6 replies to this topic

[quercuslobata]

Hi all,

 

I have some experience observing, but still just testing the water with a binoviewer. To give you a sense for where I am at, I have the very basic celestron binoviewer that I'd like to use with refractors (f6 to f7.5) and an f10 SCT. Most of the time I would plan to use a telecentric or barlow to achieve somewhere between 1.6x and whatever a 3x barlow in front of the binoviewer would get me, with I think a maximum effective field stop of around 18mm.

 

My questions:

 

Is there any advantage to using a 2" diagonal with a binoviewer given the relatively small field stops I'd be working with? I'm thinking about using a 1.25" diagonal to save weight but just wanted to see if I was overlooking something.

 

Mirror or Prism? Keeping in mind the low budget of this experiment is there any general recommendation? I have head that prism diagonals are better at f7 and up, but if I am always using a barlow anyway? or does the barlow have to go before the diagonal for this to work?

 

So far I have tried a 2" mirror diagonal and that seemed to work ok. I also tried the super basic celestron 1.25" prism diagonal at f7 with a 3x barlow.  It seemed like I had a harder time with eye placement and some weird effect that was like kidney beaning but I think caused by the prism and not the eyepiece. Just wondering if there is some theoretical reason this might be happening or if I just need a better prism diagonal.

 

Thanks

[Eddgie]

Since you don't need to use a 1.25" diagonal due to the fact you are using a telecentric Barlow, then I would go with the 2" diagonal.

 

This has nothing to do with optical quality or field illumination, it is solely to do with the fact that a standard 1.25" visual back rarely has the gripping power to keep a diagonal with a 1.25" nose from rotating in the visual back when carrying the heavy load of binoviewer and eyepiece. On an Alt-az mount this is not as much of an issue as it is on a GEM where you have to rotate the diagonal to get a comfortable viewing position. 

 

Now with the SCT, if you are going to use the 2" diagaonal, you would lose some aperture without the telecentric barlow, and how much would depend on the light path of your 2" visual back and diagonal, but typical loss would only be maybe .75".  This is not terrible but for high resolution observing such as lunar or planets, you would get your best result if you used the Barlow because this would give you your full aperture back. 

[betacygni]

For experimenting and getting your feet wet stick with what you have. If you do start going down the upgrade path the Baader t2 mirror diagonals are the best binoviewing diagonals available. They have a multitude of ways you can configure them, can be used with a 2” nosepiece for connection security, but are lighter than 2” diagonals, shorter light path, etc.

I do recommend generally sticking with mirror diagonals. Prisms can work, but you already have large prisms in binoviewers so you really have to be careful. Mirrors always work without complications, at any focal ratio.

Edited by betacygni, 25 February 2024 - 12:31 PM.

[skycreep]

For experimenting and getting your feet wet stick with what you have. If you do start going down the upgrade path the Baader t2 mirror diagonals are the best binoviewing diagonals available. They have a multitude of ways you can configure them, can be used with a 2” nosepiece for connection security, but are lighter than 2” diagonals, shorter light path, etc.

I do recommend generally sticking with mirror diagonals. Prisms can work, but you already have large prisms in binoviewers so you really have to be careful. Mirrors always work without complications, at any focal ratio.

For the SCT, isn’t it still better to “always” use a prism diagonal? Or is this just for mono viewing? Do the binoviewer prisms change that, such that a mirror would be better? Only reason I’m asking is because I would like to try binoviewing with my SCT and was thinking about the Baader T2 Zeiss spec prism diagonal with Maxbright 2...

[betacygni]

For the SCT, isn’t it still better to “always” use a prism diagonal? Or is this just for mono viewing? Do the binoviewer prisms change that, such that a mirror would be better? Only reason I’m asking is because I would like to try binoviewing with my SCT and was thinking about the Baader T2 Zeiss spec prism diagonal with Maxbright 2...

Some claim that prisms offer a bit less scatter when used as a diagonal, but this is a debated point. What’s not debated is prisms will introduce chromatic aberration and spherical aberration errors, made worse at faster focal ratios. The greater the size/length of the prism increases these aberrations. Binoviewers already introduce a fairly large prism, which is why it’s recommended to place some sort of amplifier/Barlow/glassopath/etc in front of them for high power viewing, which has the effect of slowing down the light cone, and making prism induced aberrations insignificant.

Trouble with adding a prism diagonal to the system is you now have more prism length the light needs to go through. Now with a significantly slow enough focal ratio scope, or a Barlow in front of the diagonal, this might not effect things too much, but there is the potential depending on your setup.

So that gets back to the idea that most consider high quality mirror and prism diagonals equivalent in performance (especially the newer Baader BBHS mirror diagonals), and why I wouldn’t recommend a prism diagonal for binoviewer use. If the only scope you use them on would be a slow SCT you’d probably be ok, but it’s just not warranted for a debated small benefit in my opinion (or if you end up with a faster scope at some point where it will definitely be an issue).

Edited by betacygni, 26 February 2024 - 04:18 PM.

[ABQJeff]

For the SCT, isn’t it still better to “always” use a prism diagonal? Or is this just for mono viewing? Do the binoviewer prisms change that, such that a mirror would be better? Only reason I’m asking is because I would like to try binoviewing with my SCT and was thinking about the Baader T2 Zeiss spec prism diagonal with Maxbright 2...

That is a perfect diagonal for an SCT.  Yes it better to use prism with SCT to reduce the optical path length (mirror can be a good 10-20mm longer than a prism).  F/10 ratio of SCTs is slow enough not to get CA from prism (unlike a refractor faster than F/7 where a mirror diagonal is definitely preferred for bright object/planet viewing).  The T2 Zeiss will also allow you to directly connect T2 capable BVs directly to the diagonal with a GPC inserted (again to reduce light path).

[noisejammer]

I mostly use a Zeiss-spec prism with my refractors. At f/7, I find that at low power (say 50x), the introduced CA is negligible. It becomes more obvious as I increase the magnification.

 

So that's when I use an extender or Barlow or extender. These switch the scope to ~ f/12 or f/16. This largely eliminates visible CA from the prism (up to 300x) but I still use  the GPC to correct the spherochromatism from my binoviewers. I find that atmospheric dispersion dominates things at 300x.

 

As for contrast and image quality - my Zeiss spec & Tak prisms easily outperform all my other diagonals (Astro-Physics dielectric, Tak dielectric & two Baader Maxbrights) .

 

The sole exception is the BBHS mirror which I would call a very close draw.