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8SE and WO Binoviewer

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#1 tknice

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Posted 17 June 2013 - 08:15 AM

Hey Guys,

Close to pulling the trigger on a Williams Optics Binoviewer and just wanted to make sure I'll be able to focus properly with the Celestron 8SE. Are you forced to use the barlow ahead of the diagonal to get it to work?

Also, I know it comes with 2x 20mm eyepieces but I already have a XCel XL 18mm and was thinking about picking up another one for this. Will that work well?

Is there any benefit to using an f6.3 focal reducer with the binoviewer setup?

Thanks for the guidance!

TK

#2 Eddgie

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Posted 17 June 2013 - 08:33 AM

It should focus fine without using the barlow.

Using the focal reducer is possible, but due to the long light path, it will most likely result in some aperture reduction of the system.

If you use the focal reducer with a 1.25" visual back and 1.25" diagonal, it will most likely reduce the aperture to about 7 inches or less, and if you use it with a 2" diagonal behind the focal reducer, maybe to 6.5 inches or less. The lowest power will most likely still offer a big enough exit pupil so that you get decent deep sky brightness, but for higher resolution observing (planets), you may do better by taking the focal reducer out.

It is hard to avoid aperture reduction when using binoviewers with any kind of reducer or compressor in most SCTs.

Now, the only question is whether it matters to you or not.

I would take it out for planets though. You really see the dimming even on moderately high powers when you use a reducer in front of a binoviewer.

#3 TomCorbett

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Posted 09 July 2013 - 01:10 AM

I use the Williams Optics Binoviewer with a Celestron C6 (6-inch SCT). Here are the configurations I like the best.

No Barlow
40mm TV plossl, calculated at 38x, 2.0mm exit pupil, and 1.0° FOV. These are my own calculations and I am satisfied with them--however, some of the engineers on this forum may question the accuracy of these calculations. This configuration is my best view because of maximum brightness and FOV. To get the scope magnification equivalent, I multiply the binoviewer mag by 1.35x. Thus, I calculate the 40mm TV plossl and WO binoviewer produce a scope mag equivalent of 51x.

20mm Pentax XW, calculated at 76x, 1.0mm exit pupil, and 0.6° FOV. Scope equivalent is 103x. The Pentax XW outperforms the 20mm WO plossl boxed with the WO binoviewer in sharpness, contrast, and color rendition. However, the Pentax XW adds more weight to the configuration.

20mm WO plossl boxed with the WO binoviewer, calculated at 76x, 1.0mm exit pupil, and 0.6° FOV. Scope equivalent is 103x. The 20mm WO plossl is not bad, but does fall behind the Pentax XW in sharpness, contrast, and color rendition. The most noticeable being color rendition. But the WO plossl weighs in less than the Pentax XW.

Add barlow
I sometimes add the following barlow configurations, however, the better views in my humble opinion come with using the binoviewer without a barlow. The barlow is placed between a TV Everbrite diagonal on my C6 and the WO binoviewer.

40mm TV plossl
WO barlow 1.6x -- 61x (scope equivalent 82x), 1.3mm EP, 0.6° FOV
TV Powermate 2.5x -- 95x (scope equivalent 128x), 0.8mm EP, 0.4° FOV

20mm Pentax XW or 20mm WO plossl
WO barlow 1.6x -- 122x (scope equivalent 165x), 0.6mm EP, 0.4° FOV
WO barlow 2.0x -- -- 152x (scope equivalent 205x), 0.5mm EP, 0.3° FOV

*****

I hope this helps.

#4 seawolfe

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Posted 09 July 2013 - 04:48 AM

I too have a WO Binoviewer and I'm thinking its a better deal than the Celestron one. For one thing, if you get the kit, you get two 20mm plossels, an extension plus the Binoviewer. Mine came straight from Taiwan, so it came post express and needed a signature from the person who ordered it. Since I wasn't home when the Postal person came by to deliver it, I had to claim it at the local post office and sign for it.

I have used it on my VX Edge HD 8 inch looking at Saturn, very nice! (that's when I can get the alignment to work...still working on that one but not a contributing problem from the WO Binoviewer)

#5 Eddgie

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Posted 09 July 2013 - 07:24 AM

40mm TV plossl, calculated at 38x, 2.0mm exit pupil, and 1.0° FOV. These are my own calculations and I am satisfied with them--however, some of the engineers on this forum may question the accuracy of these calculations. This configuration is my best view because of maximum brightness and FOV.



I'm not an engineer, but I would guess that your calculations for the 40mm configuration are a little off.

The focal lenght of your system is probably about 250 longer than native (no binoviewer) so your 40mm Plossl is likley giving about 44x.

But that is not really the issue. The issue is that the TV 40mm Plossls (and I own a pair of these so I know this to be true) are not giving their full true feild.

The TV Plossl has a focal plane that is so close to the rear opening of the binoviewer that the rear aperture of the binoviewer acts as a field stop. You see the edge of the field as being very sharp so it looks like it is working OK, but the apparent field is less than the 44 AFOV that the TVs can give. The apparent field is more like 38 degrees than 44.

Your real true field is more like .8 degrees.

Not that is not bad really, but it is not much wider than you would get with 26mm Plossls.

Also, if you are using a 2" diagonal, your aperture is likely to be about 7.2 inches.

If you are using a 1.25" diagonal, your apeture is likely to be about 7.6".

Still, I like the TV 40s for all of the extra brightness you get. Some objects like the Ring Nebula just stand out more because of the big exit pupil.

If you are not using a 1.25" diagonal, I would recommend that you change to one for binoviewing.

As an example, My C5 was working at 116mm with a standard 1.25" diagonal and Burgess Optics BVs (similar to the WOs).

I shortened the light path by 40 millimeters, and now it is working at 124mm.

The situsation is similar with the C6. If you let the light path get over about 200mm, you are loosing a bit of apeture.

Not trying to say anything is bad here... Only trying to give the data associated with using binoviewers in your scope.

#6 TomCorbett

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Posted 09 July 2013 - 10:07 AM

Eddgie...

Thank you for your corrections to my calculations with the 40mm TV plossl and WO binoviewer on my C6. And yes, as you suggested, the real reason I like the 40mm TV plossl is the brightness.

#7 Eddgie

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Posted 09 July 2013 - 01:46 PM

Yes, I have recommended the TV 40s on the forum for use with WO/Celestron/Burgess Binos.

I too like the extra brightness when viewing dim objects like nebula (Ring Nebula looks great in my C5 with TV 40s).

And I don't like the vignetted appearance of 32mm Plossls, but the 40s give that appearance of a very well defined field stop.

Finally this... When binovieweing, I have become much more comfortable with a narrow apparent field. Larger AFOV eyepieces often black out when you try to look at the edge. I think we tend to try to tilt our head a bit with widefields to look at the edge of the field, and when you do that, you often get a blackout with one eye.

Not having that problem with 50 degree eyepieces. You can take in the entire field easily without a lot of eye movement.

I am indeed a fan of the TV 40s in these kinds of binoviewers.






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