Baader T2 Zeiss Prism vs Baader 2 inch Zeiss Prism Diagonals

[Stu]

21 hours ago, globular said:

so your focal length will change a bit.

Optical path length will change, but not focal length. That’s only the case with SCTs.

[John]

1 minute ago, Stu said:

Optical path length will change, but not focal length. That’s only the case with SCTs.

That's what I thought as well.

 

[globular]

Thanks Stu. I was hoping someone would step in and educate me on this - I've learnt a fair bit on SCTs but not the other designs.

[Stu]

45 minutes ago, John said:

True. I have a Vixen ED102SS F/6.5 as well as the Tak and that does the very wide thing even better of course.

I think I'm talking myself into adding 2 inch fittings to the T2 prism, limiting myself to the 21 Ethos with the Tak at the low power end.

 

I think that might be the most sensible option John. The DL is not really a widefield scope, you would use the Vixen for that I assume? Somehow I think the T2 prism suits the DL better too. 2.3 degrees isn’t bad either though is it?

The other point to make is that up to a degree vignetting is not that obvious visually. I’ve used 2” eyepieces in an OMC140 Mak which would have been quite heavily vignetted by the baffle tube aperture but the results were actually quite acceptable to me.

[JeremyS]

59 minutes ago, John said:

True. I have a Vixen ED102SS F/6.5 as well as the Tak and that does the very wide thing even better of course.

I think I'm talking myself into adding 2 inch fittings to the T2 prism, limiting myself to the 21 Ethos with the Tak at the low power end.

 

Would save you a few pennies to redirect to other Astro goodies, too 🙂

 

[Deadlake]

19 hours ago, John said:

Well, I think would base it on the largest eyepiece field stop that I would be using.

It's not always easy to find that out but it can be tracked down with a bit of googling.

This spreadsheet has an estimated field stop.

https://docs.google.com/spreadsheets/d/1uriY-G3PT6K81bJYZV5pudGB9Ms9FSbOlKq6Cj3AOaM/edit?usp=sharing

putting in a 795 mm focal length f7.7 and a 1020 mm focal length f 9.2 and its hard to get a field stop > 20 mm. 

The smaller aperture should be fine.

 

Edited October 14, 2020 by Deadlake
Changed to google spreadsheet as you can specify field stop.

[John]

1 minute ago, Deadlake said:

This spreadsheet has an estimated field stop.

https://www.cloudynights.com/topic/700069-2020-eyepieces-buyers-guide/#entry10080606

 

 

Yes, Don's annual spreadsheet is a very useful source of information

[Deadlake]

Any ideas on difference in cool down between Baader prism and mirror diagonals? Mirrors in this case will be faster then the prism, but buy how much?

Edited October 16, 2020 by Deadlake

[Luminated]

Hi John, did you go for the 2” prism finally? 
I was considering this option too, for “future proofing”, but cannot seem to find a matching 2” helical focuser?

Wondering if you came across something suitable?

Thanks for any advice!

[John]

Not yet. Other priorities have taken over so I'm still using the Baader T2 Zeiss prism diagonal with my Tak.

 

[Deadlake]

Revising this thread. I have a APM XWA 17, 20 mm and 30 mm UFF EP’s. I find using a Baader 2” dialectic not enough in-focus available to gain focus. I note that the Baader 2” dialectic and BBHS mirror diagonals have an optical path length of 115 mm whereas the 2” prism has an optical path length of 100 mm. I’m hoping the 15 mm decrease in path length of the prism diagonal will be enough to gain focus with the XWA and UFF EP’s. 

Does anyone have any experience of this?

 

[globular]

I have the Baader 2" Prism diagonal and like it very much.

It does indeed have a mechanical light path of 100mm which is 12mm less than the Baader 2" Mirror at 112mm - however light passing through the prism (rather than reflecting off a mirror) does use up some of the back focus; but only a very small amount.  I'd reckon you'd save around 10mm over the mirror diagonal.

Have you considered the Baader 2"/90° Astro Amici-Prism with BBHS coating?
https://www.firstlightoptics.com/diagonals/baader-290-astro-amici-prism-with-bbhs-coating.html
This has a light path of 85mm - so you'd get another 15mm over the 2" Prism and 25mm or so over the 2" mirror.
Not sure how compromised the view is though, in order to achieve the erect image; and it is quite a bit more expensive.
All depends how much focus travel you need to achieve.

You're right to stick with 2" though - a T2 diagonal does have a shorter light path but does not have enough clear aperture (33mm) for your 30mm UFF (38mm).

[John]

On 25/11/2020 at 17:35, John said:

Not yet. Other priorities have taken over so I'm still using the Baader T2 Zeiss prism diagonal with my Tak.

 

This is still my situation so I can't help on the 2 inch versions.

Still using my 1.25 inch eyepiece set with the Tak - I do think that they suit the F/9 scope well and the 24mm Panoptic gives a 1.81 degree true field which fits most things in.

I have other options if I want to go wider .....

 

 

[Deadlake]

On 29/05/2021 at 19:01, John said:

This is still my situation so I can't help on the 2 inch versions.

Still using my 1.25 inch eyepiece set with the Tak - I do think that they suit the F/9 scope well and the 24mm Panoptic gives a 1.81 degree true field which fits most things in.

I have other options if I want to go wider .....

 

 

Did the moonlite focuser you outfitted your Vixen with reduce the optical path length? 

Another valid approach?

[John]

8 minutes ago, Deadlake said:

Did the moonlite focuser you outfitted your Vixen with reduce the optical path length? 

Another valid approach?

The Moonlite, used with the Vixen flange, was pretty much the same optical length as the original Vixen focuser.

 

 

Edited May 30, 2021 by John

[Deadlake]

On 29/05/2021 at 18:00, globular said:

I have the Baader 2" Prism diagonal and like it very much.

It does indeed have a mechanical light path of 100mm which is 12mm less than the Baader 2" Mirror at 112mm - however light passing through the prism (rather than reflecting off a mirror) does use up some of the back focus; but only a very small amount.  I'd reckon you'd save around 10mm over the mirror diagonal.

Have you considered the Baader 2"/90° Astro Amici-Prism with BBHS coating?
https://www.firstlightoptics.com/diagonals/baader-290-astro-amici-prism-with-bbhs-coating.html
This has a light path of 85mm - so you'd get another 15mm over the 2" Prism and 25mm or so over the 2" mirror.
Not sure how compromised the view is though, in order to achieve the erect image; and it is quite a bit more expensive.
All depends how much focus travel you need to achieve.

You're right to stick with 2" though - a T2 diagonal does have a shorter light path but does not have enough clear aperture (33mm) for your 30mm UFF (38mm).

It’s a good idea, the Amici is not listed here in comparison with the other available 2” diagonals:

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

Not sure if this is enough infocus.