Baader T2 Zeiss Prism vs Baader 2 inch Zeiss Prism Diagonals

[John]

I currently use a Baader T2 Zeiss prism diagonal in 1.25 inch mode with my Takahashi FC100-DL. I'm thinking of moving to one of the Baader 2 inch Zeiss prism diagonals with this scope so that I can use my 2 inch eyepieces such as the Ethos with the Takahashi.

Is there any discernible difference in the optical performance of these two prism diagonals ?

BillP had the T2 Prism fractionally ahead of the 2 inch in this report:

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

But I'm interested to see if that is the experience of other observers ?

I realise that I could use a 2 inch adapter and 2 inch barrel with the T2 Prism but the aperture is restricted to 34-35mm and I would like to use eyepieces with larger field stops than that if possible.

Thanks

 

 

Edited October 12, 2020 by John

[jetstream]

Hey John, I only have the 2" Baader/Zeiss prism diagonal but in the TSA120 I detect no scatter with it. I checked straight through as well, no discernible difference to my eyes. I used the very low scatter Zeiss zoom and Vixen Hr's to check.

[vineyard]

7 hours ago, John said:

I currently use a Baader T2 Zeiss prism diagonal in 1.25 inch mode with my Takahashi FC100-DL. I'm thinking of moving to one of the Baader 2 inch Zeiss prism diagonals with this scope so that I can use my 2 inch eyepieces such as the Ethos with the Takahashi.

Hi John, I can't answer your Q I'm afraid but in case you do make the switch & want to shift your current diagonal, I'm after something like one of those for the 102M so pls can I be v cheeky & throw my hat into the ring early in case you do! 😇 Cheers, Vin

[JTEC]

John, I have both the 2” and the 1.25.  I use them interchangeably in my TEC 140 And C11 (just the 2” in the SCT). I also have the Baader 1.25 mirror diagonal but that doesn’t get used much because, IMO, both prisms are better in terms of contrast and scatter.  The eyepieces I use are typically high-end: Tak orthos, Delos, Ethos, Tak TOE and Vixen HR and sometimes push the magnification quite high, so the prisms are being given a decent test. I use the smaller prism with the bino when using the Baader glaspath compensator in the train and the 2” when using it with the 2” TV Powermate instead of the gpc. I’ve read (and re-read) Bill P’s report. In practice, I can’t honestly see any difference in performance between the two prisms - and I’ve often gone looking for it. If there is a difference, it’s likely to be just one tiny variable among many. I do think that both have a clear edge on the mirror version though.  I’d like to try the BBHS mirror that BillP enthuses about, but it’s too pricey to buy just to find out. I have been told that both of the Zeiss Baader prisms have the BBHS coating on the hypotenuse - I’m not sure why though or what contribution it makes. Both prisms outperform the Astrophysics dielectric that I used to have - when I compared the views, the Astrophysics, top quality mirror diagonal as it is, went up for sale. Summing up, I don’t think you can go wrong with either. In practice, I tend to use the 2”  a bit more because, with the excellent Clicklock system, it is so convenient and gives up nothing in quality that I can see.

[globular]

Hi John,
Sorry I can't help with your decision (no personal experience) but I am following with interest as I put one on my wish-list a while back following some research.
Have you made a decision? I'd be interested to know which way you go and why.
And if you are going for it, have you found one at a good price? Best I can find is £360 (new).

[John]

12 hours ago, globular said:

Hi John,
Sorry I can't help with your decision (no personal experience) but I am following with interest as I put one on my wish-list a while back following some research.
Have you made a decision? I'd be interested to know which way you go and why.
And if you are going for it, have you found one at a good price? Best I can find is £360 (new).

I'm very tempted because I'd like to standardise all my scopes to having 2 inch eyepiece capability. Currently the Takahashi FC100-DL is the only one with a 1.25" diagonal (Baader T2 Zeiss Prism) but I don't want to compromise the optical quality in this move. Sounds like I won't have to though 

 

 

 

Edited October 13, 2020 by John

[globular]

I know you'll already know - but the light path of the T2 is 38.5mm + length of your eyepiece holder (40mm may be?) while the 2" is 100mm including clicklock - so your focal length will change a bit. Doubt you'll have focussing issues but if some are currently close to max they may struggle.

