TheSkySearchers

I have always been curious about binoviewers. Inspired by a recent thread about "project refractors" where lady Frac was looking take a hacksaw to the focuser to make her binoviewers work without the Barlow, I decided to get a pair myself and see what this is all about.

The WO package is great for starters, it comes with two 20 mm eyepieces and a 1.6x Barlow. My favorite shop Agena Astro did not disappoint and delivered the product with free shipping within a few days. I pulled my collapsible-converted Z12 out and, after collimating and aligning the Telrad with the 30 mm 2" GSO superview, my favorite eyepiece, I eventually found Polaris. I find Polaris all the time for AP but I set up in a slightly different place and it was nearly blocked by a tree so I mistook some other star for Polaris.

My struts (three of them) have 2 sets of holes. The lower one is for standard viewing. I drilled it to make that possible. The upper holes are about 10 cm away. I added those for AP since I still have that dual axis autoguided EQ platform that I finished but never fully tested (because I won that 127 mm ES Apo in a door prize lottery and moved to a motorized mount - long ago). I figured, I might be able to use those holes for the binoviewers without Barlow.

Indeed, I was able to reach focus with the standard viewing holes with the Barlow included, and with the AP holes with the Barlow removed. Of course, switching between the two settings is not smooth because I drilled the holes manually so they point in slightly different directions. Not much but if you want to do it right you have to recollimate and recalibrate the viewfinder and/or Barlow (both in my case).

The stars were a bit fuzzy but that is partly because the magnification is higher. I am used to the 30 mm superview that delivers very nice crisp views of the stars. The magnification by a 20 mm is quite a bit higher. I tried it on Polaris, Spica and Mizar. I know the Polaris region very well because I use an asterism for eyeballing the NCP to about 1' accuracy. It was quite a bit larger than I was used to with the 30 mm. But, the asterism was very clear.

One thing with Mizar was a bit odd. I always thought Mizar was a double star with Alcor that can be seen naked eye. I had seen it with 20x80 Skymasters, the Z12 and with AP as well. With the binoviewers I noticed something odd, namely Mizar itself looked like a double star. I was wondering if that was an optical effect so I went inside and checked Stellarium and various AP images but nothing showed the second star. Then I went to Wikipedia and lo and behold, it is a double star!

So, these binoviewers are working great especially if you have a telescope with struts that have different settings so you can use it without the 1.6x Barlow. Maybe this will convince lady Frac to buy a light bucket with some struts so she doesn't have to cut up her fracs. Ok, I said it - time to duck for cover.

Nice piece of hardware to add to the collection Henk.

Well, here I am wondering if... I should have one.
thx for the review .

messier 111 wrote: ↑Sun Apr 23, 2023 1:13 pm Well, here I am wondering if... I should have one.
thx for the review .

I'm not yet sold on it. It leads to a smaller FOV and higher magnification than I am used to. If you normally swap eyepieces a lot, it's probably not a good idea. I was lucky to have this adjustable truss setting so I could view without the Barlow, but most fracs and Newts don't have that. I still need to check if the blurriness is comparable to that of a similar eyepiece, or if the additional reflections and beam splitter contribute to it. It may just be a matter of smaller focal length than I'm used to. Some say they are best for planets not so much for star hopping. CN has several threads on it with useful thoughts.

Good to hear that they worked for you Henk. With the supplied 20 mm did you have much infocus left?
Make sure both eyepieces are fully seated and play with the IPD as that may help eliminate some abberations.
Hopefully you get some more time soon to use them.

I promise not to take a hacksaw to the new telescope when it arrives.
I plan on using my 125 mm powered angle grinder

Lady Fraktor wrote: ↑Sun Apr 23, 2023 5:20 pm Good to hear that they worked for you Henk. With the supplied 20 mm did you have much infocus left?

With my Dob and BV without Barlow I have 12 mm left so that's very good.

Trying it on my 12" GSO astrograph though, I need at least 3 cm more backfocus. It works fine with the Barlow.

Lady Fraktor wrote: ↑Sun Apr 23, 2023 5:20 pm Make sure both eyepieces are fully seated and play with the IPD as that may help eliminate some abberations.

Yep, I found out soon enough that they don't work if the eyepieces are not fully seated.

Lady Fraktor wrote: ↑Sun Apr 23, 2023 5:20 pm Hopefully you get some more time soon to use them.

I want to bring them to the Kaibab star party, it's nice to have something to show off.

Lady Fraktor wrote: ↑Sun Apr 23, 2023 5:20 pm I promise not to take a hacksaw to the new telescope when it arrives.
I plan on using my 125 mm powered angle grinder

I like that, you might as well do it right if that's what you want to do.

I just tested the BV with the Dob and astrograph outside, which is a little easier than testing under a dark sky. The nicest thing is that the BV without Barlow, 20 mm eyepieces, works better than the 30 mm 2" GSO superview with 1.5x GSO ED Barlow. Putting the cups up and taking my glasses off makes it even noticeably better. Doing that with my SV just shows a ton of dust that I need to clean out (and it's not better). That puts my fear that the resolution was worse, to rest.

