Jump to content


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Extending my binoviewing to a larger Dobsonian

  • Please log in to reply
10 replies to this topic

#1 me42


    Lift Off

  • -----
  • topic starter
  • Posts: 16
  • Joined: 25 Nov 2019
  • Loc: Germany

Posted 10 April 2021 - 08:05 AM



I am currently using a William Optics binoviewer with 8" f/5 and 12" f/5 Dobsonians. For glass path correction I use the excellent comacorrecting Gerd Düring Barlow lens element from APM, which provides a 3x magnification factor for the optical path length of the WO binoviewer. So far, I use the binoviewer only for planets and the moon with eyepieces in the 15 - 25mm focal length range.


I would like to expand my binoviewing in two ways:
1. use the binoviewer with larger Dobsonians: 16" f/4.5 and 18" f/4.
2. use the binoviewer for the brighter DSOs at magnifications down to about 100x.


I will need a glass path corrector with a lower magnification factor for these objectives. It should also correct or at least reduce the coma at f/4.5 - f/4, since my understanding is that combining the binoviewer with a standard coma corrector is not possible.


After some research I see two options:

(a) Baader 1.7x Newtonian glass path corrector.
(b) Siebert Optics OCA.


Now I am undecided which of the two options might be best:


Option a:
+ Provides coma correction

- Requires 32mm backfocus, which I can provide by slightly reducing the pole lengths of the (ATM) Dobsonians.
- Has a T2 thread that cannot be connected directly to the WO binoviewer. I need to unscrew the 1.25" nose piece of the WO and try to find an adapter from the unknown thread of the WO to T2.


Option b:
- It is unclear to me whether the cheaper Siebert OCA #1a will work well at f/4. I got no response from Harry Siebert on this subject. I might need the more expensive #3B OCA.

- does not provide coma correction


Your advice would be greatly appreciated.

Best regards,


#2 Second Time Around

Second Time Around

    Viking 1

  • -----
  • Posts: 603
  • Joined: 08 Oct 2019
  • Loc: Rural Kent, UK

Posted 10 April 2021 - 08:20 AM

Would the new APM 1.5x coma correcting telecentric Barlow be a possibility? It's a 2 inch rather than a 1.25 inch.

Edited by Second Time Around, 10 April 2021 - 11:03 AM.

#3 Spikey131



  • *****
  • Posts: 2,129
  • Joined: 07 Feb 2017

Posted 10 April 2021 - 08:48 AM

1.  Here is the adapter:  https://rafcamera.co...r-t2-female-m26


2.  At some point (around 2/2020), William Optics began gluing the threads of the nosepiece to the binoviewer body, making it difficult, if not impossible to use adapters.  Just make sure your nosepiece threads off before proceeding.  https://agenaastro.c...-6x-barlow.html


3.  I would go with Baader for the coma correction in your fast scopes.


4.  Before cutting any trusses, see if you can move your mirror back enough with your collimation screws.

#4 wcw


    Vostok 1

  • *****
  • Posts: 189
  • Joined: 14 Jun 2012
  • Loc: Red Hook, NY

Posted 10 April 2021 - 10:09 AM

I use the Siebert 1.3x OCA, attached directly to TV Binovue with 24 mm Panoptics, on my 12” f4 dob, and I see no coma at all. Provides beautiful wide fov.


The Siebert model I use is under Add#3, model B3. Model number 2OCA13x38


I did have to move my primary mirror up a little bit (less then 1/8") to allow clearance for a filter slide.



Edited by wcw, 10 April 2021 - 10:27 AM.

#5 me42


    Lift Off

  • -----
  • topic starter
  • Posts: 16
  • Joined: 25 Nov 2019
  • Loc: Germany

Posted 10 April 2021 - 01:37 PM

Thank you very much for the helpful answers!


The reference to the T2 to M26 adapter is excatly what I was locking for and the seller also offers shipping to me in Europe at a moderate price.


I was already aware of the new coma correcting 1.5x APM Barlow, but somehow did not think about it's OCA application. I took a closer look at the specs now and yes, it seems to be even more suitable than the Baader GPC, because the magnification factor is a bit lower and with 105mm (in contrast to 80mm for the Baader) it practically compensates the whole light path of the binoviewer. The only disadvantage seems to be the twice as high price.


Interesting that the Siebert Optics OCA (2OCA13x38) shows no coma on a f/4 dob. On the website it is described as equally suitable for reflectors and refractors. Then it should not include any coma correction, because that would introduce "negative" coma on a refractor as I understand it.

