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Astronomics prism 1.25" diagonal looks good, only... "not for a reflector"?

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#1 Brawlingsaz

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Posted 18 September 2020 - 01:59 PM

Hey, oh wise ones.  I've been looking to upgrade the stock 1.25" diagonal in my NexStar 8SE SCT, and in my searches I came across a confusing reference for one sold by our patron/host Astronomics:

 

https://www.astronom...m-diagonal.html

 

In the description, it has this disquieting phrase:

 

..."The lightweight (only 3 ounce) diagonal will work with any 1.25" accessory telescope other then a reflector."...  (emphasis mine)

 

So, two questions for you long-timers out there who have upgraded your diagonals...

 

1> What do you think of this "premium 1.25" Takahashi TAKSD1 compression ring prism star diagonal" (which looks pretty good, if a bit pricey... but, well, Takahashi)

 

2> My 8SE is a reflector, so - are they saying that this diagonal will NOT work with my telescope?  If not, why not?  I need to understand this in order to continue researching upgrade diagonal replacements for my stock Celestron diagonal, which many posts say is entirely adequate but not stellar (pun intended, as always). I didn't realize that this might be an issue, so please enlighten me as to the reason, or as to what they are saying in that one brief mention of a potential issue with my using this seemingly fine product.

 

Thanks in advance for your advice and assistance here, and good viewing to you all.

 

--bmr



#2 wrnchhead

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Posted 18 September 2020 - 02:08 PM

I believe that means reflector in the purest sense, ie. a Newtonian. It will work in an SCT


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#3 Rich V.

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Posted 18 September 2020 - 02:10 PM

Celestron SCTs come with a prism diagonal so the Tak should be a nice upgrade.  Regarding the "not for reflectors" part, I think they're referring to trying to use a 90° diagonal in a Newtonian reflector, which won't come to focus because of the longer light path that a diagonal would add.

 

Rich


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#4 photoracer18

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Posted 18 September 2020 - 02:49 PM

SCTs and MCTs are not technically reflectors, Newtonian type anyway, they are catadioptrics, which means both mirrors and lenses (even if the lens is a corrector plate in this case).  They are a variation of the Cassegrain reflector instead of the Newtonian reflector. There is a slight difference in the results comparing prism diagonals verses mirror ones. Most times its not enough to worry about. You might consider a 2" SCT type mirror diagonal or get a 2" visual back and use a 2" refractor type mirror diagonal. If you are like me and have a lot of scopes it saves money equipping your cats with 2" visual backs so you can use the same 2" dielectric mirror diagonals on all your scopes.


Edited by photoracer18, 18 September 2020 - 02:51 PM.

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#5 petert913

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Posted 18 September 2020 - 03:26 PM

 

I believe that means reflector in the purest sense, ie. a Newtonian. It will work in an SCT

Correct.



#6 Eddgie

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Posted 18 September 2020 - 08:04 PM

While the Tak fanboys will say give high praise, I would not recommend this diagonal for three reasons.

 

  • First, it is somewhat over-priced
  • Second, the compression ring collet can be aggravating when using eyepieces with safety undercuts
  • Last, it is not a flexible diagonal 

My recommendation would be the Baader T2 Prism diagonal and here are the reasons for that.

 

The Baader diagonal has a built in helical focuser, which can help make precision focusing easier so if the scope has any mirror play, this will aid in achieving perfect focus when doing planetary or double star work.

 

Next, the Baader uses a very strong metal housing while (if memory serves me well) the Tak uses a molded plastic housing.  This will be important to the next case.

 

The Baader has T2 connections on either end.  This means that if you ever decide to add a binoviewer with T2 connection, you will already have a T2 ready diagonal with a strong metal housing capable of easily handing the weight of even heavy binoviewers.  Any time anyone asks about improving planetary performance, I almost always say that a binoviewer can make a huge difference, and to work well in an SCT, the T2 diagonal becomes a critical component. 

 

So, for $22 more you get an all metal T2 ready diagonal with a helical focuser. 

 

And the quality of the Baader diagonal even with the standard prism is as best as I could see identical in performance to the Tak diagonal. (I have owned both so this is not someone without experience when both.)


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#7 Peter Besenbruch

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Posted 18 September 2020 - 10:31 PM

And the quality of the Baader diagonal even with the standard prism is as best as I could see identical in performance to the Tak diagonal. (I have owned both so this is not someone without experience when both.)

There are a number of bad reviews of the 32mm Baader diagonal, and for good reason. My copy never produced good detail. I have not tried the 34mm prism.


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#8 jeffmac

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Posted 18 September 2020 - 11:25 PM

While the Tak fanboys will say give high praise, I would not recommend this diagonal for three reasons.

