Prism or Mirror diagonal at f7.7?

[Franklin]

I have an f/11 102ED refractor and I was advised to use a prism diagonal with it. So I got one of the Baader T2 32mm prism diagonals with the fine focus attachment and I think they work great together. I may have the opportunity to get my hands on a Vixen SD103S, which will be a huge upgrade for me if I do. At f/7.7 will the Baader prism be ok or should I look into getting a mirror diagonal? I've read that mirror diagonals are more suited to fast scopes and prism diagonals to slower scopes. At f/7.7 the Vixen is kind of in the middle. Will I see a difference?  👍

[johninderby]

F/7.7 is about the point where a prism is advised so will be OK. At 7.7 not going to be much of a difference to a mirror diagonal though so either will be fine.

Edited October 26, 2021 by johninderby

[Franklin]

Thanks for that John, I thought around the f/8 point so that puts my mind at rest. What kind of diagonal do you use with your scope?

[dweller25]

I have tested a Baader BBHS prism and BBHS Mirror in an F/8 refractor, the prism had a slight edge in contrast to my eyes, possibly because it was slightly dimmer.

Edited October 26, 2021 by dweller25

[Franklin]

Yes the BBHS prism would be good, best start saving. I will stick with the 32mm T2 that I already have for now and I also have a 2" Altair prism, which is quite a hefty chunk of glass, very weighty. When I use that with a 2" low power eyepiece I have to rebalance the scope.

[johninderby]

8 minutes ago, Franklin said:

Thanks for that John, I thought around the f/8 point so that puts my mind at rest. What kind of diagonal do you use with your scope?

A WO mirror diagonal usually with my Tecnosky 125mm f/7.8 APO. Can’t decide if the Baader prism is any better though.The WO 2” diagonal is fairly light weight.

Edited October 26, 2021 by johninderby

[Franklin]

Right.

Well, if it's that close that you're undecided with all of your scope experience I'm not going to worry about it. I was hoping to stick with the Baader T2 32mm because,

a/ I really like the quality of it and it didn't break the bank.

b/ It's really lightweight and most of my stuff is 1.25" ( I have only one 2" eyepiece )

c/ If I do get the chance to own the Vixen then the fine focuser will come in handy as the the scope has a single speed unit.

Thanks for the helpfull replies.

[johninderby]

I use the prism with my Scopetech 80 f/15 scope or my f/12 classical cassegrains. I have tried the T2 prism with a two inch eyepiece and 2” nosepiece and it worked rather well. Love the adaptability of the T2 prism.

[Andrew_B]

I've got that 32mm Baader prism and it's great quality for the money. I felt like I was seeing a little bit more contrast and fine detail compared to my dielectric mirror diagonal but any difference was subtle although the prism has the advantage of being a bit more flexible due to its T-2 connections.

[Franklin]

2 hours ago, johninderby said:

I have tried the T2 prism with a two inch eyepiece and 2” nosepiece and it worked rather well.

I will have to get a 2" to T2 fitting and give that a go. I presume there will be some vignetting with any field stop greater than 32mm. On the other hand, my only 2" eyepiece is Baader Aspheric which comes with a 1.25" adaptor, I will have to investigate this further. I may not have to buy anything! Bonus!

[johninderby]

Yes there will be vignetting using a 2” eyepiece but not objectionable.

[Deadlake]

I have a SD103S and Baader BBHS prism/mirror diagonals. On planets the prism is the better diagonal. No need to get a new diagonal.

Note, I don’t believe speed determines to use a prism or mirror diagonal. It’s how well figured the scope is. A Mewlon 210 works better with a mirror diagonal at F11.5. 

Edited October 27, 2021 by Deadlake

[Stu]

2 minutes ago, Deadlake said:

Note, I don’t believe speed determines to use a prism or mirror diagonal. It’s how we’ll figured the scope is

I don’t see what the optical figure has to do with whether a prism or mirror would work better, can you explain?

Prisms start to show CA when used with fast scopes due to the steeper light cone. You’ve tried both in your Mewlon and prefer the mirror, how is it better?

[johninderby]

Worth a read of this.

https://www.cloudynights.com/articles/cat/articles/optical-theory/diagonals-prisms-vs-mirrors-r79

[Stu]

2 minutes ago, johninderby said:

Worth a read of this.

https://www.cloudynights.com/articles/cat/articles/optical-theory/diagonals-prisms-vs-mirrors-r79

That doesn’t indicate that scope optical figure is relevant in the choice of diagonal, but it does say prisms cause false colour in fast scopes….

[johninderby]

After searching can’t find anything about the optical figure having any effect on using a prism or mirror diagonal. 🤔

[Deadlake]

I can understand the point above, but using the speed of a scope can be a red herring in selection of a mirror vs prism diagonal. Better figured, maybe better corrected. For example:

1) When BillP was testing out planetary EP's he used a mirror BBHS with a TSA-102. I'd not just BiilP, some retailer's tell me select the prism only if your scope needs correction a prism provides.

https://www.cloudynights.com/topic/727720-toehrdelite-on-axis-planeraty-performance/?p=10483289

2) Using a u210 (F11.5) a mirror diagonal performs better then the prism flavour, see:

https://www.cloudynights.com/topic/556420-information-on-the-takahashi-mewlon-210-and-mewlon-250/page-24#entry11417328

I've found that on my SD103S (doublet) the prism gives better views of a mirror, I suspect it's correcting the scope.

With a triplet (LZOS 130/F6) the mirror diagonal gives the better views, the fact it's a F6 is a red herring, it's just better corrected.