#6 jmillsbss


    Viking 1

  • *****
  • Posts: 674
  • Joined: 03 Dec 2018
  • Loc: New Albany, Mississippi, USA

Posted 12 April 2021 - 08:56 AM

Matthias  -  THIS  VVVVVVV




Let me see if I can be brief.  I had the WO/Stellarvue BV'er and loved it!  Used the Seibert 1.3x GPC/OCA


The views w/ the Stellarvue BV3s were dim in my 10" dob.  Enter the Orion offering....  No beamsplitter.  Mirrors AND lenses, no prisms, bright mirrors and efficient light-pass optics, to get everything corrected at the actual native focal plane, hence 1x with any eyepiece ( with <22mm field stop ) in any type telescope.  Yes.  Nothing to add but eyepieces.  No OCA, GPC or any limitation on type of format OTA.  Instead of each eye getting half the illumination, each eye gets half the field (L/R) illuminated at 100%.  I don't fully understand it, but it works, and it's much brighter than the SV BVers.


I use the ExpSci coma corrector and see very little coma at the very edge with a f/4.7 scope, although I also suffer from astigmatism and use the Televue DioptRx, so I can't be sure that they're always set just perfect.  They're still kinda new to me, but it's certainly not a deal breaker for me.


The views through my Delos 17.3mm are bright and clear, and merge like a dream.  I've never looked through Denks, so I can only tell you what I've experienced.  These things are GREAT.  Open and globular clusters just float there in the dark but starry field.

They appear to work well with focal extenders, like my powermate and ES 3x FE.


The only drawback is the weight, but counterweights can solve that easy enough.  If you get a chance, get some looks through the Orion linear BVer.  It's very, very satisfying.

  • Stevenkelby likes this

#7 scarubia


    Viking 1

  • *****
  • Posts: 578
  • Joined: 03 Jul 2011
  • Loc: Toronto, Canada.

Posted 12 April 2021 - 06:08 PM



I have been messing around with all sorts of GPC's, OCA's, Barlows, Magic barlows, etc. for years.  I am currently using a TV Bino, and assessing/ evaluating a CZAS. 




-The Siebert units do not correct for coma. But they do work as advertised. The newer 38mm units work down to f2.9 - according to Harry. They work just fine in my f4.3.


-The 1.7x Badder Newt Coma correcting GPC and the APM 1.5x Coma Correcting Telecentric use up the same amount of back focus. Both do a good job of coma correcting....if you can get them to focus in your scope. I just made it with collimation bolts fully deployed in both scopes.  


-An option not mentioned, and the best combination I have found so far for coma correction, ease of use, is the TV 2x Bino Amp in combo with their paracorr, it comes to focus without having to do anything to your scope. A "neat" and compact solution. 


I'm not sure if the WO bino can be mated with the TV 2x AMP but worth consideration.


I have mated the TV 2x bino amp to the CZAS as well.

Edited by scarubia, 12 April 2021 - 06:53 PM.

  • Second Time Around likes this

#8 me42


    Lift Off

  • -----
  • topic starter
  • Posts: 16
  • Joined: 25 Nov 2019
  • Loc: Germany

Posted 17 April 2021 - 03:58 AM


the Long Perng (aka Orion Linear) binoviewer is definitely an interesting option that I would like to take a closer look at. However, I don't want to buy it unseen, so I'm waiting for an opportunity (probably after COVID) to have a look at it myself.



thank you very much for sharing your insights on the difference GPC's/OCA's!  Seems like there are even more options than I had assumed. For the interesting TV 2x AMP with Paracorr combination, I would also have to switch coma correctors because I am using the GSO coma corrector right now. Somehow the choice doesn't get any easier, but I am now leaning towards the Baader 1.7x GPC, not least due to the moderate price.

  • scarubia likes this

#9 Lentini


    Mariner 2

  • *****
  • Posts: 204
  • Joined: 18 Jun 2012

Posted 17 April 2021 - 09:59 PM

Where are you located? If you’re near me, you are welcome to give mine a try. (Washington DC area.)

#10 me42


    Lift Off

  • -----
  • topic starter
  • Posts: 16
  • Joined: 25 Nov 2019
  • Loc: Germany

Posted 18 April 2021 - 06:27 AM

Lentini, thank you very much for the kind offer. Unfortunately you are too far away as I am in Germany. My last visit to the Washington DC area was 10 years ago.

#11 Lentini


    Mariner 2

  • *****
  • Posts: 204
  • Joined: 18 Jun 2012

Posted 18 April 2021 - 07:47 PM

Es tut mir leid! My last trip to Deutschland was in the 80s, so I’m overdue to return. 

  • me42 likes this

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Recent Topics

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics