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Using binoviewers for wide field


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I am trying to work out what is the widest field I could get with a binoviewer and my 4” F7 refractor.
 

I currently have a William Optics BV and the lowest magnification I can get with the 4” frac is around 70x using the supplied 20mm 66 degree eyepieces (the 1.6x GPC won’t reach focus, unless I put it before the diagonal which results in roughly 2x). I believe the supplied eyepieces are about as low power as you can get as the field stop of the BVs is quite narrow at about 20mm.

So I don’t think I can go any lower mag or wider field with those BVs. But is there another BV setup I could use which goes wider? I understand the Baader Maxbrights have a 27mm aperture but they still have prisms which means glass path correctors which increases focal length? There are also the mirror BVs which don’t appear to need a GPC/barlow (as sold by Bresser and under various other brands) and therefore any 1.25” eyepiece could be used, so in theory I could use 24mm 68 degree eyepieces and get the full 2.3 degree field at 30x? Is that correct? Are the mirror BVs the best ones to use for low power binoviewing? 

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How much more inwards focus do you need @RobertI? The benefit of Baader binoviewers is the ability to shorten the light party by direct connection to a Baader diagonal, and if you use a T2 prism they have a very short light path which helps further.

Personally I still prefer single eyepieces for widefield viewing but others prefer binoviewing so it’s all down to personal preferences.

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5 minutes ago, globular said:

The mirror BVs only have around 17mm clear aperture so your 24mm 68 degree eyepiece will vignette down to about 41 degrees. 

Maybe some do, but not all of them.

Do you mean the linear viewers like the Bresser ? That has a field stop of 22mm

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Another option might be to use a 2" focal reducer ahead of the binoviewer.  This would compress more field into the same image circle.  Some downsides would be an increased need for in-focus and outer field distortions.

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2 hours ago, Space Hopper said:

Maybe some do, but not all of them.

Do you mean the linear viewers like the Bresser ? That has a field stop of 22mm

Rob was asking about the Linear / Mirror Binoviewer "as sold by Bresser and under various other brands".

The Bresser are marketed as 22mm but:
1) people who own them and have measured them report they are actually around 17mm 
2) all the different brandings are actually the same design and most now accept and report them as 17mm 
(e.g. FLO's own StellaLyra version  https://www.firstlightoptics.com/binoviewers/stellalyra-125-linear-erect-image-binoviewer.html )

If you know of one that has been measured closer to 22mm I'd be very interested to know.

Edited by globular
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Thanks all for the really helpful comments @globular @Space Hopper @Louis D @Stu. To answer a couple of questions, from memory I think I literally only need a few more mm to get to focus with the 1.6x GPC but I just can’t see a way of doing it - I am already using a 1.25” prism diagonal with a short light path and I have screwed down the eyepiece holders on the BV. I’ll double check though as there might be another way to get a few more mm, although even if I could manage it,  it will probably still only get me down to 57x. I was hoping for more like 40x. 

Interesting that the mirror BVs only have 17mm aperture. Even so, I think l the absence of a GPC would still mean that a 16 mm eyepiece is getting me down to 45x which is more like was I was hoping for assuming the eyepiece has an decent FOV. Is that right? 
 

@Stu It’s entirely possible I may also end up preferring the cyclops wide field views, I just wish there was an inexpensive way to find out! 😆 Perhaps I should do some more critical comparisons of cyclops v bv  at my current 70x setup, to see whether the bino experience outweighs any downsides. From memory I don’t think stars are quite as pinpoint and the background is darker implying poorer transmission, but the BV experience is SO comfortable. The mirror BVs might have better transmission too? 
 

 

Edited by RobertI
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3 hours ago, RobertI said:

Interesting that the mirror BVs only have 17mm aperture. Even so, I think l the absence of a GPC would still mean that a 16 mm eyepiece is getting me down to 45x which is more like was I was hoping for assuming the eyepiece has an decent FOV. Is that right?

Yes that's correct. 
A 16mm eyepiece with 60 degree fov will work just fine.
You may even just about get away with something like the APM UFF 18mm (giving your target 40x ?) but it will only fully illuminate 55 degrees of the 65 degree fov, so there will be some vignetting in the outer 10 degrees.  Some people report that they don't mind it, while others don't like it.

 

3 hours ago, RobertI said:

I literally only need a few more mm to get to focus with the 1.6x GPC but I just can’t see a way of doing it

Removing the nosepiece from your WO BV and directly screwing it to a diagonal instead will save you those precious few mm.
The Baader BVs (and some others too) have adapter options to achieve this.  You'd have to do some (scary) hacking to achieve it with the WO.
Or you could try without a diagonal.... might not be practical?

