4.4 go to Home built 10" F/5
    Mead supper pollsel eye pieces ES 8.8mm 18mm ES.
    Clarjo observatory 12x16 roll off.

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    Default Re: Correction within eyepieces

    There are a lot of nice eyepieces out there. Televue is only one brand, look around and you will probably find some that match your scope and eye just as well.
    Refractors: Antares 105 f/15, Celestron 150 f/8, Stellarvue NHNG 80 f/6.9, TAL 100RS f/10, TS 102 f/11, UR 70 f/10, Vixen SD115s f/7.7
    Mounts: Celestron SLT w/ pier mod & EQ-3 tripod, Celestron hypertuned CG-5 w/ tracking motor & Argo Navis, Manfrotto 028B w/ Stellarvue M2C, Manfrotto 055PRO w/ 128RC, TAL-1 HD EQ, Vixen SXP w/ HAL-130 SXG & half pier
    Diagonals: 2" Astro-Physics MaxBright, 2" Zeiss/ Baader prism, 2" Baader Herschel Wedge (Photo Version), 2"
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    Refractors kick arse precisely because they don't hide behind excuses. That is, they have no obstructions to hide behind. - Jon Isaacs (CN)

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    Default Re: Correction within eyepieces

    Quote Originally Posted by rseasy1 View Post
    Thanks guys
    I use my 18 mm E.S eyepiece the most and probably need to purchase a corrector for it but would be a nice addition
    to my collection of Astro gear it's something I think I need and can be used with my other eyepieces.
    The Televue line as dilapo mentioned sounds like would be the way to go but are a bit out of my price range.
    Time to shop!
    Thanks guys Rob
    Just to clarify, you can get coma corrector which help to address optical artifacts introduced by reflector's main mirror. I don't think you can get a corrector for individual EPs.

    Is it ES 82 degree 18 mm?

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    Default Re: Correction within eyepieces

    If you call TeleVue, speak to John or Al (Nagler). They'll lead you in the right direction.

    Clear Skies
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    Default Re: Correction within eyepieces

    EYE PIECE ABBERATIONS

    I would like to mention that "coma" is NOT an eyepiece aberration. Many people mention it when reviewing an eyepiece purchase. I find it quite laughable when they mention that they saw "coma" when viewing through a refractor. Astigmatism is the more likely culprit. Coma is a product of a parabolic mirror and the ratio of diameter to focal length. With your scope being an F4.4, coma is going to be predominate but is not an EP aberration. A poorly corrected eyepiece, especially wide fields, will tend to amplify the main mirrors coma, then add any astigmatism, field curvature, and such to that, that the EP may have.

    By "Corrected" they are designing an EP that is free of the list of abberations in the above link. Some EP's handle coma somewhat better than others but with that comes expense.

    Coma will always be present to some degree with parabolic Newtonians, unless it is specifically addressed with a special optic such as the Parracor II or Baader MPCC. The setback of these optics are that they will introduce more glass elements, and will usually require the focal length to change slightly (usually a factor of 1.2) in order to "Doctor" the light cone to reduce coma.

    JJ
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