Bottom line, with a normal Newt, even an astrograph, you must use the BV with the Barlow. A collapsible Dob where the struts can be adjusted far enough to get the backfocus needed works great without the Barlow. It's a well-constructed package. It feels very solid, and the eyepieces are individually focusable.

I just ordered two 1.25" 32 mm 52 degree GSO eyepieces from Agena Astro because I would like to have that wider TFOV. With the 20 mm I will then have 32, 20, 16 and 10 mm equivalents. I have good experience with GSO eyepieces, good for the money. I haven't done the math to check if I lose light for an exit pupil of 5 mm, with the 32 mm eyepieces.

SkyHiker wrote: ↑Sun Apr 23, 2023 8:09 pm I just ordered two 1.25" 32 mm 52 degree GSO eyepieces from Agena Astro because I would like to have that wider TFOV.

Your BVs have a clear aperture of 20.2 mm and the supplied 20 mm 66° eyepieces have a field stop of 23 mm so not very noticeable through two eyes.
The 32 mm 52° eyepieces have a field stop of 27 mm so will show considerably vignetting.

Lady Fraktor wrote: ↑Sun Apr 23, 2023 8:53 pm

SkyHiker wrote: ↑Sun Apr 23, 2023 8:09 pm I just ordered two 1.25" 32 mm 52 degree GSO eyepieces from Agena Astro because I would like to have that wider TFOV.

Your BVs have a clear aperture of 20.2 mm and the supplied 20 mm 66° eyepieces have a field stop of 23 mm so not very noticeable through two eyes.
The 32 mm 52° eyepieces have a field stop of 27 mm so will show considerably vignetting.

In theory yes. But in practice I love how 32mm Plossls perform in my prism BV. You can't really tell that they vignett.

Bigzmey wrote: ↑Sun Apr 23, 2023 9:26 pm

Lady Fraktor wrote: ↑Sun Apr 23, 2023 8:53 pm

SkyHiker wrote: ↑Sun Apr 23, 2023 8:09 pm I just ordered two 1.25" 32 mm 52 degree GSO eyepieces from Agena Astro because I would like to have that wider TFOV.

Your BVs have a clear aperture of 20.2 mm and the supplied 20 mm 66° eyepieces have a field stop of 23 mm so not very noticeable through two eyes.
The 32 mm 52° eyepieces have a field stop of 27 mm so will show considerably vignetting.

In theory yes. But in practice I love how 32mm Plossls perform in my prism BV. You can't really tell that they vignett.

What BVs are you using?
If you are looking at dim/ small objects perhaps... 27 mm of light trying to fit into a 20.2 mm hole is going to do something.

Lady Fraktor wrote: ↑Sun Apr 23, 2023 9:39 pm

Bigzmey wrote: ↑Sun Apr 23, 2023 9:26 pm

Lady Fraktor wrote: ↑Sun Apr 23, 2023 8:53 pm

Your BVs have a clear aperture of 20.2 mm and the supplied 20 mm 66° eyepieces have a field stop of 23 mm so not very noticeable through two eyes.
The 32 mm 52° eyepieces have a field stop of 27 mm so will show considerably vignetting.

In theory yes. But in practice I love how 32mm Plossls perform in my prism BV. You can't really tell that they vignett.

What BVs are you using?
If you are looking at dim/ small objects perhaps... 27 mm of light trying to fit into a 20.2 mm hole is going to do something.

I have Orion's prism BV, very similar design to WO's. I use them mainly for Lunar and planets. 32mm is the only way how I can fit the whole lunar disk within FOV of my 8" SCT. There is plenty of light, vignetting some of it probably is a good thing.

I was just looking around and Orion does not even state the clear aperture in their manual.
Have you ever timed them?

Lady Fraktor wrote: ↑Sun Apr 23, 2023 9:55 pm I was just looking around and Orion does not even state the clear aperture in their manual.
Have you ever timed them?

I believe they are in the same park of 20-22mm aperture as many other basic prism BVs (WO, TV, Celestron, etc.).

This quote is from a Kasai linear binoviewer review. So I'm not exactly certain that this applies, but it may.

The only problem with erecting 1:1 relay lenses is that the field diameter at the second focal plane is limited. In the case of this product, the effective field of view is φ17.4mm, which is slightly smaller than a normal binocular device (approximately φ22mm). Using a rough formula, focal length (mm) x apparent field of view (degrees) = about 1,000 or more will cause vignetting, and below that, the entire field of view can be used.

Lady Fraktor wrote: ↑Sun Apr 23, 2023 8:53 pm

SkyHiker wrote: ↑Sun Apr 23, 2023 8:09 pm I just ordered two 1.25" 32 mm 52 degree GSO eyepieces from Agena Astro because I would like to have that wider TFOV.

Your BVs have a clear aperture of 20.2 mm and the supplied 20 mm 66° eyepieces have a field stop of 23 mm so not very noticeable through two eyes.
The 32 mm 52° eyepieces have a field stop of 27 mm so will show considerably vignetting.