Edited October 27, 2021 by Deadlake

[Stu]

2 hours ago, Deadlake said:

I can understand the point above, but using the speed of a scope can be a red herring in selection of a mirror vs prism diagonal. Better figured, maybe better corrected. For example:

1) When BillP was testing out planetary EP's he used a mirror BBHS with a TSA-102. I'd not just BiilP, some retailer's tell me select the prism only if your scope needs correction a prism provides.

https://www.cloudynights.com/topic/727720-toehrdelite-on-axis-planeraty-performance/?p=10483289

2) Using a u210 (F11.5) a mirror diagonal performs better then the prism flavour, see:

https://www.cloudynights.com/topic/556420-information-on-the-takahashi-mewlon-210-and-mewlon-250/page-24#entry11417328

I've found that on my SD103S (doublet) the prism gives better views of a mirror, I suspect it's correcting the scope.

With a triplet (LZOS 130/F6) the mirror diagonal gives the better views, the fact it's a F6 is a red herring, it's just better corrected.

Trying to work out what you have actually experienced and what is just read elsewhere. Do you have a Mewlon 210?

First off there is a real effect with prism diagonals and faster scopes, it’s just physics so it’s not a red herring. I note you ignore the comment and calculations from old speckled hen. F6 may still be on the cusp in terms of being affected.

Your use of the word dogma seems intended to offend so I’m uncertain why you do it?

For what it is worth, I found Zeiss and Tak prisms gave better views in my Tak f7.4 than a BBHS mirror but have always assumed my Tak is reasonably well corrected.

Unless I missed it, Bill doesn’t mention prism correction or it’s benefits. What correction are they providing?

[johninderby]

Physics wins for me re: fast refractors and prisms. However wonder if seeing conditions could affect results using a prism or mirror at longer focal lengths.

Anyway as I have both available not a problem. 🙂

Edited October 27, 2021 by johninderby

[Deadlake]

4 hours ago, Stu said:

Trying to work out what you have actually experienced and what is just read elsewhere. Do you have a Mewlon 210?

The Mewlon performance is a quote from CN, the point being about the Mewlon being a slow scope and viewing with a mirror diagonal gives reportedly better result. Given the Mewlon is F11.5 I would not of expected that.

4 hours ago, Stu said:

First off there is a real effect with prism diagonals and faster scopes, it’s just physics so it’s not a red herring. I note you ignore the comment and calculations from old speckled hen. F6 may still be on the cusp in terms of being affected.

I'm not disagreeing about a fast scope having a steeper light cone, I'm trying to make the point that a prism will also effect the light path. If the scope is well corrected you can end up overcorrecting the observed image. That seems to be the effect of a prism on my LZOS scope looking at Jupiter, it makes the belts look blurry.

4 hours ago, Stu said:

Your use of the word dogma seems intended to offend so I’m uncertain why you do it?

Dogma, didn't know that was an offensive term. Just making a choice on prism or mirror diagonal based on speed of scope may not end up with the best results. I suspect that F7 slower triplet might be better with a mirror diagonal than a prism. 

Regards to BillP, I was thinking of his report here, section 3E on Jupiter:

https://www.cloudynights.com/articles/cat/user-reviews/the-baader-bbhs-sitall-silver-diagonal-r3038

The fact that the BBHS mirror was showing more detail on Jupiter was a surprise to me, his 152 mm APO (F7.7??) should be better with a prism, but he is reporting this is not the case.

 

Edited October 27, 2021 by Deadlake

[vlaiv]

Is BBHS Baader prism diagonal just marketing trick?

Prism diagonals don't really need silver coated surfaces to do their thing, so why put expensive BBHS (and it must be expensive because mirror treated that way is more expensive than others) coating if it does nothing?

[johninderby]

What Baader says about the BBHS coatings.

https://www.baader-planetarium.com/en/blog/baader-bbhs-reflective-properties/

Edited October 27, 2021 by johninderby

[Stu]

29 minutes ago, vlaiv said:

Is BBHS Baader prism diagonal just marketing trick?

Prism diagonals don't really need silver coated surfaces to do their thing, so why put expensive BBHS (and it must be expensive because mirror treated that way is more expensive than others) coating if it does nothing?

I’ve often thought that too Vlaiv. I guess John’s response below answers it, although with total internal reflection does it matter what is on the outside? I guess Baader know what they are doing just a bit more than me though so I’ll go along with it. It begs the question as to whether you would see any difference on new prisms though, theoretically not.

Actually, from reading that it seems that their prisms have always had this coating, does anyone know if that’s correct? I have one which is not labelled as BBHS so have always assumed it doesn’t have it.

[Deadlake]

13 minutes ago, johninderby said:

What Baader says about the BBHS coatings.

https://www.baader-planetarium.com/en/blog/baader-bbhs-reflective-properties/

Good link, in-particular:

The question of "mirror or prism" for a telescope is accompanying our product development since ages. We have gained some intricate knowledge about the preferences of certain optical configurations for either prisms or mirrors over the years - ask us. Many of these insights we regard as proprietary. Hence we do leave it to the findings - and taste - of gifted astronomers, to let these experienced observers decide on their own what they prefer to use. Check out the various test reports published about our stuff on forums such as CN and SGL - for instance those done by William Paolini.

 

[vlaiv]

9 minutes ago, johninderby said:

What Baader says about the BBHS coatings.

https://www.baader-planetarium.com/en/blog/baader-bbhs-reflective-properties/

I'm not buying that

It certainly is not Sitall prism as Baader themselves say it is Bak4 glass used in their prism.

Wiki article on prism star diagonal states:

"Also a prism will never degrade over time as a mirror will since there is no reflective metal coating to degrade from oxidation."

Indeed - reflection in prism is achieved by https://en.wikipedia.org/wiki/Total_internal_reflection

mechanism - it works on water as well, not just glass, so I don't see how it can degrade unless actual surface is scratched / destroyed