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    Earthshine's Avatar
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    Unhappy What is the definition of "Flat Field"? [Paradigm Eyepiece]



    I recently bought two Astro-Tech Paradigm eyepieces, the 15mm and 25mm. They are advertised as having a very flat field, which I take to mean that they will focus clearly from edge to edge. Last night was my first chance to try them out on stars. I tried to check out the Pleiades.

    They certainly are a bit clearer (brighter?) than my cheap eyepieces that came with the scope, but flat field? They are far worse than the give-away eyepieces when it comes to achieving an even focus across the view. The center 40% (radius) is pretty good, but once I get out beyond that it is totally useless. The edge stars have huge flares shooting outwards from the center of the image. (I mean huge....like several degrees long) It looks like you're flying through space at warp 2! Whereas the cheap Celestron 25mm eyepiece that came with the scope is pretty crisp from edge to edge...just minor flaring in the outer 5 degrees of view.

    So my question is, first of all, am I doing something wrong? Are these eyepieces just not appropriate for my scope for some reason? I'm using a Celestron Powerseeker 127EQ which is a Bird/Jones Newtonian with a 1000mm focal length and 127mm (5") aperture.

    Or my second question: If this is what one gets from an eyepiece advertised as having a flat field, what is the definition of a flat field? If this is flat I'd hate to see curved. Any advice would be most appreciated. Thanks.
    Proud owner of: 25mm & 15mm Astro-Tech Paradigm EPs; Celestron Powerseeker 127EQ; Celestron CG-2 Equatorial Mount w/ RA Motor Drive; CHDK modified Canon SC2-IS; Meade 60-AZ-A TeleStar (used as spotting/guiding scope)

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  3. #2
    MadCrawdad's Avatar
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    I'm not an expert, but my guess is that the problem stems from the fact that the flat-field EPs are trying to flatten a field that's already been flattened, due to the design of your telescope... pushing the system past its original correction, and giving you problems on the other side.
    Mike

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    Earthshine (10-29-2011)

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    Quote Originally Posted by MadCrawdad View Post
    I'm not an expert, but my guess is that the problem stems from the fact that the flat-field EPs are trying to flatten a field that's already been flattened, due to the design of your telescope... pushing the system past its original correction, and giving you problems on the other side.
    Thats a good theory. I wonder if the Barlow/corrector lens that is installed in any Jones/Bird design doesn't already give good results with the cheap eyepieces that are supplied? Unfortunately the advertisment of these Pradigm ED eyepieces doesn't say "a sharp and flat field....unless your scope is already pretty good, then they will screw it up".

    AstroTech Website Advertisement: The six lens/four group design and two ED (Extra-low Dispersion) glass elements of the 15mm Astro-Tech Paradigm provides all of the optical virtues described at the top of this catalog page – high contrast, a sharp and flat field from edge to edge, excellent color fidelity, and exceptionally low astigmatism and barrel distortion.
    Proud owner of: 25mm & 15mm Astro-Tech Paradigm EPs; Celestron Powerseeker 127EQ; Celestron CG-2 Equatorial Mount w/ RA Motor Drive; CHDK modified Canon SC2-IS; Meade 60-AZ-A TeleStar (used as spotting/guiding scope)

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