[Deadlake]

I'd presume larger aperture is preferable, are the smaller aperture models with a smaller optical depth really just for binoviewing? The prism diagonal also look cheaper, I'd of thought it would of been the other way around as the prism diagonals are  meant to deal with stray light better, but take longer to acclimate.

See:  https://www.baader-planetarium.com/en/downloads/dl/file/id/1617/product/1687/baader_star_diagonals_t_2_and_2.pdf

page 23

models 2456117  vs 2456095

 

Edited October 13, 2020 by Deadlake

[John]

2 hours ago, Deadlake said:

I'd presume larger aperture is preferable, are the smaller aperture models with a smaller optical depth really just for binoviewing? The prism diagonal also look cheaper, I'd of thought it would of been the other way around as the prism diagonals are  meant to deal with stray light better, but take longer to acclimate.

See:  https://www.baader-planetarium.com/en/downloads/dl/file/id/1617/product/1687/baader_star_diagonals_t_2_and_2.pdf

page 23

models 2456117  vs 2456095

 

Thanks for that link. Baader do have a wide variety of products, don't they !

I currently use the Baader T2 Zeiss Prism which has a clear aperture of 35mm. One of these:

https://www.harrisontelescopes.co.uk/acatalog/baader-t-2-zeiss-prism.html?gclid=EAIaIQobChMIy9HV_5qx7AIVxbHtCh20bwE0EAQYAiABEgKr9fD_BwE

I currently have the Baader 1.25" click stop on the eyepiece side and the Baader 1.25" push fit adapter on the scope side.

I could put a 2" eyepiece side adapter on the eyepiece side and a 2" push fit barrel on the scope side and it would be OK for 2 inch eyepieces with field stops up to 35mm in diameter. 

I have been wondering if moving to the "full" 2 inch Zeiss prism diagonal would result in any performance loss and the feedback seems to be that is not likely to be the case. I also tend to think that a 2 inch diagonal would provide a firmer foundation for heavier eyepieces rather than the smaller diagonal plus adapters approach.

Thanks for the feedback so far - it's been very useful

[Deadlake]

58 minutes ago, John said:

Thanks for that link. Baader do have a wide variety of products, don't they !

I currently use the Baader T2 Zeiss Prism which has a clear aperture of 35mm. One of these:

https://www.harrisontelescopes.co.uk/acatalog/baader-t-2-zeiss-prism.html?gclid=EAIaIQobChMIy9HV_5qx7AIVxbHtCh20bwE0EAQYAiABEgKr9fD_BwE

I currently have the Baader 1.25" click stop on the eyepiece side and the Baader 1.25" push fit adapter on the scope side.

I could put a 2" eyepiece side adapter on the eyepiece side and a 2" push fit barrel on the scope side and it would be OK for 2 inch eyepieces with field stops up to 35mm in diameter. 

I have been wondering if moving to the "full" 2 inch Zeiss prism diagonal would result in any performance loss and the feedback seems to be that is not likely to be the case. I also tend to think that a 2 inch diagonal would provide a firmer foundation for heavier eyepieces rather than the smaller diagonal plus adapters approach.

Thanks for the feedback so far - it's been very useful

OK for 2 inch eyepieces with field stops up to 35mm in diameter. This is really the deciding reason to select between the two sizes? The scope I have creates an imaging circle of 44 mm, the larger aperture prism of 43 mm could make use of this, i.e. not throwing away light?

Edited October 13, 2020 by Deadlake

[John]

4 hours ago, Deadlake said:

OK for 2 inch eyepieces with field stops up to 35mm in diameter. This is really the deciding reason to select between the two sizes? The scope I have creates an imaging circle of 44 mm, the larger aperture prism of 43 mm could make use of this, i.e. not throwing away light?

I my case the scope will be 100mm F/9.

 

[Deadlake]

6 minutes ago, John said:

I my case the scope will be 100mm F/9.

 

It's an aperture of 103 mm with a 44 mm imaging circle on the back.

[John]

18 minutes ago, Deadlake said:

It's an aperture of 103 mm with a 44 mm imaging circle on the back.

Sorry but I don't know what an imaging circle is - I'm purely a visual observer.

