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lowest power ep's in a bviewer w/o barlow in a C8

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#1 Itz marcus

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Posted 03 November 2013 - 01:05 PM

Hi what is the lowest power ep that I could use in a binoviewer +c8 w/o vignetting? I'm talking about a 1.25 diagonal and ep.
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#2 Eddgie


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Posted 03 November 2013 - 01:56 PM

The vignetting will come from the Binoviewer before it comes from the scope.

As a general rule, you can use a eyepeice with a field stop sllightly larger than the aperture of the binoviewer and not easily see vignetting.

So, if you are using a binoviewer with 23mm apetures a 25mm Plossl will likley work.

There is one trick though. If you really really want low power, you can use a pair of 40mm Televue plossls.

These eyepecies have a field stop that is very close to the end of the chrome barrel.

Becuase of this, the focal plane of the the eyepeice is so close to the rear aperture of the Binoviewer that the rear apeture of the BV almost acts like a field stop for the eyepeice.

This means that when you use it, you see a fairly sharp outer edge.

The price though is that the apparent field is reduced from about 44 degrees that the eyepiece would normally have to about 37 degrees.

That is a pretty stiff price to pay, but the field is still the same true size as it would be with an eyepiece with a 23mm field stop.

If your binoviewer has a 26mm or 27mm apertures you can use 32mm or 40mm Plossl, or Ultia 35 (Parks is similar).

Sadly the Ultima does not have the same sharp off axis performacne of these others in fast scopes. In my EdgeHD 8 and the binoviewers they are terrific. Not so much in my other scopes.

So, rule of tumb is that you can use a field stop slighly larger than the rear apeture of the BV and not experience severe vignetting.

#3 Joe Aguiar

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Posted 04 November 2013 - 02:50 PM

normally 4x the power per inch is what you dont wanna go under or youll see the 2nd mirror but this is without the extra focal length of the bino viewer

#4 jimandlaura26


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Posted 05 November 2013 - 09:19 AM

I agree with Eddgie's points. I use a set of 25 mm Antares Elites Plossls (similar to Orion Ultrascopics and old Celestron Ultima's). They possess about a 25 mm field stop and 17 mm eye relief, with a 52 degree aparent field of view; and overall are well matched to my Denkmeier bino. You can pick them up used, since they no longer seem to be widely available.

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