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Binoview-Curious...

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#1 Dave Bush

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Posted 01 August 2019 - 11:02 AM

I've got an EdgeHD8 and love it.  

 

I've looked through binoviewers only briefly in the past but I've been thinking about them recently.   I have some questions...

 

  1. I've got astigmatism in my left eye.  Is it possible to null that out with any of the available binoviewers or would I have to use a Dioptrx w/TeleVue eyepieces?
  2. Used natively at 2125mm, with 1.25" eyepieces I don't expect I'd be able to get fields much wider than 0.8º.   I know there are 2" binoviewers but I'm unsure of the physical issues with using two, 2" eyepieces.  What are folks typically doing to achieve the widest field?    Now, I know I could use the 0.7 reducer on the scope which would give me a wider field of view but that leads to my next question, back focus...
  3. What are the issues with regards to back focus.  I know these devices will add to that.
  4. Finally, generally what's the consensus on which units are best?

 

Thanks in advance.

 



#2 Eddgie

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Posted 01 August 2019 - 11:51 AM

First, binoviewer will no do much for astigmatism and the best way to manage that is either find long eye relief eyepieces or use Dioptrix.

 

Next, I am going to say a lot of things the might discourage you, but I am going to suggest that you still try it out for planetary observing because this is where BVs really improve visual acuity. 

 

With binoviewers in the C8, even with the most optimized light path length (96mm for a William Optics or similar BV, a T2 adapter (2mm), a T2 diagonal (38mm), and a 15mm SCT to T2 connector, the light path will be 149mm, and this will make the focal lenght of the EdgeHD 8" about 45mm longer than the nominal 2125mm so figure 2175mm, and this is using a short light path binoviewer. 

 

The concern I have about the EdgeHD is that in mine, the aperture was decreased if the back focus was allowed to go over 170mm.  This means that if you use a standard 1.25" visual back (about 25mm of light path) an standard 1.25" prism about 65mm of light path) and a 96mm light path binoviewer (WO and similar) you are over 170mm, so you might get some aperture reduction. (also, the 185m of light path raises the focal lenght by another 80mm or so (maybe a bit more). 

 

Now, I measured mine, but someone else said that they measured theirs and did not see reduction until over 200mm of back focus, so maybe it is a sample to sample variation thing. The only way to know for sure would be to measure it. 

 

No reducer configuration will give full aperture and that means that the secondary obstruction goes up as a percentage of the (remaining) aperture.   Also, the spherical aberration may increase slightly. 

 

Now as I said, I still think you should try it out, and here is my advice.

 

Buy an inexpensive used William Optic or similar binoviewer and get a pair of eyepieces that best match your typical seeing conditions.  For example, if you normally use a 12mm eyepeice to good advantage, then get a pair of 25mm Plossls. 

 

Use the 1.9x or 2x Barlow  on the binoviewers and use a 1.25" standard prism diagonal. This will fix any concern about aperture loss or SA.

Try it on Jupiter.  If you like it, then worry about the path forward.  If you don't you can sell the BVs pretty much for what you paid for them.  If you do like it, you can still sell the BVs for what you paid for them and upgrade to a better BV and a good T2 (T2 prism, and better BV.

 

I would say that trying it out should be your first step and if you like it, you can decide where you want to go.

 

For planets, even an inexpensive BV and the included 2x Barlow, when used in your 1.25" standard diagonal, and a couple of inexpensive Plossls might be all you need (I recommend zooms as next purchase for planetary work). 

 

Now you can use this without the Barlow but again, you might (or might not) loose a little aperture, but you still get a wider field, and that will be worth 3-4mm of aperture loss.   Use the Barlow for planets though.


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#3 Eddgie

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Posted 01 August 2019 - 11:52 AM

And with a reducer, expect some meaningful aperture loss (1" to 1.5" depending on configuration). 



#4 Dave Bush

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Posted 01 August 2019 - 12:04 PM

Thanks Eddgie for the detailed info.

 

I'm 95% deep sky and maybe only 5% lunar/planetary so it's sounds like maybe I wouldn't be getting the most out of one.

 

And the astigmatism thing is a bummer.  I'd have to have a Dioptrx on the left eyepiece and none on the other.  

 

Not sure it's going to be worth it.



#5 Eddgie

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Posted 01 August 2019 - 02:13 PM

Well, you can certainly use binoviewers for deep sky, but you do loose about 30% brightness, but for an SCT, the real negative is the small true field you get locked in to.  