 

  • First, it is somewhat over-priced
  • Second, the compression ring collet can be aggravating when using eyepieces with safety undercuts
  • Last, it is not a flexible diagonal 

My recommendation would be the Baader T2 Prism diagonal and here are the reasons for that.

 

The Baader diagonal has a built in helical focuser, which can help make precision focusing easier so if the scope has any mirror play, this will aid in achieving perfect focus when doing planetary or double star work.

 

Next, the Baader uses a very strong metal housing while (if memory serves me well) the Tak uses a molded plastic housing.  This will be important to the next case.

 

The Baader has T2 connections on either end.  This means that if you ever decide to add a binoviewer with T2 connection, you will already have a T2 ready diagonal with a strong metal housing capable of easily handing the weight of even heavy binoviewers.  Any time anyone asks about improving planetary performance, I almost always say that a binoviewer can make a huge difference, and to work well in an SCT, the T2 diagonal becomes a critical component. 

 

So, for $22 more you get an all metal T2 ready diagonal with a helical focuser. 

 

And the quality of the Baader diagonal even with the standard prism is as best as I could see identical in performance to the Tak diagonal. (I have owned both so this is not someone without experience when both.)

Eddgie is right here on most counts. However, when using it with an SCT, it has an unintentional built-in helical focuser. If you leave the collet loose instead of tightening it around the eyepiece, you can rotate the collet in or out some. In effect, it will act like a sort of helical focuser for fine focus. Since the scope is not on an equatorial mount, the diagonal and eyepiece remain constantly pointed up, negating the likelihood of dumping the eyepiece on the ground.  


Edited by jeffmac, 18 September 2020 - 11:26 PM.

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#9 Bean614

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Posted 19 September 2020 - 07:12 AM

There are a number of bad reviews of the 32mm Baader diagonal, and for good reason. My copy never produced good detail. I have not tried the 34mm prism.

A noted CN member did a detailed comparison of about a dozen diagonals a few years back.  The results of that 'review' were, basically, accepted as 'Bible' by CN members.  I fell for it too, but luckily didn't sell ANY of my 'disparaged' diagonals until I could compare myself, and with a LOT of different Refractors, Reflectors, and Catadioptrics.  

    My own tests confirmed what Peter just said, and others have noted, and save for a small handful of the VERY pricey, 'Premium' Brand Diagonals, found that several of the 'lower priced' models were every bit as good as all the others, no matter the price.

    I am a big fan of the venerable Celestron #94115A Prism, and also have been extremely impressed by the Dialectric Diagonals offered by our Hosts, Astronomics.  In fact, the ONLY mirror diagonals I now use are the 2" and 1.25" Astronomics, and I regularly use, and am very happy with, the Celestron Prism (I have 3 of them!).

  Now, before you ask, YES, I have used the AP Maxbright, ALL the Televue's, ALL the APM's, ALL the Baaders & Baader/Zeiss variants, etc., etc.  

    Funny thing is, with the 'low priced' models, there is a sample-to-sample variation, as you would expect, but that sample-to-sample variation ALSO exists in the $500+ models!


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#10 dusty99

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Posted 19 September 2020 - 12:54 PM

The stock Celestron prism diagonal is fine (of course, Celestron QC has at times been lax, so I don't doubt that some bad ones have gotten through to the public).  I have the Tak and four #94115-As and while the Tak might be a touch better, the advantages are small.  I do really like the compression ring instead of tension screws, but as Eddgie wrote, it's a pain with eyepieces that have big, square undercuts like Meade UWA, etc. (if they are seated all the way down they will not snug up).  Some of the newer non-slip eyepiece barrels like those on the Baader Morpheus work better.  Celestron makes a 1.25" mirror diagonal with twist-lock, and they should probably update the prism to twist-lock, too.


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#11 Brawlingsaz

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Posted 20 September 2020 - 10:26 AM

A noted CN member did a detailed comparison of about a dozen diagonals a few years back.  The results of that 'review' were, basically, accepted as 'Bible' by CN members.  I fell for it too, but luckily didn't sell ANY of my 'disparaged' diagonals until I could compare myself, and with a LOT of different Refractors, Reflectors, and Catadioptrics.  

    My own tests confirmed what Peter just said, and others have noted, and save for a small handful of the VERY pricey, 'Premium' Brand Diagonals, found that several of the 'lower priced' models were every bit as good as all the others, no matter the price.

    I am a big fan of the venerable Celestron #94115A Prism, and also have been extremely impressed by the Dialectric Diagonals offered by our Hosts, Astronomics.  In fact, the ONLY mirror diagonals I now use are the 2" and 1.25" Astronomics, and I regularly use, and am very happy with, the Celestron Prism (I have 3 of them!).

