Binoviewer in SCT

[globular]

I’m contemplating trying binoviewers and I have a SCT.
Great, I’ll have none of the focusing issues often reported with binoviewers; there is lots of focus travel in my SCT.
But wait. If I take the baader maxbright mark II as an example; this has a light path of 110mm. And adding light path to a SCT adds focal length. So my F/10.6 is going to be a F/12.3 when binoviewing.

Boring maths bit:
*  8” EdgeHD has published fl of 2032 and F ratio F/10
*  But this figure is only obtained when using the stock visual back and diagonal that celestron fitted to their original C8s - light path of which was 103.4mm
*  With the EdgeHD stock diagonal (light path 137mm) the fl is actually 2032 + 3.1 x (137 - 103.4) = 2136.3 and so it’s really F/10.5
*  I have a clicklock visual back and a 2” prism diagonal with combined light path of 143.4; so I have fl 2156 or F/10.6.
*  adding binoviewers with extra 110mm of light path will add  3.1 x 110 = 341mm to focal length; so fl is 2497 or F/12.3

Now, I could seek out a 1.25” diagonal when using the binoviewers. So I could save maybe 50mm light path.
This puts me at 2497 - 3.1 x 50 = 2342 fl or F/11.5.  Still a lot different to my cyclops configuration.

But then so what? Does the F ratio moving a bit really matter?  I can just pick different fl EPs to get the magnification I want, right?
Or will it do other nasty things too?  
The larger focus movements required will move the primary mirror a lot closer to the secondary… that doesn’t sound good in terms of contrast or aberrations?
Will that spoil the benefits of binoviewers?

Does the Bresser Binoviewer Deluxe solve these issues? (If they really are issues?)
This binoviewer claims; 
“Thanks to the newly patented optical system you can get into focus with any telescope. 
Since the BRESSER BinoViewer Deluxe does not require any additional light path (back focus), it can be used with any telescope - including Newtonians and short focus refracting telescopes, which traditionally suffered when using standard binoviewers. No light path corrector or similar accessory is necessary. If a standard 1.25'' eyepiece can be focused, the binoviewer will also work. The eyepiece focal length and field of view size of the eyepieces used are fully retained”

Maybe this is the binoviewer I should try in my SCT?
Anyone already tried it?

Sorry for the long post.  Hopefully some of you will have got this far and can put me straight on any mistakes in my logic and / or on the workability of binoviewers in SCTs.
 

Edited August 25, 2021 by globular

[jetstream]

12 hours ago, globular said:

Maybe this is the binoviewer I should try in my SCT?

I bought the Binotron 27s and couldnt have been happier, no calculating just plug and play in my truss dobs and refractors. The prisms are top notch, show almost no polarization and have a nice opening.

https://www.denkmeier.com/creatures

Edited August 25, 2021 by jetstream

[Louis D]

Yes, moving your mirror forward to reach focus with binoviewers will add some spherical aberration to the image.  You'll probably never notice it at lower powers, but you might notice it at higher powers.  In that case, you might want to swap in a Barlow element to reach focus instead of moving the mirror forward.  It has the added benefit of allowing you to use your lower power eyepieces at higher power.  That, and it can be difficult to merge high power eyepiece views due to slight collimation issues in the binoviewer not visible with lower power eyepieces.

[globular]

1 hour ago, jetstream said:

I bought the Binotron 27s and couldnt have been happier, no calculating just plug and play in my truss dobs and refractors. The prisms are top notch, show almost no polarization and have a nice opening.

https://www.denkmeier.com/creatures

Thanks @jetstream. 
It looks a lovely instrument and I like the power switch and the filter slider option too. 
It's a bit expensive though.  I would definitely have to have tried something else first to make sure I get on with binoviewers before laying out over £1200.

Also, while they do a SCT 'version', I think this is simply a minimised light path option (with their own diagonal and connectors) rather than something that prevents the need for lots of inward focus movement (and hence large primary mirror movement).
Indeed their own FAQs state:
"The collimating of your SCT must be done with the binoviewer in place if the best possible images are to be produced. This is because the primary mirror of your SCT must be moved toward the secondary convex mirror (negative) in order to push the focal out of the exit aperture of the tube assembly in order to reach focus with the binoviewer/eyepieces. I have seen a very big difference in the quality of planetary images after fine tuning the collimation of our company's Celestron SCT with a Denk Binoviewer in place. The reality is that the scope becomes a different instrument when switching from single eyepieces to binoviewer usage. This must not be ignored if you want the best images possible with the binoviewer setup."