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@RobertI I've a thread (don't know how to link to it) in the member equipment review section giving first impressions on the Stella Lyra linear Binoviewers (are these the mirror ones you refer to?) and comparing them to the WO ones.

I think I've been through the same process as you. Have the WO Binoviewers but wanted to use them without the GPC to get low power and wide FOV. I eventually got there with a Tak 100DC but it involved unscrewing just about everything that would unscrew from the OTA and replacing them with a couple of Baader pieces. Bit of a faff so I decided to try the Stella Lyra ones.

They do come to focus without a GPC. I haven't compared the largest FOV between them yet. I can try tonight if you'd be interested. However, unfortunately, I do not find the view through them to be as sharp and contrasty as that through the WO. 

It's a trade-off between a poorer view than the WO against being able to use them without a GPC. Another plus is I found merging the images in the SL easier than in the WO.

Hope this helps (rather than confuse!)

Malcolm 

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59 minutes ago, MalcolmM said:

@RobertI I've a thread (don't know how to link to it) in the member equipment review section giving first impressions on the Stella Lyra linear Binoviewers (are these the mirror ones you refer to?) and comparing them to the WO ones.

I think I've been through the same process as you. Have the WO Binoviewers but wanted to use them without the GPC to get low power and wide FOV. I eventually got there with a Tak 100DC but it involved unscrewing just about everything that would unscrew from the OTA and replacing them with a couple of Baader pieces. Bit of a faff so I decided to try the Stella Lyra ones.

They do come to focus without a GPC. I haven't compared the largest FOV between them yet. I can try tonight if you'd be interested. However, unfortunately, I do not find the view through them to be as sharp and contrasty as that through the WO. 

It's a trade-off between a poorer view than the WO against being able to use them without a GPC. Another plus is I found merging the images in the SL easier than in the WO.

Hope this helps (rather than confuse!)

Malcolm 

Link here:

 

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17 hours ago, Stu said:

How much more inwards focus do you need @RobertI? The benefit of Baader binoviewers is the ability to shorten the light party by direct connection to a Baader diagonal, and if you use a T2 prism they have a very short light path which helps further.

Personally I still prefer single eyepieces for widefield viewing but others prefer binoviewing so it’s all down to personal preferences.

@RobertI I found a thread which shows how you can attach Baader binoviewers directly to a Baader T2 prism to shorten the light path. Might be worth exploring in future.

 

DF1B8353-DDCC-4477-90E2-0910AD949A0B.jpeg

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5 hours ago, globular said:

Removing the nosepiece from your WO BV and directly screwing it to a diagonal instead will save you those precious few mm.
The Baader BVs (and some others too) have adapter options to achieve this.  You'd have to do some (scary) hacking to achieve it with the WO.
Or you could try without a diagonal.... might not be practical?

Thanks globular. My 1.25" prism diagonal has T2 connectors, but unfortunately I think the problem lies on the BV side - the BV thread into which the nosepiece goes is very small and non-standard (at least none of my numerous adaptors fitted!) so I can't see how I could connect the BV directly to the T2 connector of the diagonal.

2 hours ago, MalcolmM said:

 I've a thread (don't know how to link to it) in the member equipment review section giving first impressions on the Stella Lyra linear Binoviewers

Great review and very informative. Sounds like the SL BVs might not be the way forward, although FLO make it safe to try with their excellent returns policy. :) 

53 minutes ago, Stu said:

I found a thread which shows how you can attach Baader binoviewers directly to a Baader T2 prism to shorten the light path. Might be worth exploring in future.

Interesting. I had been investigating the Baader MBs anyway and they would probably attach nicely to my existing T2 diagonal. Interestingly, in that thread I said I could get 56x with my BVs, but I think that was theory, when I tried I believe I could not reach focus. 

I have managed to eek out another couple of mm on my existing setup by changing the 2" to 1.25" converter - I'll give it a go later to see if that helps. :)  

Edited by RobertI
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19 minutes ago, RobertI said:

I think the problem lies on the BV side - the BV thread into which the nosepiece goes is very small and non-standard (at least none of my numerous adaptors fitted!) so I can't see how I could connect the BV directly to the T2 connector of the diagonal.

I just received the adapter below for my TS/Omegon binoviewer.

https://rafcamera.com/adapter-t2f-m28-5x0-6-to-m26x0-75m

It fits the thread perfectly and has a T2 and a 1.25" filter thread. Be sure to measure the thread of your binoviewer first, as it might not be the same! The same website sells different adapters for other binoviewer threads. Delivery takes quite a while, but I believe it's fair for the price. It's really well made.