  Now, before you ask, YES, I have used the AP Maxbright, ALL the Televue's, ALL the APM's, ALL the Baaders & Baader/Zeiss variants, etc., etc.  

    Funny thing is, with the 'low priced' models, there is a sample-to-sample variation, as you would expect, but that sample-to-sample variation ALSO exists in the $500+ models!

Thanks for the insights and opinions.  Two questions, though.

 

Since Astronomics resells most/all brands.  When you say "the Dialectric Diagonals offered by our Hosts, Astronomics.  In fact, the ONLY mirror diagonals I now use are the 2" and 1.25" Astronomics,", are you referring to the Astronomics "house brand", Astro-Tech?  Or am I missing something, and does Astronomics have its own line of products outside of the Astro-Tech brand that it appears to own?

 

And also, since you own and use both mirror and prism diagonals, I'm curious as to why?  I had the impression that one of the two types was superior, and that for highest quality viewing, one would utilize the superior optic type.  But I don't know which is considered 'superior', and after reading your comments, it would seem that both have a place in your equipment repertoire.

 

So, can you (or anyone, all comments welcome) expand on when you prefer prism vs. mirror diagonals, what causes you to use one vs. the other - viewing conditions?  What you're looking at?  Any chromatic or "it just looks better/clearer/etc" reasons?  I thought it just came down to price and quality, but your comment implies that there is more to the story.


Edited by Brawlingsaz, 20 September 2020 - 10:47 AM.


#12 Bean614

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Posted 20 September 2020 - 10:39 AM

Yes, AstroTech!



#13 TG

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Posted 20 September 2020 - 12:04 PM

A diagonal is close to the eyepiece. Image aberrations due to its imperfect flatness will be smallish unless it's egregiously bad but scatter is another matter which is where prism diagonals provide an advantage.

 

Speaking of prism diagonals, I felt that I didn't understand them enough, so I did some calculations from first principles. This is what I found:

 

  • For a typical "regular glass-line" glass, focal ratios above 8 are sufficient to not introduce unwanted aberrations. See the polychromatic strehl vs. f-ratio graph below for three different glass types: BK7, FPL-53 and N-PK52A (why this one? see below)
  • At f/8 a 25mm BK7 diagonal will give a polychromatic strehl of 0.93. At f/10, it's 0.97 and at f/12 it's 0.98, which is sensibly perfect. So "regular" prism diagonals are not for perfectionists with A-P and Tele Vue refractors but should do well in SCTs. A BK-7 diagonal would not be diffraction limited below f/6.
  • What if a low dispersion glass like FPL-53 was used? Such a diagonal could do much better in terms of polychromatic strehl because of its low dispersion but at the same time, because of its low index, it cannot reflect rays from field points much off the axis making it useless as a diagonal material. It would not even reflect all the rays from the marginal zones (of the aperture) till about f/21, which would reduce the effective aperture, making it pretty useless as a diagonal material, so scratch this off the diagonal glass list.
  • To understand the previous bullet point, you have to understand the critical angle, at which the light ray inside the prism reflects off the diagonal side instead of passing through. The critical angle is a function of the refractive index and higher indices will have lower critical angles. Here are the critical angles for some glasses:
    • N-BK7: 41.2°
    • N-BAK4: 39.6°
    • FPL53: 44°
    • N-PK52A: 41.9°
  • For most (all?) glasses, the ray on axis (red ray in the diagram below) will always reflect since the critical angle is less than 45° but one angled to the axis (green ray) may not and will instead pass through. E.g., in the below diagram, the angle V has to be greater than the critical angle for the glass for total internal reflection to occur. As is apparent, as the off-axis angle U increases, which can be from faster f-ratios or wider fields, eventually V will fall below the critical angle and the ray will pass through. For BK-7, you have a leeway of about 4° before off-axis rays start passing through but for FPL-53 this value is only 1.4°. Therefore, a high index glass is necessary to reflect a rays from wider off-axis angles. However, for glasses on the "regular" glass line, index is roughly proportional to dispersion. So with high index also comes lower polychromatic strehl but better field illumination at wider fields or faster f-ratios. BTW, this is the reason binoculars tout BAK4 prisms, which is a higher index than BK7, because field illumination is better.
  • So, what is the better glass for prisms? You need high index to give better illumination over wider fields or faster f-ratios but you don't want high dispersion so polychromatic strehl can stay high. In some ways this is the dual of an ED glass, which tends to be low dispersion and low index. There aren't many glasses that fit the bill here but Schott's N-PK52A seems to be one and it seems to be in production. At f/8 such a glass would give 0.96 Strehl while at f/10 it would be sensibly perfect. It would be diffraction limited till about f/5. I don't know if any diagonals are manufactured using this glass but it wouldn't surprise me that there aren't any.
  • A minor point of interest: some prism diagonals will have a flocked backing because off-axis light can pass through the diagonal face and bounce off the housing.

