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    Default Any long eye relief planetary eyepieces?



    I have a 6.7mm explore scientific 82 degree which offers 14mm of eye relief. That is plenty for me, as I don't wear glasses, by my 8 year old does, and even 14mm is a bit tight for her (in comparison she has absolutely no problem with the 22mm relief provided by my 30mm GSO superview). Is 14mm as good as I should expect at this short focal length or are there any reasonably priced eye pieces out there that will provide in the 18mm+ eye relief at short focal lengths?

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    To maximize eye relief and still get high magnification, your best bet is to barlow a longer eyepiece. So if you can find a good 12-14mm eyepiece with 20+mm eye relief, you can put it into a 2x barlow and get 6-7mm focal length, but still get that long eye relief.

    However, unless your daughter has astigmatism, she can correct for her vision using the telescope's focuser. I'm not sure if 8 is old enough for her to do her own focusing, but it might be something to try before you go out and buy a new eyepiece and barlow.

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  3. #3
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    Default Long eye relief eyepieces

    If you are looking for planetary eyepieces, try these 20mm eye relief SPL ones from William Optics (get the set of 3 - much better deal):

    Internally baffled, All lenses' edges and internal spacers blackened, Fully multicoated
    Model SPL 3 SPL 6 SPL 12.5
    Focal Length 3mm 6mm 12.5mm
    Lens Structure (#Elements/#Groups) 7/4 7/4 7/4
    Field of View 55° 55° 55°
    Eye Relief 20mm 20mm 20mm
    Barrel 1.25" 1.25" 1.25"
    Coating FMC FMC FMC
    Parfocal Yes
    Yes

    Yes
    Weight 200g 150g 150

    Dick

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    Approved

    Hi and welcome to our forums Dick, maybe drop by the Welcome Forum and introduce your good self.

    Clear Skies

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    You may want to give these a look. The 12.5mm provides an eye relief of 20mm and has an AFOV of 55 degrees. They offer various sizes in this series as well.

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    Quote Originally Posted by jproffitt1 View Post
    You may want to give these a look. The 12.5mm provides an eye relief of 20mm and has an AFOV of 55 degrees. They offer various sizes in this series as well.
    Thanks. I was looking at those (along with the Zhummel Z series and the Pro-optic long relief....all of which are basically the same design). That leads me to this question: My understanding is that eye relief is measured based upon how close you have to be to see the entire field of view. So moving my eye back, I can still use the eyepiece I just can't see to the field stop. So is it the case that if I move my eye back 30% further that my field of view would be reduced by the same 30%? Because if that is the case my 82 degree ES eyepiece would give about a 57 degree FOV at 20mm. So these 20mm 55 degree eyepieces wouldn't really give me any benefit over what I already have. Or is that incorrect?

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    You pose a great question. I would like to hear the responses from those who have the eyepiece mathematics sorted out weigh in on this.

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    Default

    To use glasses you need from 15-20mm of eyerelief.

    The Televue Radians have about 20mm of eyerelief.

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    Default

    Several brands of EPs have eyeglass useful eye relief {Radians, Delos,...}

    You can get a 4X Barlow/PowerMate and stick the 30mm GSO in it for an equivalent to 7.5mm with the same ER as the 30 GSO (a tad more with a Barlow, spot on with the PowerMate).

    As far as the math goes, the size of the eye lens needs to be big enough that it does not vignette the Exit pupil subtended over the apparent FoV. Which basicaly means that the longe the ER the bigger the eye lens. A 31mm Nagler is, in essence, a 60-odd mm EP with large FoV and medium ER attached to a special Barlow that creates the curved image field that the EP focuses precisely. This special Barlow is called a Smyth lens.

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    One thing you could do would be to view the apparent field of the 14mm while holding back for 20mm eye relief and then compare this apparent field with eyepieces of various fields of view to see about what size you are achieving, such as plossl 50 deg, perhaps if you have a 60 deg field eyepiece as well, though I think the comparison with the plossl would be telling enough.
    I might also look at the Owl Enhanced Superwide eyepieces, the 6 and 9mm have 17mm eye relief and a decently corrected 66 degree field, even at the f/ratio of your Zhummel 10". I wouldn't recommend the 15mm as it has shorter eye relief and the 20mm as its edge correction is not as good as the others.

 

 
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