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Thread: Are GSO Eyepieces FOV Bogus?

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    Default Are GSO Eyepieces FOV Bogus?



    Came across this review of the GSO 42mm 65 degree eyepiece. What do you think?
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    Default Re: Are GSO Eyepieces FOV Bogus?

    Quote Originally Posted by refractordude View Post
    Came across this review of the GSO 42mm 65 degree eyepiece. What do you think?
    I have never really checked this before, and as you would expect it is cloudy so I can't do a star-drift check: but my GSO 42 Superview "Wide field" eyepiece has a (newly measured!) retaining ring/"field stop" diameter of around 41.5mm (not 38mm as in the quoted review). This corresponds to an AFOV of around 57°... (This is the same as someone measured on the other forum. )

    It doesn't have a true field stop though: the retaining ring is inside the focal point of the eyepiece, and so there is no clear field stop when you look through the eyepiece. At the true focal point the inner diameter of the tube is ~48mm. If this was the true field stop, it would give an AFOV of 65° (or 68°!) as advertised.

    So, without doing a star drift test to confirm I would say the 65/68° claimed AFOV is bogus. (Although nowhere on the eyepiece itself is the AFOV mentioned!) It is not as bad as the 53° claimed in your quoted review, but it is still in the order of 57°, not 65°...

    It is a nice eyepiece for the money though (I think I paid around $A50 for mine...): it is good in f8 or slower scopes, but just don't expect it to shine in fast scopes!

    All the best,

    Dean
    Last edited by DeanD; 06-07-2018 at 06:44 AM.

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    Default Re: Are GSO Eyepieces FOV Bogus?

    I think a lot of claims by distributors/manufacturers have a little bit of artistic licence lol.
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    TS Optics: 150mm, f/6 Newtonian (GSO SVD series). Altair: Starwave 102ED-R. Sky-Watcher: SkyMax 102mm, SkyMax 127mm, Evostar 72ED DS Pro, Evostar 80ED DS Pro, ST80 (TS Focuser), ST102 (TS Focuser), EQ5 Deluxe, AZ5 Deluxe. Vixen: Porta II, SXG Hal-130. Orion: 90mm StarMax , ST80 (TS Focuser). Celestron: NexStar Evolution 9.25". William Optics: E-BINO-P Binoviewer. A shed load of eyepieces including: Antares, Astro Hutech, Baader, Bresser, BST, Bushnell, Celestron, Explore Scientific, GSO, Kokusai, Meade, Orion, Sky-Watcher, Solomark, SvBony, TeleVue, Vixen & one or two I'm not sure of.

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    Default Re: Are GSO Eyepieces FOV Bogus?

    Quote Originally Posted by DeanD View Post
    I have never really checked this before, and as you would expect it is cloudy so I can't do a star-drift check: but my GSO 42 Superview "Wide field" eyepiece has a (newly measured!) retaining ring/"field stop" diameter of around 41.5mm (not 38mm as in the quoted review). This corresponds to an AFOV of around 57°... (This is the same as someone measured on the other forum. )

    It doesn't have a true field stop though: the retaining ring is inside the focal point of the eyepiece, and so there is no clear field stop when you look through the eyepiece. At the true focal point the inner diameter of the tube is ~48mm. If this was the true field stop, it would give an AFOV of 65° (or 68°!) as advertised.

    So, without doing a star drift test to confirm I would say the 65/68° claimed AFOV is bogus. (Although nowhere on the eyepiece itself is the AFOV mentioned!) It is not as bad as the 53° claimed in your quoted review, but it is still in the order of 57°, not 65°...

    It is a nice eyepiece for the money though (I think I paid around $A50 for mine...): it is good in f8 or slower scopes, but just don't expect it to shine in fast scopes!

    All the best,

    Dean
    My estimate come to ~56 deg, so yes GSO like few other vendors stretching the truth quite a bit. In my experience only Televue reports accurate FOV. ES, Celestron, Meade, GSO add some degrees here and there.
    SpyderwerX and Gabby76 like this.

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    Default Re: Are GSO Eyepieces FOV Bogus?

    A star drift test will not confirm apparent field of view. The key is the word, "apparent". Some eyepieces, particularly widefield eyepieces, have pincushion distortion that "stretches" the AFOV out to the stated AFOV.

    Classic examples include the Panoptics and the T5 and T6 Naglers. For instance, the 41mm Panoptic has a stated 68°-AFOV, but also a 46mm field stop. The TRUE field of view in any eyepiece is determined by:

    TFOV = (field stop, mm) * (180/pi) / telescope focal length.

    Let's look at an 8" f/5.9 (1,200mm focal length) dob with a 41mm Panoptic. Using AFOV as a guide, many would say the TFOV = 41mm * 68° / 1,200mm, or 2.32°.

    But in actuality, the TFOV = (46mm field stop) * (57.3) / 1,200mm, or 2.20°.

    If one did a drift test and came up with 2.20°, they might conclude the AFOV is only 64.3°, but the *apparent* field of view is indeed 68°. This can be confirmed with a flashlight, a nearby wall and some trigonometry. A 41mm eyepiece with a 46mm field stop would, without distortion, only have a 64°-AFOV. But the distortion "stretches" out the edges of the field of view.

    Clear Skies,
    Phil

 

 

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