Binoviewers on a dob?

[cajen2]

Is there any particular reason why nearly all BVs seem to be used solely on refractors and not dobs, apart from balance issues?

[Whistlin Bob]

It's usually because they will struggle to reach focus- the light usually need to go a fair bit further. 

I do use binoviewers in my 8" and 14" dibs, but to achieve focus I use slim barlows  that drop into the focuser (the 8" needs one, the 14" needs two).

[PeterC65]

I have the William Optics Binoviewer and while the views of any objects exhibiting structure (Moon, M42) are much better I think, achieving focus is very hit and miss.

With the refractor (and short 1.25" diagonal) I can only achieve focus with a Baader x2.25 Barlow (not with the included WO x1.6 Barlow).

With the Newtonian (6") I can achieve focus with the WO x1.6 Barlow (with no extension tube) and with the Baader x2.25 Barlow (with a 35mm extension tube), but not without a Barlow.

With the Mak I can always achieve focus with any combination, with and without Barlows and filter wheels!

 

[cajen2]

Thanks for that, WB. I have a 35mm focus extender plus a couple of Barlows....maybe that'll help.

Edited April 21, 2022 by cajen2

[cajen2]

Thanks, Peter. Our posts crossed. I suppose I'll suck it and see. I just wondered if there was any other reason.

[cajen2]

I have a Vixen 30mm NPL which is a great, cheap EP. With all these Barlows, etc, presumably it isn't a good idea to use too high a mag EP. Do you suppose buying another would do the job?

Edited April 21, 2022 by cajen2

[PeterC65]

Just now, cajen2 said:

I suppose I'll suck it and see. I just wondered if there was any other reason.

I have tested all of my scopes with all possible Binoviewer / Barlow combinations in daylight so that I know what will and will not work.

A Binoviewer on a longer Barlow hanging out of a focuser is quite a hefty thing that may unbalance a Newtonian or cause concern about the stresses on the focuser. I use screw in Barlows (both the WO and the Baader) which will pass right through the eyepiece clamp and sit inside the focuser so that the Binoviewer doesn't stick out so far. I also think you need to use short and lightweight eyepieces in the Binoviewer.

[Stu]

One of the main reasons I use binoviewers with my refractors is to reduce the visibility of floaters. These get to be a real problem with the small exit pupils you get at high powers in refractors. For example x200 in a 100mm frac is a 0.5mm exit pupil, definitely floater visibility territory if you suffer from them.

In an 8” dob you would still be at 1mm, and obviously the larger you go, the larger the exit pupil at a given magnification. So, as well as being harder to reach focus, binoviewers are perhaps just not needed as much in a dob.

A lot of emphasis is put on achieving focus with binoviewers natively. For me personally I find the cyclops views through a good widefield eyepiece at low power to be preferable to binoviewers; I think there is a purity to the stars that even excellent binoviewers don’t match. So I only really use mine at mid and high powers as they are more comfortable and give much better solar and lunar views. Planetary views I’m still 50/50 on and use whatever I feel like on the night.

Even more than eyepieces, I think binoviewers are a very personal experience and often take time to get used to, and to get the best out of.

[PeterC65]

2 minutes ago, cajen2 said:

I have a Vixen 30mm NPL which is a great, cheap EP. With all these Barlows, etc, presumably it isn't a good idea to use too high a mag EP. Do you suppose buying another would do the job?

Do you mean to use in a Binoviewer? I did lots of research before buying my Binoviewer and the opinion seems to be that most Binoviewers have a limited clear aperture so there is little point in going much lower than 20mm magnification, and that they don't work well at higher than 10mm magnification. They are quite sensitive to eyepiece positioning so I keep the WO 20mm eyepieces permanently in the Binoviewer and use Barlows to change magnification. I find the Binoviewer works best for seeing detail in objects with clear structure, so generally I'm wanting around x100 to x150 magnification.

 

[Stu]

One more thing, a counter balance at the bottom end of the dob can really help achieve balance when using a binoviewer.

[Stu]

1 minute ago, PeterC65 said:

Do you mean to use in a Binoviewer? I did lots of research before buying my Binoviewer and the opinion seems to be that most Binoviewers have a limited clear aperture so there is little point in going much lower than 20mm magnification, and that they don't work well at higher than 10mm magnification. They are quite sensitive to eyepiece positioning so I keep the WO 20mm eyepieces permanently in the Binoviewer and use Barlows to change magnification. I find the Binoviewer works best for seeing detail in objects with clear structure, so generally I'm wanting around x100 to x150 magnification.

 

That’s exactly what I do. I use 25mm eyepieces and then a combination of GPC, barlow and extension tubes to reach the desired mag. I find it much more comfortable and the images merge easily. 

[cajen2]

24 minutes ago, Stu said:

One more thing, a counter balance at the bottom end of the dob can really help achieve balance when using a binoviewer.

My StellaLyra 8" has adjustable alt pivots which I'm sure will help.

 

23 minutes ago, Stu said:

That’s exactly what I do. I use 25mm eyepieces and then a combination of GPC, barlow and extension tubes to reach the desired mag. I find it much more comfortable and the images merge easily. 

That's the sort of thing I had in mind.

My BVs are a cheap second-hand Celestron pair, so if they don't work, I'll just sell them on and hopefully not lose much.

Thanks, all of you, for your input.

[cajen2]

By the by, my 1.25" 'Barlow' is actually an ES 2x Focal Extender and I also have my StellaLyra 1.25/2" Barlow. Is one preferable to the other?

[jetstream]

The Binotron 27s focus in any scope including my dobs. The 15" sees the most use of them and on the moon and planets. Yes, the binoviewers can de collimate a dob through focuser flex, but knowing how far racked out you can go will mitigate the issue, same goes for using light eyepieces. I really prefer low power binoviewing on the moon and the planets see a modest 250-300x on them when high up.

I still prefer mono viewing for some things.

[Louis D]

If your Dob is balanced to handle 30mm 82 degree, 40mm 68 degree and 17mm 92 degree eyepieces that each weigh over 2 pounds plus a coma corrector, you'll do fine with a binoviewer loaded with a Barlow/OCS/OCA/GPC nosepiece and two compact 15mm to 20mm eyepieces that your nose will fit between.  I find that at 3x with the 2x Meade 140 Barlow nosepiece in my BV, I don't use my GSO coma corrector because it just adds axial spherical aberration without noticeably improving the edges.  This lightens the load on focuser a bit.

At an effective f/18, I find the SVBONY 68° Ultra Wide Angle 20mm eyepieces to work great:

 

[Don Pensack]

One other thing to consider is that:

--binoviewers with glass path corrector lenses typically generate high powers, so no low-power wide field views.

--binoviewers are often found on refractors because refractors are small scopes and users tend to concentrate on Moon, planets, double stars.

--binovewers lose a lot of light in the path (inches of glass, less than perfect coatings, and dividing the image) so are best on Moon, planets, double stars.

 

Now, a bino-scope is a different animal:

--low powers with wide fields are possible

--they work fine on faint deep-sky objects

--the light isn't divided between the eyes--each eye sees the full brightness.

 

Personally, rather than binoviewers in the scope, I'd look for a pair of binoculars with 80mm (or larger) lenses where they can accept 1.25" eyepieces.

But, if the whole point of binoviewers for you is Moon/planets/double stars, then they work great.  In most applications, they need a magnifying glass path corrector/OCA,SCT and MCT scopes being the exceptions.