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Takahashi and WO Binoviewers

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There are many questions on using Binoviewers (including my own) so I thought I would put down my experience of Williams Optics Binoviewers and Takahashi FC100DC. The exact same applies to WO and Tak FS60CB. I have not yet used the WO on the Tak 76DC but I would suspect the same would apply to that combination.

I found it confusing and tricky at first getting everything focusing and working well but when it all comes together the views are fantastic, especially for the sun, moon and planets; arguably better than mono. They are also much easier on the eyes and relaxing to use than mono, particularly for longer viewing sessions.

The caveat is that some people simply do not seem to get on with them and so they are not for everybody! Also, what applies to this particular combination may not work for other combinations of telescope and Binoviewer.

1. I cannot use the WO natively (without a GPC) due to the increased light path through the Binoviewers. I have been told, but have not tried it, that removing the extension tube from the 100DC will shorten the light path enough to use the WO natively (the bit ringed in red in the pic). The problem with this I suspect is that you may not then be able to focus easily in mono mode.


2. So to get it working I need to use the 1.6x GPC (that comes with the WO) attached to the diagonal nosepiece. The GPC threads do not match the Takahashi diagonal but I have successfully used both a Sky-Watcher 1.25" Di-Electric Star Diagonal and (as in picture) a Baader T2 Prism Star Diagonal.

3. The bits are (see pic below) - (A) Baader T-2 Prism Star-Diagonal 32mm (B) Baader T-thread to 1.25" Helical Focuser (C) Baader Nosepiece 1.25" - T2 which has the WO 1.6 GPC screwed into it. The Helical Focuser is a waste of time and would be better with a click lock. When you rotate the Helical Focuser the whole Binoviewers rotate! I also use this combination successfully on the FS60CB. I have not tried it with the FS60Q, FC76DC/Q.


4. I have the 2x GPC from WO as well. If I use it instead of the 1.6x in the Diagonal nosepiece I cannot reach focus. If I screw the 2x WO GPC into the Binoviewer nosepiece (as well as the 1.6x on the diagonal) I can double the magnification though I am told this is misusing the GPC, a dedicated barlow would be better. I now never do this and change the eyepieces when I want a different magnification.

5. Eyepieces. I use pairs of Tak 28mm Erfle, the standard WO 20mm, Tak 10 LE, 10mm Baader Classic Ortho, and Tak 7.5 LE. The 28mm and 20mm are always easy to use and rarely do I have image merge problems. I got a second Erfle to pair up with one I already had to try and get a wider FOV. I do get a wider FOV with them but probably not wide enough to justify the expense! I can usually merge the 10mm eyepieces but sometimes it takes a bit of tweaking. I have only managed to merge the 7.5's a couple of times so far but got a great view of the moon when I did. 7.5mm is generally considered too much. Many people would not recommend anything more powerful than 10mm and to use a barlow for extra magnification. I have never tried this, not owning a barlow!

6. Techniques. First get the focus right. Close one eye and use the scope focus to come to focus. Then close the other eye and use the eyepiece focus control on the Binoviewers to get a sharp image. Then open both eyes :) and adjust the Inter Pupil Distance on the WO to suite your eyes. Finally for me, in order to merge sometimes, I have to loosen the compression ring of the right eyepiece holder. If this fails I start fiddling with all the above and sometimes it just suddenly works!

I have found using Binoviewers a bit of a Black Art, takes a bit of experimentation to get right and, for me, much self questioning about whether something is possible, should be possible, do I just need to buy another bit ... But in the end, I think the view is worth it!



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Good to hear you are getting some nice views.

Like you say, getting the results you want usually means quite a bit of experimentation.

Its often quite tricky getting a refractor to focus natively : you usually need an 'imaging optimised' scope with a big focuser, lots of back focus

to accommodate accessories like filter wheels, then finding a binoviewing system with as short a lightpath as possible, and then having a good

focusing range so it also focuses at higher powers. 

Removing that extension tube will probably do the trick, but i understand it can be a faff in the dark screwing and unscrewing various sections.

Is there a way to couple the WO bino directly onto the prism like with Baader systems ?

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1 hour ago, Space Hopper said:

Is there a way to couple the WO bino directly onto the prism like with Baader systems ?

Good question! I've just had a look and the nosepiece does actually screw out of the binoviewers. I hadn't realised that. So with a bit of ingenuity it might be possible! I still think that in itself may not shorten the light path enough but definitely that idea is worth pursuing. Thanks!


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Super write up, really helpful especially as I’m currently practising with binoviewers and my Tak. I think I need some sessions with the moon or planets in the sky so I can play around with the BV settings and focusing.

I think I will upgrade to a Baader prism & clamp system eventually, it’s just not a priority at the minute. I need more premium eyepieces first 😅


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@IB20 glad it might be of some help. I floundered around for a lot of months trying to get to grips with it all. I'm sure I could do lots more experimentation but I've got a way of working now that works for me!


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On 22/04/2022 at 21:26, mikeDnight said:

If you remove the spacer between the scope and focuser, you can still reach focus in mono mode providing you use the black extension tubes supplied with the DC.

I have managed to get the WO to focus natively but it involved removing a lot of bits from the light path!

Removing all the extension tubes and basically attaching the diagonal pretty well directly onto the focus draw tube does the trick.

In the photo below, (1) is an Astro Essentials 56mm to T2 adapter. Unfortunately the thread spacing is not quite right for Taks (1mm rather than 0.75) so the fit is not great and I have not tightened it too much for fear of ruining the threads on the draw tube. (2) is a Baader Ultrashort 1¼" / T-2 Eyepiece Clamp.

So this works but is not ideal due to the incorrect M56 thread spacing.

However I had a quick look at the sun and the views were fabulous, particularly with the 28mm Erfles. The FOV is roughly doubled from using the 1.6 GPC and I'm looking forward to using this on some DSO and star clusters. The sun really looked fabulous, filling less than half the FOV (roughly) and it did have an almost 3D look to it.


I then did as @mikeDnight suggested, fitted one the the extension tubes and and was able to focus in mono mode. See next pic:


I think I can get a neater solution by using a Baader 2" Click Lock adapter with Takahashi M56 x 0.75 and then using a 2" to 1.25" adapter. I think this will not introduce too much light path. 

My only problem is I really dislike screwing and unscrewing the tubes and adapters every time I want to switch from Bino to mono. Apart from the hassle of doing this mid observing session I would be worried of damaging the threads on the draw tube - if it's never done it'll never break!

I think I'll give the Baader Click Lock and 2 to 1.25 adapter a go so I at least have the option but I suspect I will nearly always have it in the original configuration and use the 1.6 GPC.


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