I don't want to have to collimate my SCT when switching between cyclops and binoviewers!
This issue is not just with the Denkmeier, of course.  Nearly all binoviewers will require significant primary mirror movement to achieve focus.
All except the Bresser Binoviewer Deluxe, which, if I'm reading their claims correctly, do not move the focal point when inserted into any telescope. 
So my perfectly collimated scope in cyclops mode will also be optimal for binoviewing. 
And no big mirror movements mean no added spherical aberration (thanks @Louis D for confirming this).

I just don't know what the Bresser glass is doing to achieve this.... is it really going to achieve this without introducing aberrations itself?
It feels too good to be true.
Anyone know how they've done it?

I'd love to hear from anyone with one.... whatever scope they use it in.
@Knighty2112 have you had first light with yours yet?

 

[jetstream]

44 minutes ago, globular said:

I don't want to have to collimate my SCT when switching between cyclops and binoviewers!
This issue is not just with the Denkmeier, of course.  Nearly all binoviewers will require significant primary mirror movement to achieve focus.

Yes, checking this out is a must for sure- when I tried to figure out how to focus my truss dobs I gave up and called Russ...we checked focus points for a reference EP (21E) and he made a custom adapter extension for free. I then ordered his system and its truly plug and play.

I figured there might be some info on his site that could be of use.

[Louis D]

1 hour ago, globular said:

I'd love to hear from anyone with one.... whatever scope they use it in.
@Knighty2112 have you had first light with yours yet?

You could start by reading this mini-review of the Orion equivalent by Tom Dey (a retired optical designer) on CN.

[Deadlake]

The linear Orion BV does look a good option if you do not want to modify the light path length. @Louis D beat me to the CN review post.

Do all SCT, Cassiegrains and CATS need to be collimate when using a BV?
 

[Louis D]

I know I'm not going to mess with the near perfect factory collimation of my 127 Mak for BV usage.  It works well enough as-is for my purposes.

[globular]

54 minutes ago, Deadlake said:

Do all SCT, Cassiegrains and CATS need to be collimate when using a BV?

When these scopes are collimated everything is optimised at that primary mirror position (i.e. the position during collimation).
Focusing different EPs, Binoviewers, Cameras, etc, is achieved by moving the primary mirror and things start moving away from optimal.
Most EPs, even when not par focal, don't require that large a change of focus; so you don't notice any difference.
Add something to the light path that significantly moves the focal plane and you might start to notice.
A (conventional) binoviewer falls into this category.  The new(ish) linear binoviewers don't.

What I don't know.... hence this thread..... is how bad it is in reality.  
I suspect / hope it's not that bad.
I certainly won't be fiddling with my collimation to fix it - so if it's bad I won't use one.  (or maybe I'll try a linear one).

[globular]

12 minutes ago, Louis D said:

I know I'm not going to mess with the near perfect factory collimation of my 127 Mak for BV usage.  It works well enough as-is for my purposes.

My SCT came perfect from the factory and has stayed that way since - I check it regularly but have never needed to tinker. 
I don't intend to start now for a binoviewer - but would like to give one a try.

[jetstream]

5 minutes ago, globular said:

but would like to give one a try.

Years ago YKSE used a Maxbright in his C8, Yong is a very precise astronomer and it might be worth looking up his threads, cant remember them specifically offhand. Not active right now I dont think unfortunately.

[globular]

27 minutes ago, jetstream said:

Years ago YKSE used a Maxbright in his C8, Yong is a very precise astronomer and it might be worth looking up his threads, cant remember them specifically offhand. Not active right now I dont think unfortunately.

Thanks Jetstream

I found a few of his threads.

There is one where he painstakingly measures all combinations of different visual backs and diagonal, with and without binoviewer.
Good to see his measurements give the same as my formulaic theoretical approach.

And another where he lists all his baader accessories that he accumulated in search of the shorted path possible for use with binoviewers.

Looks like he got it working but only by keeping everything as short as possible.
In other words I suspect he could tell the difference if things were too long. 

[jetstream]

18 hours ago, globular said:

Looks like he got it working but only by keeping everything as short as possible.
In other words I suspect he could tell the difference if things were too long. 

Yes, he did end up getting excellent views. He was also top notch on how eyepieces work and their design and recommended the VIP barlow to me which is beyond compare in the barlow world.