Ingen tilgængelig beskrivelse.

Here's the adapter mounted with a GPC in the 1.25" thread:

Ingen tilgængelig beskrivelse.

Now with an added T2 to 2" nosepiece, ND3.0 and continuum filter for white light:

Ingen tilgængelig beskrivelse.

Unfortunately it still doesn't reach focus for white light with my 2" Lacerta wedge, but the images are just to prove how versatile the adapter is. You can of course also screw the diagonal straight into the T2 thread for a budget/DIY Baader T2 thread system:thumbright:

Victor

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The shortest light path achievable with your scope would be to use Maxbright binos directly attached via T2 to a Baader 32mm prism diagonal fitted with a 2" nosepiece so there would be no need for the 2"-1.25" adaptor supplied with the scope. This set up may reach focus without GPC or barlow but if not then a Baader GPC 1.25x would probably do the trick. That said, eyepieces with a field stop greater than 21mm will vignette. There was an adaptor available and I think it was made in Russia which allowed the WO binos to be directly attached via T2, it was sold on Fleabay.

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Very unscientific FOV comparison. Tak 100DC + WO + 1.6x GPC + Tak 28mm Erfle (60°) the FOV was roughly one and a half moons. Same setup with the SL (no GPC) and the FOV was roughly two and a half moons. I did not notice (surprisingly) any vignetting though there was a blue ring round the very edge of the view.

Malcolm 

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Well something surprising happened last night. I tried the BV with the 1.6x GPC in the ‘proper’ place (attached to BV nosepiece) to see how many mm short of focus I was, and was amazed to find that it came to focus with about  4 or 5mm to spare. I don’t think my slimline 2” to 1.25” adapter was what solved the problem, so perhaps I cocked up when I first tried it. Anyway that’s great news as I can now observe at 57x and that is giving some really nice views of open clusters and Milky Way. I will try it on a dark night (sans moon) and try some other types of DSO, and compare to cyclops viewing to see how I get on.
 

Thanks @Victor Boesen and @Franklin for the heads up on the adaptor. I did try unscrewing the BV nosepiece and e/p holder on the diagonal, and held the BV directly against the diagonal prism to see if I might be able to get focus with the adapter and without the GPC - sadly as always seems to be the case, I was a few mm short , so I don’t think the adaptor will work without a GPC. 

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On 10/08/2022 at 20:59, Peter Drew said:

I found that my hacksaw solved all of these problems including air transport.   🙂

I think a hacksaw would certainly be a permanent solution one way or the other! I have an old Tal 100 RS which is probably a good candidate for shortening - trouble is with a FL of 1000mm I’d still be at 50x which isn’t exactly wide field!

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21 hours ago, MalcolmM said:

Very unscientific FOV comparison. Tak 100DC + WO + 1.6x GPC + Tak 28mm Erfle (60°) the FOV was roughly one and a half moons. Same setup with the SL (no GPC) and the FOV was roughly two and a half moons. I did not notice (surprisingly) any vignetting though there was a blue ring round the very edge of the view.

Malcolm 

At the moment I’m getting 1.2 degrees with the WO BV, and looks like I’d be getting 1.8 with the SL BVs which is a pretty decent improvement. 

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On 11/08/2022 at 21:49, RobertI said:

At the moment I’m getting 1.2 degrees with the WO BV, and looks like I’d be getting 1.8 with the SL BVs which is a pretty decent improvement. 

Some more unscientific tests; counting bricks on a distant chimney!

I compared the FOV of a number of eyepieces: mono, WO + 1.6x GPC, WO Native and SL.

The results are very rough but are similar to the first experiment so your quoted text would seem to be in the right ballpark.

- Native WO is very similar to mono (even with 28mm 60 deg Erfle which surprised me; I though I might have run into field stop issues)

- Native WO gives approximately a 10% wider FOV than SL. Not sure the reason for this.

- WO + 1.6x GPC is approximately 0.6ish the FOV of the SL. Maybe makes sense in that 1/1.6 ~ 0.6?

Again I stress - very unscientific and approximate!

In the absence of clear skies I'm studying chimneys! How sad is that :)

Malcolm

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2 hours ago, MalcolmM said:

In the absence of clear skies I'm studying chimneys! How sad is that

I've been trying out different eyepiece/barlow/gpc combos in my binos lately and to gauge the fov I've been counting cows bums. They all congregate around the feeder at the top of the field and stand still for ages. Reckon that wins on the sado scale!

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