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    Question Increase eye relief by using a barlow?



    I always assumed eye relief remained the same for a given eyepiece even when using a barlow. But apparently that is not always the case. See this post #33. I thought eye relief was built in to the eyepiece and that was that. Can someone explain how this works? I have no idea.
    Last edited by reborn; 10-13-2011 at 12:48 AM. Reason: Add a few words
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    Releif is pretty much is built into the eyepiece, a Barlow will double the image size - with a cheaper lens, it may create a different exit pupil according to the way it's built.
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    I’ve never tested it myself because I bought a TeleVue Powermate instead of a Barlow, but their web site uses the fact that Powermates don’t increase the eye relief as a selling point. The way they put it, on longer focal length EPs, the eye relief can become too long, causing vignetting.

    Here is the link: Tele Vue Optics: Powermates

    And here is what it says:
    While Barlows are powerful tools, the negative element defining them also limits their ability. The simple negative element Barlow increases eyepiece eye-relief. With short focal length eyepieces the effect is negligible. However, on long focal length eyepieces the exit pupil position moves well beyond the designer's intended position, resulting in vignetting with many eyepieces. This is why "Shorty" Barlows in particular, with their strong negative element often vignette and degrade long focus eyepieces. My 4-element form picks up where the Barlow concept can go no further. Powermate™ is a technically universal solution, using a positive field lens to redirect field rays. The result is an exit pupil that stays where the eyepiece designer intended. With freedom from aberrations, greater magnification potential, and compact size, I hope you'll agree Powermate™ raises the standard for image amplification. — Al Nagler
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  5. #4
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    Quote Originally Posted by reborn View Post
    I always assumed eye relief remained the same for a given eyepiece even when using a barlow. But apparently that is not always the case. See this post #33. I thought eye relief was built in to the eyepiece and that was that. Can someone explain how this works? I have no idea.
    It doesn't seem to come into play until you get into very long eyepiece focal lengths. I just did another series with my 25mm, 32mm, and 40mm plossls. There wasn't much difference with the 25mm and 32mm, but the 40mm showed an increase. If anyone wants to quantify this, the measurement across my eyeglasses is 49mm. You can almost animate the 3 photos by clicking the next button.

    To repeat: my method is to move my eye towards the EP until I can just see the field stop all around, and take a photo.
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  7. #5
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    With short focal length eyepieces the effect is negligible. However, on long focal length eyepieces the exit pupil position moves well beyond the designer's intended position, resulting in vignetting with many eyepieces.
    This is precicely what I have found with my empirical measurements. It's nice to be backed up by Al Nagler.
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    Thankyou for that link Sam. WWPierre have you done any tests with TeleVue Powermates to see if their claim is accurate? I have a better understanding of this issue now thank you. So basically it only gets to be a problem with the longer focal length eyepieces. I no longer own a "barlow" but was saving for a Powermate 2.5 x. So i guess im still good to go on that. Learn someting new on this forum almost every day. What a great thing to have such knowledgeable people here to help. Thanks again.
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    This topic is discussed in "Telescope Optics" Rutten and vanVenrooij

    Normally, the light cones converge from the objective towards the focal plane and point normal (90 degrees) or ever so slightly inward. EPs are designed to collimate these light cones and pass the resulting image through an exit pupil.

    When a barlow in inserted in the light path, the convergine light cones a) now convergine at a slower rate, b) diverge from the optical axis inorder to produce an image plane significantly larger than the unBarlowed image plane. This divergence causes the optical prescription in the EP to render the exit pupil farther away then when the light cones are normal tothe focal plane.

    It is true that higher magnifications (smaller focal lenght EPs) have a smaller exit pupil displacement but this is the result of imaging a smaller focal plane consisting of light cones that diverge less that the light cones that would have been image with a wider FoV EP or a longer FL EP (or both).

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