I'm trying to understand this. If the F ratio of the BV tube is the same as the F ratio of the telescope then 100% of the central light beam will hit the retina. But non-central light beams will be vignetted by the BV tube. The field stop size plays into this too (as you pointed out in relation to the focal length of the eyepiece) so it's not easy to explain without a drawing. The only way in which 100% will reach the retina is if the F ratio of the scope is a sufficient amount larger than the F ratio of the BV tube (simplified, assuming the field stop size is the same as the BV tube diameter). For instance, for the case of the Williams with a BV tube of 100 mm long and 20 mm wide, its F ratio is F/5 so perhaps an F/7 won't vignette. But since my Dob is an F/5 it will vignette no matter what if the field stop is 20 mm (or larger of course). With a Barlow there will be less vignetting, probably zero, because it effectively multiplies the F ratio of the telescope by its multiplier. So I should have gone with that Barlow of your other thread, ideally. Maybe I will trade up some day.

The binoviewer does not have a focal ratio, it has a lightpath length that can be modified using different OCA. It only directs the light.
The field stop is a physical size limitation.

The use of the OCA is to help adjust the focus distance needed (negate the lightpath of the BV) though changing magnification is a side effect it still does not change the clear aperture of the unit.

I will post a couple of images later, maybe it will help a bit.

Lady Fraktor wrote: ↑Mon Apr 24, 2023 3:34 pm The binoviewer does not have a focal ratio, it has a lightpath length that can be modified using different OCA. It only directs the light.

What I mean by the binoviewer tube focal ratio is, simplifying it as a tube that the light has to go through, its length divided by its diameter. In the diagram below I have a simplified view of the scope (focuser tube left out) that represents an OTA with binoviewer. The dimensions as drawn are totally out of scale but otherwise the BV tube becomes too small to make the point, visually.

It shows the central light bundle in black and the upper and lower ones in blue and orange (the upper and lower ones reach the field stop border indicated by the small box to the left representing the eyepiece). The central one is partially obstructed by the BV tube, the other two even more. Clearly, if the BV tube becomes shorter and/or thicker (its focal ratio as I defined it gets smaller), the vignetting will be less and eventually will go away. If the BV focal ratio is exactly that of the OTA the central light bundle will be unobstructed but the outer ones are still partially obstructed. Hope this explains the thought that I wanted to get across.

Let's consider practical outcome. The purpose of using 32mm is to maximize the FOV while using BVs. It will serve the purpose. Due to vignetting the edges of the FOV may get darker, but you still will be getting the max FOV available for BVs and TFV (even if augmented by vignetting) will still be wider than with 20mm 66d deg EPs.

JayTee wrote: ↑Mon Apr 24, 2023 5:35 am This quote is from a Kasai linear binoviewer review. So I'm not exactly certain that this applies, but it may.

The only problem with erecting 1:1 relay lenses is that the field diameter at the second focal plane is limited. In the case of this product, the effective field of view is φ17.4mm, which is slightly smaller than a normal binocular device (approximately φ22mm). Using a rough formula, focal length (mm) x apparent field of view (degrees) = about 1,000 or more will cause vignetting, and below that, the entire field of view can be used.

Good to know for linear BVs but the formula does not apply to prism BVs unfortunately.

Bigzmey wrote: ↑Mon Apr 24, 2023 10:28 pm Let's consider practical outcome. The purpose of using 32mm is to maximize the FOV while using BVs. It will serve the purpose. Due to vignetting the edges of the FOV may get darker, but you still will be getting the max FOV available for BVs and TFV (even if augmented by vignetting) will still be wider than with 20mm 66d deg EPs.

I agree that you are trying to maximize TFOV but you will not do that with a set of 32 mm eyepieces due to vignetting.
Better to pick a set of eyepieces that give maximum FOV but only exceeding the clear aperture by 1-2 mm, no major vingetting that way and you are getting the maximum resolution from the BVs.
Using the prism in a SC negates the use of a OCA so will not be the same as using a newtonian or refractor.

This is a chart made by EdZ from CN who has worked out for prism BVs.

SkyHiker wrote: ↑Mon Apr 24, 2023 7:35 pm Clearly, if the BV tube becomes shorter and/or thicker (its focal ratio as I defined it gets smaller), the vignetting will be less and eventually will go away. If the BV focal ratio is exactly that of the OTA the central light bundle will be unobstructed but the outer ones are still partially obstructed. Hope this explains the thought that I wanted to get across.
bv.jpg

I will admit that I am a bit confused but I think it is mixing terms that do not apply.
The BVs have no focal ratio but they have a focal length, so the f/r will not match between BVs & telescope.
Using the OCA just negates the f/l of the BVs so focus can be achieved.
When you used your newtonian, you had the mirror in a position that would equal the focal length of your BVs, if you put the mirror in its other position you would need to use the OCA or a stronger one depending if the mirror location was +/- focal length.

The picture is pretty much what is happening if you exceed the clear aperture of 22 mm