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Thread: 1 1/4" Wide Field Eyepiece Options.

  1. #1
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    Default 1 1/4" Wide Field Eyepiece Options.



    I have an 8" SCT and am looking for a wide-field eyepiece without going to a 2" option. My 32mm has an actual FOV of about 0.7 degrees. How wide can a 1 1/4 eyepiece reach? Is there a way to calculate this? Any recommendations?
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    Default Re: 1 1/4" Wide Field Eyepiece Options.

    I myself did an excel worksheet to compare what eyepiece will give me the widest FOV vs price. The formula is simple
    1. Divide the focal lenght of the telescope in the eyepiece focal lenght and you have the eyepiece magnification.
    2. Every eyepiece has an apparent field of view given by the manufacturer so take that number and divide by the magnification and then you have the true field of view that eyepiece can give you.

    I could not attach the excel, but this is the example how it works...

    Untitled.png

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    Default Re: 1 1/4" Wide Field Eyepiece Options.

    IIRC, the 32mm Plossl is a most popular eyepiece providing the widest TFOV possible in the 8" SCT in 1.25".

    The most precise way to calculate TFOV would be by the eyepiece Field Stop (FS) diameter. Eyepiece Field Stop, mm/Telescope Focal Length, mm*57.3 = TFOV*

    So, if your scope FL=2032mm, than out of eyepieces available new the widest TFOV=0.89* will give you the 31mm or 36mm Baader Hyperion Aspheric (FS=31.7mm); TFOV=0.82* 35mm Parks Gold (Celestron Ultima) (FS=29mm); TFOV=0.77* 24mm ES68 (FS=27.2mm). Of course, the magnification and an eye pupil will be different than in the 32mm Plossl. Some of these eyepieces show vignetting, so the easiest way to max out TFOV without undesirable effects would be, probably, to install 2" back and use 2" eyepieces, if possible.

    The data on FSs if available can be find on manufacturer's, vendor's sites or, e.g in Don Pensack's Eyepiece Byuer's Guide posted here or elsewhere. In some cases you can measure it yourself with a caliper. If the FS data are not available or FS can't be measured directly (it's *hided* inside many wide angle eyepieces), than you can use approximation described by musiclucho above or use direct measurement through star drift timing.

    There are a variety of opinions on this subject but you may also find useful e.g. reading this thread.
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    Default Re: 1 1/4" Wide Field Eyepiece Options.

    this is a usefull tool as well:
    TeleVue EP calculator

    Funny.. when I bought my Evo9.25 I also splurged on the 24mm Panoptic which is the EP that gives you the widest possible true field for 1.25" EPs. Let me just say that EP is a joy. Friendly to use and wonderful views. Its my workhorse. Not just a wide go find it EP, this is the kind of EP that gets stuck in your scope and there is no reason to remove it.

    That being said I just bought a 2" diag because I want more, so there is a 35mm Nagler in my costly future. Once I complete the sale of my kidney I'll pick up another 24mm pan for binos. That should be pretty magical
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    Default Re: 1 1/4" Wide Field Eyepiece Options.

    Quote Originally Posted by musiclucho View Post
    I myself did an excel worksheet to compare what eyepiece will give me the widest FOV vs price. The formula is simple
    1. Divide the focal lenght of the telescope in the eyepiece focal lenght and you have the eyepiece magnification.
    2. Every eyepiece has an apparent field of view given by the manufacturer so take that number and divide by the magnification and then you have the true field of view that eyepiece can give you.
    In general, this works, but it does not tell you the scope's maximum TFOV. It is possible to find eyepieces that should give a larger TFOV according to this formula, but that can't because it would exceed the scope's maximum.

    Quote Originally Posted by CloudyNight View Post
    The most precise way to calculate TFOV would be by the eyepiece Field Stop (FS) diameter. Eyepiece Field Stop, mm/Telescope Focal Length, mm*57.3 = TFOV*
    This is the correct formula. No eyepiece can give you more than 0.8 degrees TFOV in a 1.25" barrel. That is why they make 2" eyepieces.
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    Default Re: 1 1/4" Wide Field Eyepiece Options.

    32 mm is as wide as you can get in 1.25". 40 mm would give you less magnification but the same AFOV.
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    Default Re: 1 1/4" Wide Field Eyepiece Options.

    Quote Originally Posted by KeithBC View Post
    This is the correct formula. No eyepiece can give you more than 0.8 degrees TFOV in a 1.25" barrel. That is why they make 2" eyepieces.
    I did not know that Keith, thanks for the info. But now I am rather confused because I have a pair of sketches made with my Astromaster 130 and a 25 kellner eyepiece that shows almost 2 degrees in the field of view. I dont have that set anymore but I am sure I could see more than 0.8 degrees....Or are you talking the 0.8 are the widest TFOV using the C8 Scope?
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    Default Re: 1 1/4" Wide Field Eyepiece Options.

    Quote Originally Posted by musiclucho View Post
    I did not know that Keith, thanks for the info. But now I am rather confused because I have a pair of sketches made with my Astromaster 130 and a 25 kellner eyepiece that shows almost 2 degrees in the field of view. I dont have that set anymore but I am sure I could see more than 0.8 degrees....Or are you talking the 0.8 are the widest TFOV using the C8 Scope?
    I'm not Keith, but let me answer

    Of course it's for C8. Look at the denominator, it uses the scope FL so the result is scope specific. Your scope has FL=650mm (max TFOV 2*23') vs 2032mm in C8 which max TFOV in 1.25" focuser is less than a degree (~45').
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    Default Re: 1 1/4" Wide Field Eyepiece Options.

    Quote Originally Posted by CloudyNight View Post
    I'm not Keith, but let me answer

    Of course it's for C8. Look at the denominator, it uses the scope FL so the result is scope specific. Your scope has FL=650mm (max TFOV 2*23') vs 2032mm in C8 which max TFOV in 1.25" focuser is less than a degree (~45').
    Thanks CloudyNights,

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