 

[Deadlake]

18 minutes ago, John said:

Sorry but I don't know what an imaging circle is - I'm purely a visual observer.

 

Imaging circle is the diameter of light coming out the back of the telescope (2"/44 mm), maybe someone will correct me? I presuming if you have a 2" focuser you should go for the larger prism otherwise you are dropping all those photons.... I'm visual and had to look up field stop. 😀

[John]

1 hour ago, Deadlake said:

Imaging circle is the diameter of light coming out the back of the telescope (2"/44 mm), maybe someone will correct me? I presuming if you have a 2" focuser you should go for the larger prism otherwise you are dropping all those photons.... I'm visual and had to look up field stop. 😀

Thanks but I'm not sure it works like that

 

 

 

Edited October 13, 2020 by John

[JeremyS]

7 hours ago, John said:

I currently use the Baader T2 Zeiss Prism which has a clear aperture of 35mm.

John, I have the T2 BBHS mirror diagonal with 2-inch fittings both ends. Has clear aperture of 33 mm. Fine for most of my 2-inch eyepieces, but depends which ones you want to use (field stop diam). My N T5 26 mm has 35 mm field stop, so technically too wide, but can't see a problem. Would not go with wider field stop eyepieces tho. I do have a true 2-inch Baader diagonal too, but that lengthens the optical path length (means I have to remove one of the Tak extension tubes on the TSA 120 in the same way as when I use binoviewers) and is bulkier. For most applications I prefer the compact T2 diagonal.

Edited October 13, 2020 by JeremyS

[andrew s]

When the field stop is internal to the belly of the beast it can be larger than the entrance pupil of the eyepiece.  So your N T5 may not be limited as you suspected @JeremyS .  It would of course depend of  the details of both the scope/diagonal and the eyepiece.

I though you were having a nap (sorry meditating)  after all that unboxing?

Regards Andrew 

[JeremyS]

2 minutes ago, andrew s said:

When the field stop is internal to the belly of the beast it can be larger than the entrance pupil of the eyepiece.  So your N T5 may not be limited as you suspected @JeremyS .  It would of course depend of  the details of both the scope/diagonal and the eyepiece.

I though you were having a nap (sorry meditating)  after all that unboxing?

Regards Andrew 

Ah, good point Andrew. I'm glad someone is wide awake 🀄🥋

[John]

I have 3 eyepieces with field stops larger than 35mm:

Ethos 21mm = 36.2mm

Nagler 31mm = 42mm

Aero ED 40mm = 47.5mm

I guess the Ethos 21 would work OK with the Baader T2 Zeiss prism plus it has an internal field stop of course which will also be in it's favour.

The Aero ED 40 has a field stop below the field lens of the eyepiece so I think will need a full aperture diagonal. I suspect that the Nagler 31 might as well.

Of course I could just decide stop at the Ethos 21 mm with the Tak and stick with the T2 prism with 2 inch fittings either end. That would be showing a very respectable 2.3 degree true field

[globular]

2.7 degrees and 3.0 degrees with the other two though John!

[John]

6 minutes ago, globular said:

2.7 degrees and 3.0 degrees with the other two though John!

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.

 

[globular]

Do you know the back focus to focal length ratio for your scope?

If it's large then the extra optical length of the 2" may make your F/9 something like F/9.9 - which you may or may not like.

[John]

6 minutes ago, globular said:

Do you know the back focus to focal length ratio for your scope?

If it's large then the extra optical length of the 2" may make your F/9 something like F/9.9 - which you may or may not like.

I've no idea, sorry.

I have used 2 inch diagonals with the Takahashi before - I have an Astro Physics and a couple of Tele Vue Everbrights in that fitting and they work fine with it but I particularly like the Zeiss prism performance with that scope which is why I was asking if the 2 inch Baader zeiss prism is the equal of the T2 version. 

 

Edited October 13, 2020 by John

[Deadlake]

3 hours ago, John said:

Thanks but I'm not sure it works like that

 

 

 

That’s why I was asking, how do I work out if I go for the 33 or 43 mm diagonal? 

[John]

11 minutes ago, Deadlake said:

That’s why I was asking, how do I work out if I go for the 33 or 43 mm diagonal? 

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.