 

That being said, I used binoviewers on my EdgeHD 8" and really used to enjoy them, but again, very limited true field of view.  If I wanted a bigger field than about .76 degrees, I had to either go to mono-vision or change scopes. 

 

If you need your glasses for all observing regardless of exit pupil though, the BV is not going to really fix that so yes, you might be locked into using the Dioptrix. 



#6 rkelley8493

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Posted 03 August 2019 - 02:10 PM

I've got astigmatism in my right eye, along with a small blind spot right in the center. My left eye does a really good job of compensating for what the other lacks. I can only really tell the right one has problems when I close my left eye. So using a binoviewer is kind of similar to using both your eyes in the real world. That was my limited experience anyways. It's very similar to using binoculars, but it's just setup in a different way.

As for field of view [in my limited experience*], anything over 76° is kind of a waste due to the limitations of the human body. I wasn't able to see to the edge of the 82° eyepieces that I used in bino's. Could be because of inter-pupillary distance or my facial features, but I couldn't tilt my head in such a way to keep the view and see the edge. However, I had the best success with the ES 24/68° on most targets. 

Back focus wasn't really an issue with my SCT. There was a big focus difference in mono vs bino, but there wasn't an issue.

I eventually gave up on binoviewing. The 3D effect was really cool the first few time I used it, but this effect faded quickly for me. In the end, I preferred the brighter, sharper image produced by a single eyepiece & objective. Just personal preferences really, same reason I don't care for zoom eye-p's. YMMV of course.



#7 REC

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Posted 05 August 2019 - 11:05 AM

When Jupiter, Saturn and the moon are available for observing lik now, the BV are the best way to go, IMO. I think once you view the moon target with any BV you will say, only way to observe the moon! A good tool to have in your toolbox. I also like some of the brighter nebulas in a BV.


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#8 Dave Bush

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Posted 05 August 2019 - 11:22 AM

When Jupiter, Saturn and the moon are available for observing lik now, the BV are the best way to go, IMO. I think once you view the moon target with any BV you will say, only way to observe the moon! A good tool to have in your toolbox. I also like some of the brighter nebulas in a BV.

 

Actually, I've been interested in the Moon lately and could see myself spending more time observing it.  

 

So without stressing over back focus and stuff, would I be able to simply used a less premium binoviewer like say, Celestron's along with decent quality eyepieces (ES82) ?



#9 Eddgie

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Posted 05 August 2019 - 05:15 PM

Yes, sure.

 

If you are going to use if for planets, just use a 2x Barlow and then you don't have to worry about all of that other stuff.

 

Without the Barlow though, if you don't manage the back focus, you could experience as much as a .75" aperture loss (which makes the central obstruction go from 33% of 203mm to 35% and the spherical aberration will change by as much as 1/3rd of a wave so the scope will fall well below the diffraction limit.

Now that does not stop one from doing it though and you get to use both eyes, but the focal lenght of your scope will typically be 200mm to 300mm longer than stated and if there is aperture loss, the image could be further dimmed (the binoviewers dim the image by 30%).

 

You could always try it and if you like using both eyes, you could later come back and optimize your setup, but if the view is disappointingly dim and you used a poor configuration, then it would not be a fair comparison to the views you get in mono-vision. 

 

For planets though,  I highly recommend that you use a 2x Barlow.

 

And better yet, I recommend either the 2" 2x Powermate, or the 1.25" 2.5x powermate.  These will make the Binoviewer roughly parfocal to a regular eyepiece.

 

A standard 1.25" or 2" Barlow will work though. 

 

.


Edited by Eddgie, 05 August 2019 - 05:24 PM.


#10 Dave Bush

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Posted 05 August 2019 - 08:42 PM

Yes, sure.

 

If you are going to use if for planets, just use a 2x Barlow and then you don't have to worry about all of that other stuff.

 

Without the Barlow though, if you don't manage the back focus, you could experience as much as a .75" aperture loss (which makes the central obstruction go from 33% of 203mm to 35% and the spherical aberration will change by as much as 1/3rd of a wave so the scope will fall well below the diffraction limit.

Now that does not stop one from doing it though and you get to use both eyes, but the focal lenght of your scope will typically be 200mm to 300mm longer than stated and if there is aperture loss, the image could be further dimmed (the binoviewers dim the image by 30%).

