Binoviewer & dob combo question

[BS269]

I’m thinking of buying a binoviewer to go with my 12” skywatcher flex tube dob (fl 1500mm). I know This combination would go great for planetary and lunar with the use of a Barlow to bring it to focus, but I would really like to try and utilise it for some DSO. Because of this I would preferably want one with a clear aperture of 27-30mm, allowing me to use the widest field 1.25” eyepieces.

i found this large prism binoviewer for a good price, which is important because I can’t afford the top end right now - https://www.teleskop-express.de/shop/product_info.php/info/p11593_TS-Optics-1-25--Wide-field-Binoviewer-with-30-mm-Prisms-for-Telescopes.html

Due to the design of the skywatcher flex tube I can shorten the OTA which should allow me to reach focus, which for this binoviewer would be 4-5”. But obviously with shortening the tube I lose some of the aperture, I just don’t know how much. Is there anyway to calculate how much I would lose? If it’s only 1-2” I would be happy going ahead with it, but any more and I feel like I’m losing too much light gathering capacity, given views through a bino are dimmer due to the light splitting also.

could anyone help with my aperture theoretical question? As it would be my first binoviewer purchase, if this wide field idea isn’t feasible I would rather pay low end like the WO and just use it for lunar/planetary purposes.
 

cheers!

[Mick H]

3 hours ago, BS269 said:

Is there anyway to calculate how much I would lose?

Hi, I cannot help with your question but this might help:

https://agenaastro.com/articles/guides/eyepieces-binoviewers/choosing-astronomy-eyepieces-for-binoviewers.html

I have the WO binoviewer, but wished I had gone for the Baader MaxBright's to pair up with the Baader T2 diagonal which would make shortening the light path easier like in the pic below.

 

[Louis D]

You keep all of your aperture but experience vignetting in the outer field by shortening your truss poles.

[BS269]

3 hours ago, Mick H said:

Hi, I cannot help with your question but this might help:

https://agenaastro.com/articles/guides/eyepieces-binoviewers/choosing-astronomy-eyepieces-for-binoviewers.html

I have the WO binoviewer, but wished I had gone for the Baader MaxBright's to pair up with the Baader T2 diagonal which would make shortening the light path easier like in the pic below.

 

I read this article yesterday, was a very interesting read but I’m not sure it has the info I need sadly. Did give me a lot of other knowledge though! This TS binoviewer I’m looking at says it has a T2 connection also! 

[BS269]

2 hours ago, Louis D said:

You keep all of your aperture but experience vignetting in the outer field by shortening your truss poles.

Is there a way of calculating how much vignetting there will be?

[Mick H]

BS269, Just trying to help and found another bit of info:

There are two types of vignetting to

consider -- vignetting at the center of

the field, and vignetting at the edge of

the field. For planetary work you only

care about the field center, but for other

stuff you care about both.

 

Vignetting at the field center does not

depend on the eyepiece focal length / field

size. It only depends on the scopes F ratio

and the size of the prisms. Most of the binoviewers

add about 5 inches to the optical path. So a

unit with say one inch prisms should be OK

with focal ratio 5" / 1" = F5 or slower.

For one of the binoviewers with small

prisms its 5" / (1/2") = F10 or slower.

 

Vignetting at the field edge is more complex.

It depends on the scope light cone, the size of

prisms in the binoviewer, and the size of the

eyepiece field stop. If the binoviewer is one of the

microscope derived units with 12mm prisms, and

you use an eyepiece with a 27mm field stop,

the field edges are going to be completely

vignetted (dark) no matter what the

scopes focal ratio is. Fast scopes will probably

have some vignetting at the field edge, even

with large prisms -- maybe 50% light loss at the

edge -- detectable but not terrible.

[Louis D]

The human eye is pretty tolerant of edge vignetting.  Even 50% is difficult to detect while looking in the center of a wide field eyepiece.  Complete vignetting generally happens when the clear aperture of the binoviewer is significantly less than the field stop of the eyepieces because they're optically very close together.

2 hours ago, BS269 said:

Is there a way of calculating how much vignetting there will be?

Try this calculator.

[BS269]

21 hours ago, Mick H said:

BS269, Just trying to help and found another bit of info:

There are two types of vignetting to

consider -- vignetting at the center of

the field, and vignetting at the edge of

the field. For planetary work you only

care about the field center, but for other

stuff you care about both.

 

Vignetting at the field center does not

depend on the eyepiece focal length / field

size. It only depends on the scopes F ratio

and the size of the prisms. Most of the binoviewers

add about 5 inches to the optical path. So a

unit with say one inch prisms should be OK

with focal ratio 5" / 1" = F5 or slower.

