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Diagonals & Prisms - Dramatic or Subtle Differences?

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#1 rkelley8493

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Posted 16 July 2019 - 12:36 PM

I've read Bill P's reviews on the MIRROR VS. DIELECTRIC VS. PRISM DIAGONAL COMPARISON as well as the Baader BBHS-Sitall Silver Diagonal review. Very well written and informative waytogo.gif  But I wanted to get some others' points of view on the subject. How much of a difference are the views produced by the different types of diagonals? Are they subtle differences or dramatic differences?  Do the diagonals enhance the image in any way, or should they be "invisible" in the optical train? My primary scope is a 130mm f/7 Apo refractor. 



#2 JGass

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Posted 16 July 2019 - 01:58 PM

The smoothness or lack thereof to the dielectric's substrate can contribute to light scatter around your targets, especially bright ones.

 

In faster scopes, the relative steep incidence of the light cone can result in some chromatic dispersion.  At f/7, there may be some of that, but possibly not too strong.

 

The light throughput for a prism can be pretty good.  So, the dielectrics may not be much brighter than prisms.

Obviously, the light path lengths will be different in prism vs. dielectric mirror diagonals.  Just a minor bother, if one is switching for A/B comparisons.

 

The dielectric coatings should last a long time, IMO.  I don't know if there is a significant difference between the coatings on diagonal prisms over the years.  So, it may be that an older prism may not perform quite as well as a new one.  I'm not sure. 

 

In all diagonals, there is also the question of how well collimated they are.  So, one example of a poorly collimated diagonal vs. a well-collimated diagonal of the other type may affect your results.


Edited by JGass, 16 July 2019 - 01:59 PM.

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#3 Peter Besenbruch

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Posted 16 July 2019 - 02:35 PM

I've read Bill P's reviews on the MIRROR VS. DIELECTRIC VS. PRISM DIAGONAL COMPARISON as well as the Baader BBHS-Sitall Silver Diagonal review. Very well written and informative waytogo.gif  But I wanted to get some others' points of view on the subject. How much of a difference are the views produced by the different types of diagonals? Are they subtle differences or dramatic differences?  Do the diagonals enhance the image in any way, or should they be "invisible" in the optical train? My primary scope is a 130mm f/7 Apo refractor. 

Viewing straight through is better than using a diagonal, any diagonal. The goal here involves getting the least amount of optical degradation when using one. Yes, we as a people have become soft, trading convenience and comfort for the purity of straight through viewing. That is why our world is in the shape it is in; we use star diagonals.4.gif

 

At f7 you can use either a prism, or a mirror. I use a 1.25" for planetary, but get good views with a GSO dielectric. There is more scatter, but the views are still good. The issue isn't with the substrate used, it's with the dielectric coating itself. Even when applied with care, the dielectric coatings will produce more scatter. That was a major point of BillP's article.

 

That said, dielectrics match or beat prisms when it comes to durability. I call them "dishwasher safe." The BBHS silver diagonal from Baader I won't buy due to repeated issues with over-coated diagonals, particularly the silver ones. That's sort of OK when you pay $100 for one, but not $500.


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#4 Astrojensen

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Posted 16 July 2019 - 02:38 PM

The differences can range from extremely subtle to very easily seen, depending on the difference in quality. The single best way to tell is simply to compare directly at the eyepiece.

 

 

Clear skies!
Thomas, Denmark


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#5 Kent10

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Posted 16 July 2019 - 02:56 PM

Viewing straight through is better than using a diagonal, any diagonal. The goal here involves getting the least amount of optical degradation when using one. Yes, we as a people have become soft, trading convenience and comfort for the purity of straight through viewing. That is why our world is in the shape it is in; we use star diagonals.4.gif

 

At f7 you can use either a prism, or a mirror. I use a 1.25" for planetary, but get good views with a GSO dielectric. There is more scatter, but the views are still good. The issue isn't with the substrate used, it's with the dielectric coating itself. Even when applied with care, the dielectric coatings will produce more scatter. That was a major point of BillP's article.

 

That said, dielectrics match or beat prisms when it comes to durability. I call them "dishwasher safe." The BBHS silver diagonal from Baader I won't buy due to repeated issues with over-coated diagonals, particularly the silver ones. That's sort of OK when you pay $100 for one, but not $500.

Peter, what are the issues with the BBHS diagonals?  Thanks.

