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    Default Optical match for a Dob



    Are the following EPs a good optical match for my F/4.6 Dob?

    ES 2" 40mm 62 deg series
    TV 1.25" 40mm Plossel
    TV 2" 55mm Plossel
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    Default Re: Optical match for a Dob

    Yes. However, I believe, I'm not sure about the TV, but most 40mm plossls are cut off by the field stop of the barrel and offer the same field as a 32mm plossl. Different magnification but same field of view. All of them will work well in the scope.

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    Default Re: Optical match for a Dob

    I think they might all produce too much exit pupil in a F4.6 dob.

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    Default Re: Optical match for a Dob

    I was going to say the same as Don. You'll be getting an exit pupil of about 9mm with a 40mm eyepiece and 12mm with a 55mm eyepiece, which is way too much. So effectively, you're eye pupil will become a stop. You won't see any real benefit from a 40mm in that scope. The objects will just be smaller. You don't really need to go lower than a 30mm. If you want a wider view, go with a wider angle eyepiece. The ES 30mm 82° is perfect for that. It will have the same field of view as a 55mm Plossl eyepiece and the objects in it will be just a bright, but bigger.
    Rob

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    Default Re: Optical match for a Dob

    Quote Originally Posted by Don Alvarez View Post
    I think they might all produce too much exit pupil in a F4.6 dob.
    Quote Originally Posted by Robrj View Post
    I was going to say the same as Don. You'll be getting an exit pupil of about 9mm with a 40mm eyepiece and 12mm with a 55mm eyepiece, which is way too much. So effectively, you're eye pupil will become a stop. You won't see any real benefit from a 40mm in that scope. The objects will just be smaller. You don't really need to go lower than a 30mm. If you want a wider view, go with a wider angle eyepiece. The ES 30mm 82° is perfect for that. It will have the same field of view as a 55mm Plossl eyepiece and the objects in it will be just a bright, but bigger.
    Oh, yes. Not great matches at all. A 35mm Panoptic would be my limit. Sky background would be brighter even from a dark site with excessively large exit pupils.
    j.gardavsky, DeanD, 10538 and 2 others like this.

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    Default Re: Optical match for a Dob

    And with exit pupils that big, you're going to see the shadow from the secondary mirror...
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    Default Re: Optical match for a Dob

    Quote Originally Posted by not_Fritz_Argelander View Post
    Oh, yes. Not great matches at all. A 35mm Panoptic would be my limit. Sky background would be brighter even from a dark site with excessively large exit pupils.
    NFA is right on the money! A 35mm Panoptic is as low as I can go with the 14g.
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    Default Re: Optical match for a Dob

    Even a 35mm Panoptic will give an exit pupil of 7.6mm at the maximum field of 1.3°, so you will lose some of the light even if your pupil dilates to 7mm...

    Perhaps a 27mm Panoptic, which will give you a 1.1° field and 5.9mm exit pupil, or a 32mm TV plossl, which will give a .93° field (the maximum possible for a 1 1/4" eyepiece) at a 7mm exit pupil.

    The maximum field for a 2" eyepiece with a smaller than 7mm exit pupil will come from a 31mm Nagler (1.5° at 6.7mm), or very slightly smaller, but much cheaper, an ES 30mm 82°

    Have you looked at the TV eyepiece calculator? Tele Vue Optics: Calculator

    Have fun with the new scope!

    - Dean

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    Default Re: Optical match for a Dob

    Quote Originally Posted by DeanD View Post
    Even a 35mm Panoptic will give an exit pupil of 7.6mm at the maximum field of 1.3°, so you will lose some of the light even if your pupil dilates to 7mm...

    Perhaps a 27mm Panoptic, which will give you a 1.1° field and 5.9mm exit pupil, or a 32mm TV plossl, which will give a .93° field (the maximum possible for a 1 1/4" eyepiece) at a 7mm exit pupil.

    The maximum field for a 2" eyepiece with a smaller than 7mm exit pupil will come from a 31mm Nagler (1.5° at 6.7mm), or very slightly smaller, but much cheaper, an ES 30mm 82°

    Have you looked at the TV eyepiece calculator? Tele Vue Optics: Calculator

    Have fun with the new scope!

    - Dean
    True, true. But the 35mm Panoptic is still useful as a finder and I'm pretty sure that my eyes don't dilate to 7mm anymore either. A little stopping down is acceptable IMO for finding and extreme framing needs. For observing rather than finding is the 27mm Pan.
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    Default Re: Optical match for a Dob

    Ok, I should have mentioned why I was asking this question since I figured I get everyone spun up about exit pupil ...

    I'm looking at these for use with NV afocal viewing ....

    turns out that I can double the speed of my F/4.6 Dob, turning it into a f/2.25 scope using a 55mm EP and a 40mm EP would turn it into a F/3.1 scope using NV afocal viewing

    the equation is: Effective telescope FR = focal length of NV objective/ scope-EP exit pupil

    a 55mm Plossel & my F/4.6 dob yield an 11.95mm Eit pupil. The focal length of the NV device is 27mm; this yields a f/2.25 system. A 40mm Plossel yields a f/3.32 system.

    The reason to do this is for image scale ... on very large emission nebulae it's not really possible to see the entire structure without this technique ....

    I'm not sure I'll be doing this as prime focus NV is much more straight forward WRT making all the connections work but if magnifications besides what the 27mm NV objective produce are desired than afocal is required so having optically matched EPs to your scope is now in play again ...
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