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binoviewer friendly refractor?

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#1 jprideaux

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Posted 01 October 2019 - 10:32 PM

I see posts where people first mention their refractor and then ask whether it will work (without barlow or OCA for widest FOV) with a binoviewer while using a diagonal to achieve focus at infinity. 

 

In my case, I'm in the market for a 100mm to 127mm refractor and want to select one that is designed to already be binoviewer friendly.  Perhaps one with a tube section that can optionally be unscrewed (before the focuser) to shorten the tube for use with a binoviewer (or leave it screwed in for non-binoviewer use).  Do any such products exist?      



#2 petert913

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Posted 02 October 2019 - 12:49 AM

Just tonight I took a look through a friends Vixen 4” ED scope.  It had a Zeiss BV on a diagonal and the view was stunning.  No extended or Barlow needed.  



#3 emilslomi

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Posted 02 October 2019 - 06:54 AM

Check on the backfocus of the scope. That's also what a removable section changes. If it is 180 mm or more you should be fine with most regular binoviewers. Less than 160 will only work with the shortest combinations of diagonal and binoviewer. Many vendors list the available backfocus with the product description. If not, they should know or be able to provide it within a reasonable time frame. If still not, it's either a product not worth having or a vendor not worth dealing with.

 

Emil


Edited by emilslomi, 02 October 2019 - 10:17 AM.


#4 Eddgie

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Posted 02 October 2019 - 08:07 AM

First, even many of the refractors that are "Binoviewer Ready" will still require you to use a T2 diagonal with the binoviewer connected directly to the top of the diagonal (either a T2 ready or T2 modified).

 

Second, my advice would be to post a question like "show me your binoviwer ready refractor with the binoviewers on it.)  The reason I recommend that you do this is because the Binoviewer forum does not get enough traffic to get a speedy, in-depth answer to your question.

 

As to your question, here are the two refractors that I can tell you that I used that I was able to get to work with a binoviewer.

 

The ES 127ED reached focus with one of the focuser tube extender removed.  I was using the Baader standard Prism diagonal along with a Mark V and I only had a very few millimeters left, so this means that no all binoviewers would have reached focus.  For example, the Televue Bino Vue is T2 ready, but it has a light path that is around 130mm and I am not positive that it would have reached focus. I only had about 5mm left with the Mark V, and for the size prism, the Mark V has a tidy light path.

 

The next scope was the Stellarvue 110. This BV easily reached focus with any T2 prism and any T2 binoviewer I put on it. The SV110 comes with a 110 extension (as I recall) on it, and I took that off and tried to do it native, but that made for two problems and there is on important note on this configuration.  For this to work, I had to use a 2" diagonal.  There was not enough out-focus for the 1.25" diagonal (This could be solved with some kind of extension tube on the T2 diagonal, but I did not see the point of that.  The next issue was that the focuser tube was so far into the light cone that the field illumination falloff was quite high  Now the 2" diagonal is good news in that almost everyone has a 2" diagonal, so that means nothing else to buy, but again, my issue was that the field illumination was pretty high (vignetting).  Not everyone is bothered by field illumination falloff though.  If you are under very dark skies, you probably would not notice it so much, but under bright skies, you are viewing a bright hole through a dark doughnut.

 

The configuration I eventually used was to order a 55mm extension from Stellarvue (I think it was 55mm, not positive).  With this on, I was able to reach focus with any T2 setup and as I recall, I could even remove the BV and go to a 2" diagonal and still reach focus.

 

Now I have converted two refractors to be binoviewer ready and this is quite inexpensive. to do.  Both were 4" achromats and I simply cut about 2" off of the tube of each one.   I sold both of them and since they were inexpensive achromats, the suffered no penalty for the modification because in the end, both were bought by people that wanted a binoviewer ready 4" refractor. 

 

Not all larger telescopes are refractor ready.   If the scope was designed for imaging, then there is a good chance it will work.  Many of these scopes have extra in-travel to accommodate focal reducers and many have oversize focuser tubes to provide good illumination to large sensors.

I would be wary about the advice that almost all larger refractors will reach focus with binoviewer though because none of the larger telescopes I have ever owned would do so without using some kind of GPC. 

 

Again, I would recommend that you post this on the refractor forum. I know this seems like the right place for it, but this forum simply does not get enough traffic for you to hear about all of the candidates and hear about what configuration was being used.


Edited by Eddgie, 02 October 2019 - 08:09 AM.

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#5 Eddgie

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Posted 02 October 2019 - 08:08 AM

And in fact, if you ask for a picture, this gives a record for people that ask the same question in the future.

 

People love to show pictures of their refractors.. 



#6 jprideaux

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Posted 02 October 2019 - 04:50 PM

Thanks.  I'll post a similar question in the refractor forum in a day or so for specific working examples of refractors being used (with pictures) of their complete binoviewer setup (listing the refractor, diagonal, binoviewer, (and any other connecting information).  I'll wait a day just in case a moderator elects to move this question to the refractor forum.  If not, I'll post a new question.  



#7 noisejammer

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Posted 02 October 2019 - 05:07 PM

In the meantime, my FLT110 (with Moonlite CFL), APM 115/805 (with FTF3035) and TOA150 (with FTF3545) all allow binoviewer usage at unity power.

 

My setup goes focuser

- Baader 72mm - 2" ClickLock (50mm)

- Baader / Zeiss T2 Prism (42mm)

- Baader T2 Dovetail (10mm)

- Baader Mk V (110mm).

 

... in other words, I needed about 202mm on each scope. In practice, I set the system up with about 235mm which allows weird eyepieces like the 35 Masuyama to come to focus.



#8 Kunama

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Posted 02 October 2019 - 05:16 PM

Tak TSA and TOA have enough back focus (227mm+)



#9 REC

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Posted 03 October 2019 - 12:31 PM

I have a C102 with the Baader set up and I don't need anything to achieve focus. Do what Eddgie say's, he is a BV Guru!




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