I'm curious what you come up with and the views you achieve- I wonder if you can "lock" the mirror and focus using a short focuser or something? I would never touch a well collimated SCT...

Maybe keep us updated here?

[MarkRadice]

On 25/08/2021 at 02:21, globular said:

Maybe this is the binoviewer I should try in my SCT?
Anyone already tried it?

I bought the Linear Binoviewer - although I gotmine from Telescope Express in Germany. 

First impressions are favourable.  I put together a short review on youtube and summarised my thoughts below: 

-  It arrived in a great pelicase with space for a number of eyepiece pairs. 

 

- I love the fact it does not need a corrector to reach focus.  If your eyepiece can focus, the binoviewer will focus. 

- That being said, I'm not sure I like the reflections looking at bright objects.  It takes some getting used to for sure. Picture of Jupiter below snapped with smartphone.

 

- On the other hand, for deep sky observing, it's a winner.  Quite a pleasant way to observe!

The review is here for further interest: https://youtu.be/ExdkGG9MXRM

Spoiler - the binoviewer is quite good!

[globular]

3 hours ago, MarkRadice said:

I bought the Linear Binoviewer - although I gotmine from Telescope Express in Germany. 

First impressions are favourable.  I put together a short review on youtube and summarised my thoughts below: 

-  It arrived in a great pelicase with space for a number of eyepiece pairs. 

 

- I love the fact it does not need a corrector to reach focus.  If your eyepiece can focus, the binoviewer will focus. 

- That being said, I'm not sure I like the reflections looking at bright objects.  It takes some getting used to for sure. Picture of Jupiter below snapped with smartphone.

 

- On the other hand, for deep sky observing, it's a winner.  Quite a pleasant way to observe!

The review is here for further interest: https://youtu.be/ExdkGG9MXRM

Spoiler - the binoviewer is quite good!

Thanks Mark.  Great to hear from a user of the linear binos.

Which brand did you get?  Is it the TS Optics one?
There was some chatter in another place that the 'cheaper' ones suffer more with reflection than the more expensive Orion one.
"I tried the $399 model. Sent it back. Bought the Orion. Less reflections and spurious color."
Not sure if there is in fact any technical differences between the different brands.... I understand they are all built in the same factory.... I'd have thought it's just the branding that's different.  

I am really tempted by this binoviewer.... avoiding moving the primary a long way to focus being it's biggest draw.
However I suspect this design would be better in a shorter focal length telescope than mine.... where the 17mm field stop will not be as restricting as it would be for for me.  The maximum TFOV I'd get with them is 0.45 degrees.  And the biggest exit pupil 1.9mm.
And if I can't look at bright stuff either because of reflections.... well it all seems a little too compromised.

<shrug>

 

Edited August 26, 2021 by globular

[globular]

4 hours ago, jetstream said:

Yes, he did end up getting excellent views. He was also top notch on how eyepieces work and their design and recommended the VIP barlow to me which is beyond compare in the barlow world.

I'm curious what you come up with and the views you achieve- I wonder if you can "lock" the mirror and focus using a short focuser or something? I would never touch a well collimated SCT...

Maybe keep us updated here?

I did think I could collimate for the binoviewer and then add an extension into the light path for cyclops viewing...  similar to your locking the mirror idea.
But I soon decided against it.  2125mm F/10.5 is where you should be using an 8" EdgeHD and I just can't bring myself to deviate too far from this optimal position.

I'll keep this thread going with my progress.

[jetstream]

37 minutes ago, globular said:

I'll keep this thread going with my progress.

Thanks, I like to loosely keep track of solutions in case they are needed. I was looking at C8's a while back to purchase.

[Deadlake]

Good thread here on SCT and Orion linear BV:

https://www.cloudynights.com/topic/782572-c925-or-c11-and-orion-linear-binoviewers/

This part:

"I can’t comment on 11” vs 9.25” but I can say that you need to choose eyepieces carefully for the linear. Keep the eyepiece FL x afov at or under 1000. Stated a different way, make sure the eyepiece field-stop is at or under 17.4mm. For example, I use 18mm afov 52 degree Baader orthoscopics with my linears and they work well. Others have used 25mm 40 degree eyepieces and liked those. I find that eye placement becomes more finicky with shorter focal-length eyepieces. I would recommend spending some time with an online fov calculator to see how various objects would show up with either the 11” or 9.25” scope for your intended eyepiece. It is possible to use both a reducer and an extender with the linear. I typically use my linear with lower powers and go to a different binoviewer when going for higher powers."