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    pe51c's Avatar
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    Default What type of eyepiece is this



    It came with Celestron Astromaster 130. It is 20mm and also includes correcting prism.I tried to identify it but can not find match online. Looks like field lens is double (glued together?) and there is one eye lens. Lenses look coated and made of glass. View is clear but narrow. Would buying 20 mm brand name plossl eye piece be improvement?
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  2. #2
    Carlos_dfc's Avatar
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    Default

    Almost certainly a 'Kellner' design
    Kellners have 3 lenses, with 2 of them cemented together as a achromatic doublet, and give an apparant field of view of around 44° to 46°
    They do give pretty sharp views in most scopes - but performance towards the edge of the field deteriorates in scopes faster than about f/6

    Plossls have 4 lenses - two cemented pairs - and give an AFOV of between 50° and 52°
    A good plossl will perform better than a Kellner, and can also handle faster f/ratio scopes.

    edit.
    I just re-read your original post, and realised you say it's the field lens which is doubled.
    That changes things somewhat.....
    A doublet field lens would suggest a simplified Orthoscopic design, similar to a 'RKE' or a 'Konig'.
    The narrow FOV also supports this - 'Ortho's (and similar EPs) are intended for high-magnification planetary viewing, where sharpness and contrast are far more important than field of view. They work best in long focal ratio scopes, and as with Kellners, edge performance suffers in a fast scope

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    Last edited by Carlos_dfc; 04-24-2010 at 02:37 PM.
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  4. #3
    pe51c's Avatar
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    I learned a lot from these forums thanks to you and many other helpful guys.
    Astromaster 130 is f4.9 so I belive I can get decent wide views with 20 or 25mm plossl eyepice.

 

 

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