 



#6 Kent10

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Posted 16 July 2019 - 02:59 PM

You sometimes hear that prism diagonals have lower light throughput than mirror diagonals.  Does anyone know what that is for the 2" BBHS Prism Diagonal compared to the 2" BBHS Mirror Diagonal?  Thanks. 


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#7 photoracer18

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Posted 16 July 2019 - 03:02 PM

My info from testing, using, and collimating diagonals while I worked at Hands On Optics is this: most mirror diagonals that have been used are likely off a little over time (maybe not enough to really tell without testing). Those are the good ones. Some cheap ones did not even come well collimated. Most, if not all, prism diagonals (RA type) are at or near perfect collimation all the time new or used. This is because the prism is just held in tension against both the in and out tubes so it can't really get out unless the housing machining is off. Don Yeier told me once that if you start out with a 1/20 wave diagonal mirror and then lay down dielectric coatings, the surface accuracy drops to as much as 1/10th or less. Even the slightest particle on the surface will get much larger once all the coatings get on so a clean room is mandatory. I used to have a test rig that included a very narrow laser and a 50' target to collimate diagonals, after which then I put them in a scope. I really did not want to know what the collimation looked like before that. Most that were off had machining issue in their housings not optical issues. I only found one diagonal that was impossible to collimate and that had a housing issue.

All that being said your own eyes have more to do with what can be seen than the telescope optical train. Since my eyes are not perfect I don't sweat it much. I use mostly dielectric ones because they are easy to get and don't cost a fortune. In fact I also have an SVA130T (#0002) and currently use an SV 1/10th Lambda dielectric simply to keep it in the family. I also own GSO, AT, and another SV dielectric.


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#8 Frisky

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Posted 16 July 2019 - 03:10 PM

"Viewing straight through is better than using a diagonal, any diagonal."

 

Not the last time I compared views. I removed my mirror diagonal, a highly polished one that came on my scope, and I added an extension tube so I could view Jupiter straight through. I got that red/white/blue effect (atmospheric dispersion?). I put the diagonal back on and it was gone. Had a nice view. I did notice a darker background without the diagonal.

 

Joe


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#9 Kunama

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Posted 16 July 2019 - 03:21 PM

.....The BBHS silver diagonal from Baader I won't buy due to repeated issues with over-coated diagonals, particularly the silver ones. That's sort of OK when you pay $100 for one, but not $500.

I have yet to hear of anyone having issues with the Baader BBHS diagonals or prisms as far as their durability goes so I assume you're referring to issues with other manufacturer's products made in years gone by.

The Baader BBHS coating has been used on their Zeiss spec prisms for two decades, I have one of their early versions and a near new one and I would challenge anyone to tell which is which when viewing through them.

 

I realise that the BBHS coating on their first surface diagonals is recent but I wouldn't malign the item without some evidence....


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#10 Jon Isaacs

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Posted 16 July 2019 - 03:36 PM

My experience:

 

Unless something is dramatically wrong, the differences are subtle.  

 

Even miscollimation has relatively little effect because, I think, the diagonal is so close to the focal plane.  Just about every inexpensive 1.25 mirror diagonal I have ever tested has been significantly out of Collimation but removing the mirror and reattaching it so it's aligned a square only makes a slight improvement.

 

Roland Christen has pointed out that due to closeness to the focal plane, the importance of the flatness of the diagonal is much reduced. Only a small portion of the diagonal contributes to any individual point on the focal plane.

 

Of course this is the eyepiece forum so subtle is the word of the day. Mole hills are made into mountains.  Dramatic in my world is the difference between an ST-80 and an ED-80.

 

Jon

 

 


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#11 rkelley8493

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Posted 16 July 2019 - 03:46 PM

......

 

Of course this is the eyepiece forum so subtle is the word of the day. Mole hills are made into mountains.  Dramatic in my world is the difference between an ST-80 and an ED-80.

 

Jon

Couldn't have said it better waytogo.gif  I was trying to think of a comparison on how the differences would be between a BBHS & Dielectric but couldn't come up with something. So that was what I was wondering, if it would be like going from a cheap Achromat to an Apo.  


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#12 russell23

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Posted 16 July 2019 - 03:52 PM

The main thing I have noticed comparing different diagonals is that the low cost dielectric diagonals do have more scattered light.  It is not dramatic, but it is noticeable.   My Vernonscope Quartz diagonal is 1/26th wave and I mostly use that diagonal when I use a 2" diagonal.  After I got the Vernonscope the AP Maxbright became my "backup" diagonal.