For one of the binoviewers with small

prisms its 5" / (1/2") = F10 or slower.

 

Vignetting at the field edge is more complex.

It depends on the scope light cone, the size of

prisms in the binoviewer, and the size of the

eyepiece field stop. If the binoviewer is one of the

microscope derived units with 12mm prisms, and

you use an eyepiece with a 27mm field stop,

the field edges are going to be completely

vignetted (dark) no matter what the

scopes focal ratio is. Fast scopes will probably

have some vignetting at the field edge, even

with large prisms -- maybe 50% light loss at the

edge -- detectable but not terrible.

Thank you very much! I think I may end up going for the smaller TS or WO now, as it’s just to get me started into binoviewing to see if I enjoy it. If I do I will end up upgrading to a much better one at some point I’m sure. I’m quite worried about the eyepieces though now. The WO bundle is a great price but I’m worried I’d buy it and the EPs that come with it won’t handle the fast focal ratio of my scope. If they wouldn’t, I would probably go for the TS one and buy EPs separately.

[Paz]

21 minutes ago, BS269 said:

Thank you very much! I think I may end up going for the smaller TS or WO now, as it’s just to get me started into binoviewing to see if I enjoy it. If I do I will end up upgrading to a much better one at some point I’m sure. I’m quite worried about the eyepieces though now. The WO bundle is a great price but I’m worried I’d buy it and the EPs that come with it won’t handle the fast focal ratio of my scope. If they wouldn’t, I would probably go for the TS one and buy EPs separately.

With regard to the focal ratio and if particular eyepieces will handle it, the focal ratio hitting the eyepiece will depend in of you are using a Barlow or glass path corrector. If say you have to use a 2x Barlow to reach focus then an f5 light cone from the objective becomes f10 coming out of the Barlow and would be less demanding on eyepieces. 

Edited August 22, 2020 by Paz

[Louis D]

And, the smaller binoviewers have a shorter optical path due to using smaller prisms.  Thus, there will be less vignetting if you shorten your truss poles to reach focus.

If you go the route of the barlow to reach focus, I use the optical nosepiece from a 90s vintage Meade 140 2x barlow to good effect.  It works out to a 3x boost.  At that focal ratio, the 23mm Vite/Svbony aspheric 62 degree eyepieces work very well in them for maximum field viewing.

[BS269]

3 hours ago, Paz said:

With regard to the focal ratio and if particular eyepieces will handle it, the focal ratio hitting the eyepiece will depend in of you are using a Barlow or glass path corrector. If say you have to use a 2x Barlow to reach focus then an f5 light cone from the objective becomes f10 coming out of the Barlow and would be less demanding on eyepieces. 

I’m more worried about not using a Barlow actually, as I would expect to not need one if I collapse the dob by 4 inches or so, hence keeping it at f4.9

i will use Barlows too; I already own a 2.25x so having a 1.6x would be a nice add on which should give me a few different magnifications!

[BS269]

3 hours ago, Louis D said:

And, the smaller binoviewers have a shorter optical path due to using smaller prisms.  Thus, there will be less vignetting if you shorten your truss poles to reach focus.

If you go the route of the barlow to reach focus, I use the optical nosepiece from a 90s vintage Meade 140 2x barlow to good effect.  It works out to a 3x boost.  At that focal ratio, the 23mm Vite/Svbony aspheric 62 degree eyepieces work very well in them for maximum field viewing.

I want to go down both routes really... it will give me a good range of magnitudes with a minimum of 75x for the stock EPs if I shorten the tubes. If I use my 2.25x Barlow and don’t shorten them I’m guessing it will give a 3.5-4x effect, which then Takes me to 260-300x mag. Or I could do a mix of both shortening and use the 1.6x Barlow, which (assuming it gives a 2.5x effect) takes me to 180x mag. And that’s just with one eyepiece pair! Obviously it isn’t going to be that easy sadly, as I don’t know how badly collimation will be affected.

i also own a baader Hyperion zoom mk III which could tempt me into buying another, though 2 of these plus a binoviewer could really strain my focuser. The mk IV was better designed for binoviewers but it is what it is.

i really wish I could just go straight for something like the binotron I’m that confident I’ll love them, but as I’m meant to be saving for travelling next year and not doing a very good job at it (because of this bloody hobby), I can’t really afford spending triple the price 😞

[Louis D]

Get a lower priced pair to start with to see if you even like using them.  Some folks just don't get on well with binoviewers no matter the type or brand.