 

You could always try it and if you like using both eyes, you could later come back and optimize your setup, but if the view is disappointingly dim and you used a poor configuration, then it would not be a fair comparison to the views you get in mono-vision. 

 

For planets though,  I highly recommend that you use a 2x Barlow.

 

And better yet, I recommend either the 2" 2x Powermate, or the 1.25" 2.5x powermate.  These will make the Binoviewer roughly parfocal to a regular eyepiece.

 

A standard 1.25" or 2" Barlow will work though. 

 

.

 

OK. so, on my EdgeHD 8, using the supplied back and a 1.25" diagonal, the focal length is 2125mm

So say I use a 1.25" ES Focal Extender.  Should I place it ahead of the diagonal or behind (between diagonal and BV)?

 

And, how then do I calculate what the resulting focal length would be?



#11 REC

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Posted 06 August 2019 - 10:48 AM

OK. so, on my EdgeHD 8, using the supplied back and a 1.25" diagonal, the focal length is 2125mm

So say I use a 1.25" ES Focal Extender.  Should I place it ahead of the diagonal or behind (between diagonal and BV)?

 

And, how then do I calculate what the resulting focal length would be?

Behind it, in front it makes it perform more like a 3x barlow. If the barlow glass unscrews from the barrle, it's more like a 1.5x I think?

Eddgie, is a 1.5x barlow ok too?



#12 Eddgie

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Posted 06 August 2019 - 12:17 PM

My answers are for high resolution observing (solar system). 

 

It would be best to use the 1.9x or 2x Barlow that comes with the binoviewer and mounts to the nose unless you plan on using a 2" diagonal. 

 

Remember, ever mm back focus you eliminate decreases the focal length by (about) 3.1mm.  Every 25mm of back focus you save reduces spherical aberration by 1/23rd wave. 

 

If you are using a 2" diagonal, I recommend that you use the 3x Barlow that comes with these, or a regular 2x barlow, which will give more like 2.6x  and  will shorten the light path by about 50mm or more, or a powermate that will shorten the light path by 75mm or more (I am estimating this.  I have not used the Powermate in an SCT) 

 

If you want to work at 2x, then I recommend a 2" Powermate, or a 1.25" powermate coupled to the front of the binoivewer using the optional top for the 2.5x powermate that converts it to T2.  Then you can use the RAF adapter to convert the binoviewer to T2.   This can be used in a standard 2" and will give about 2.2x with a William Optics type binoviewer. 

 

The Powermate is the Mercedes Benz solution. While it will not be quite parfocal with binoviewers, the change in focus is only going to be about 10mm- 20mm.   This means that going back and forth between mono-vision and binoviewers will not take a lot of refocus.  It also means that it will allow most refractors to reach focus.  



#13 Eddgie

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Posted 06 August 2019 - 12:25 PM

And many people don't understand the benefit of the Powermate over standard barlows.  

With a Barlow, the light rays diverge and the power goes up as the separation between the barlow and field stop increases. Most 2x standard Barlows are going to give more like 2.6x to 3x, though in the case of the SCT, this is actually a good thing because it also shortens the light path more.

 

With the Powermate, there seperation only raises the power a little.   With the 2x Powermate, the power would only go to about 2.2x.  If one used the 1.25" 2.5x powermate, the power actually falls with increase in spacing, so the typical binoviewer would give between 2.1x and 2.2x. 

The nice thing about the 2.2x is that it can be converted to T2 connection.  The eyepiece holder is removed, and the T2 connector is screwed on.  This allows it to connect directly to any binoviewer that has been made T2 ready. 

 

(SCTs and MCTs are peculiar cases.  They are really optimized for a specific spacing between the front corrector and the primary mirror.  If you move the focal plane back by moving the primary forward, if this is carried too far, it can damage the image.  Also, you can run into aperture cutoff.. For example, cutoff on the standard C9.25 is only something like 160mm, so  a 2" diagonal and binoviwer will reduce the aperture. The more Barlow power you use, the shorter the effective light path of the binoviewer so the less the mirror has to be moved forward to reach focus. For general observing, none of this is all that critical but for planetary observing, it can make a difference. I don't invest myself in what other people choose to do or not do.  My effort is only explaining the behavior of the system and how to configure it for best performance for a given application.)


Edited by Eddgie, 06 August 2019 - 12:48 PM.



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