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#13 Kunama

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Posted 16 July 2019 - 03:52 PM

Couldn't have said it better waytogo.gif  I was trying to think of a comparison on how the differences would be between a BBHS & Dielectric but couldn't come up with something. So that was what I was wondering, if it would be like going from a cheap Achromat to an Apo.  

I have had the Baader BBHS prism and Maxbright diagonals side by side in identical scopes and the differences are very subtle. Some targets favoured the BBHS, some the Maxbright. We are essentially talking of differences that don't matter... 


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#14 Jon Isaacs

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Posted 16 July 2019 - 03:55 PM

Couldn't have said it better waytogo.gif  I was trying to think of a comparison on how the differences would be between a BBHS & Dielectric but couldn't come up with something. So that was what I was wondering, if it would be like going from a cheap Achromat to an Apo.  

 

:waytogo:

 

I was trying to think of an analogy for a subtle improvement.. the best I could come up with for subtle was the difference between a decent diagonal and a perfect diagonal.

 

Jon


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#15 rkelley8493

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Posted 16 July 2019 - 03:57 PM

I have had the Baader BBHS prism and Maxbright diagonals side by side in identical scopes and the differences are very subtle. Some targets favoured the BBHS, some the Maxbright. We are essentially talking of differences that don't matter... 

Is the Maxbright dielectric? My current diagonal is the 2" Baader ClickLock dielectric. Would the differences still be subtle?



#16 Kunama

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Posted 16 July 2019 - 04:41 PM

Yes, 140 layer coating
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#17 Esso2112

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Posted 16 July 2019 - 05:01 PM

I have a 30 year old Vernonscope mirror diagonal and it still gives some of the best views. The mirror is still beautiful after all these years. I also have the Baader BBHS prism and it is excellent as well. I use both in my f8 scopes and can’t say as I’ve seen much difference between them. Now, in my FCT-125 (f5.6), that’s a different story and the mirror wins. 


Edited by Esso2112, 16 July 2019 - 05:02 PM.


#18 Peter Besenbruch

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Posted 16 July 2019 - 05:57 PM

Peter, what are the issues with the BBHS diagonals?  Thanks.

The issue I would have is with over-coated silver that might fail in 5-10 years. I have had a variety of over-coated diagonals fail, both aluminum and silver. These include Celestron, Lumicon, and Televue. I will not spend $500 to see if Baader's efforts with last.

 

In actual performance, a new BBHS silver is very close to a good prism. I say this from experience, and side by side comparisons.


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#19 Peter Besenbruch

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Posted 16 July 2019 - 06:10 PM

I have yet to hear of anyone having issues with the Baader BBHS diagonals or prisms as far as their durability goes so I assume you're referring to issues with other manufacturer's products made in years gone by.

The Baader BBHS coating has been used on their Zeiss spec prisms for two decades, I have one of their early versions and a near new one and I would challenge anyone to tell which is which when viewing through them.

 

I realise that the BBHS coating on their first surface diagonals is recent but I wouldn't malign the item without some evidence....

1) As you say, there is a difference between coating a prism, and over-coating a mirror.

 

2) Over-coating is vital to protect a front surface mirror. I prism will still function with failed coatings. Mirrors won't very well. They tarnish around the area of failure.

 

3) Mirrors have been over-coated for many decades. Longevity depends partly on the quality of the coating, and partly on environment the mirror is used in.

 

4) Time is a greater enemy of mirrors than prisms. A mirror after ten years may look fine, but it will not perform like new. A prism will continue to perform at a high level, longer.

 

5) I understand that dielectric diagonals achieve their reflectivity through nothing but "over-coating." There is no metal surface to protect. Hence, I suspect they will last, and perform at a higher level over time.

 

Keep all this in mind when you consider shelling out $500 ($800 for Vernonscope) for a new diagonal.


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#20 Starlease

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Posted 16 July 2019 - 07:02 PM

If you look at the planets much you must have the Baader T2 Zeiss prism.

It is so much better than anything else on planets, a dramatic difference.

 

Cannot afford it then most any other prism are really good.

Next aluminum coatings.

Last and worst are dielectrics, never used a good one. Even the BBHS silver mirror with dielectric coatings is just as bad as any other dielectric.

 

If you rarely look at planets then anything works good. But the BBHS mirrors shows the red stars better than anything. All those red Giants in the double cluster show up nicely in even small scopes.


Edited by Starlease, 16 July 2019 - 07:03 PM.

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#21 Gavster

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Posted 16 July 2019 - 07:27 PM

If you look at the planets much you must have the Baader T2 Zeiss prism.

It is so much better than anything else on planets, a dramatic difference.

 

Cannot afford it then most any other prism are really good.

Next aluminum coatings.

Last and worst are dielectrics, never used a good one. Even the BBHS silver mirror with dielectric coatings is just as bad as any other dielectric.

 

If you rarely look at planets then anything works good. But the BBHS mirrors shows the red stars better than anything. All those red Giants in the double cluster show up nicely in even small scopes.

My refractors are relatively fast (f7 and less) and so I don’t tend to use my Baader t2 Zeiss due to potential for CA. But your comment that it’s a dramatic difference does make me want to have another compare with my mirror bbhs diagonal. 

I’ve been very pleased with my mirror bbhs but with the planets so low at the moment it’s been a while since I’ve had good planetary views. Maybe Mars next year.


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#22 jaraxx

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Posted 16 July 2019 - 08:11 PM

Viewing straight through is better than using a diagonal, any diagonal. The goal here involves getting the least amount of optical degradation when using one. Yes, we as a people have become soft, trading convenience and comfort for the purity of straight through viewing. That is why our world is in the shape it is in; we use star diagonals.4.gif

I just knew something was wrong....


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#23 Jeff Morgan

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Posted 16 July 2019 - 10:43 PM

My refractors are relatively fast (f7 and less) and so I don’t tend to use my Baader t2 Zeiss due to potential for CA. But your comment that it’s a dramatic difference does make me want to have another compare with my mirror bbhs diagonal. 

I’ve been very pleased with my mirror bbhs but with the planets so low at the moment it’s been a while since I’ve had good planetary views. Maybe Mars next year.

 

Two friends and I had a dinner and astronomy get-together last night, one of them items on the menu was Jupiter through KW's AP Stowaway. The diagonal was a Zeiss 1.25" prism belonging to the third member of our group, GG. The prism performed well in the Stowaway even at f/6.

 

Regarding some other posts on the "durability" of dielectrics. You can surf over to the Astro-Physics website and see this statement:

 

"The Astro-Physics 2" diagonal has a very high-tech dielectric coating that was developed originally for military optics used in hostile environments such as blowing desert sand." 

 

Sounds indestructible. But it's not. I would not count on a dielectric being any more robust than an aluminized one. If I have time tomorrow or the next day I will take apart my Astro-Physics MaxBright photograph the sleeks in mine from regular cleaning, and post them to this thread.

 

To the OP: Yes, BillP seems to be correct in his assessment of the BBHS. Jupiter does seem more saturated. With my NV eyepiece, the BBHS definitely has more pop on star fields since dielectrics drop off beyond 700nm and the image intensifier reaches down beyond 950nm. (The NV eyepiece was my primary motivation for the BBHS, happy coincidence it appears equal or superior with regular eyepieces.)

 

And the ClickLock is a revelation all by itself. Just ordered one for my AP focuser drawtube collar, plus the ClickLock 1.25" adapter.

 

Jury is out on BBHS longevity. If AP's marketing prose is "optimistic" I am not counting on Baader's to be any more realistic. Even so, the BBHS has earned high marks already. I really don't care if I do have to replace it every five years.


Edited by Jeff Morgan, 16 July 2019 - 10:45 PM.

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#24 fate187

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Posted 17 July 2019 - 01:35 AM

When I got my Baader 2" BBHS prism, the prism was loose in its housing and moving a lot (like 2 mm)... "rattling sound issue". Hence there was a major astigmatism visible. Sent it back to fix it. However, I cannot see much of a difference compared to my WO dielectric.

 

regards

Michael


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#25 bmurphy495

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Posted 17 July 2019 - 11:05 AM

I have a BBHS Mirror diagonal as well as a few others, a 1.25 Orion with a dielectric mirror and a 2" WO and the one SkyWatcher includes with their 120mm refactors.  

 

I don't think my eye is discerning enough to really see any difference. I'm glad I purchased it, and I really like the click lock feature, but I'm not sure the juice is worth the squeeze. However I have not conducted a shoot out with them, switching back and forth, to really determine if one is better than the others. Might be a fun exercise at some point this Summer. 

 

B


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