 

TL; DR: BK-7 works well for f/ratios above 8 but there exist special glasses which would make for better diagonals.

 

Addendum: the polychromatic strehl values were computed assuming a perfect, aberration-free lens in OSLO. It's possible that there are lenses with aberrations which would balance out the prism induced aberrations giving a higher polychromatic strehl ratio.

 

TG

I7zSSLal.png

dxUvXwEl.pngAKSJDbgl.png


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#14 Old Speckled Hen

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Posted 20 September 2020 - 02:41 PM

A diagonal is close to the eyepiece. Image aberrations due to its imperfect flatness will be smallish unless it's egregiously bad but scatter is another matter which is where prism diagonals provide an advantage.

 

Speaking of prism diagonals, I felt that I didn't understand them enough, so I did some calculations from first principles. This is what I found:

 

  • For a typical "regular glass-line" glass, focal ratios above 8 are sufficient to not introduce unwanted aberrations. See the polychromatic strehl vs. f-ratio graph below for three different glass types: BK7, FPL-53 and N-PK52A (why this one? see below)
  • At f/8 a 25mm BK7 diagonal will give a polychromatic strehl of 0.93. At f/10, it's 0.97 and at f/12 it's 0.98, which is sensibly perfect. So "regular" prism diagonals are not for perfectionists with A-P and Tele Vue refractors but should do well in SCTs. A BK-7 diagonal would not be diffraction limited below f/6.
  • What if a low dispersion glass like FPL-53 was used? Such a diagonal could do much better in terms of polychromatic strehl because of its low dispersion but at the same time, because of its low index, it cannot reflect rays from field points much off the axis making it useless as a diagonal material. It would not even reflect all the rays from the marginal zones (of the aperture) till about f/21, which would reduce the effective aperture, making it pretty useless as a diagonal material, so scratch this off the diagonal glass list.
  • To understand the previous bullet point, you have to understand the critical angle, at which the light ray inside the prism reflects off the diagonal side instead of passing through. The critical angle is a function of the refractive index and higher indices will have lower critical angles. Here are the critical angles for some glasses:
    • N-BK7: 41.2°
    • N-BAK4: 39.6°
    • FPL53: 44°
    • N-PK52A: 41.9°
  • For most (all?) glasses, the ray on axis (red ray in the diagram below) will always reflect since the critical angle is less than 45° but one angled to the axis (green ray) may not and will instead pass through. E.g., in the below diagram, the angle V has to be greater than the critical angle for the glass for total internal reflection to occur. As is apparent, as the off-axis angle U increases, which can be from faster f-ratios or wider fields, eventually V will fall below the critical angle and the ray will pass through. For BK-7, you have a leeway of about 4° before off-axis rays start passing through but for FPL-53 this value is only 1.4°. Therefore, a high index glass is necessary to reflect a rays from wider off-axis angles. However, for glasses on the "regular" glass line, index is roughly proportional to dispersion. So with high index also comes lower polychromatic strehl but better field illumination at wider fields or faster f-ratios. BTW, this is the reason binoculars tout BAK4 prisms, which is a higher index than BK7, because field illumination is better.
  • So, what is the better glass for prisms? You need high index to give better illumination over wider fields or faster f-ratios but you don't want high dispersion so polychromatic strehl can stay high. In some ways this is the dual of an ED glass, which tends to be low dispersion and low index. There aren't many glasses that fit the bill here but Schott's N-PK52A seems to be one and it seems to be in production. At f/8 such a glass would give 0.96 Strehl while at f/10 it would be sensibly perfect. It would be diffraction limited till about f/5. I don't know if any diagonals are manufactured using this glass but it wouldn't surprise me that there aren't any.
  • A minor point of interest: some prism diagonals will have a flocked backing because off-axis light can pass through the diagonal face and bounce off the housing.

 

TL; DR: BK-7 works well for f/ratios above 8 but there exist special glasses which would make for better diagonals.

 

Addendum: the polychromatic strehl values were computed assuming a perfect, aberration-free lens in OSLO. It's possible that there are lenses with aberrations which would balance out the prism induced aberrations giving a higher polychromatic strehl ratio.

 

TG

I7zSSLal.png

dxUvXwEl.pngAKSJDbgl.png

 

 

Thank you for taking the time to produce and share that, it is very interesting [if somewhat detailed for a mortal such as myself] however the last few bullet points are the most telling. frown.gif

 

Edit.. https://www.cloudyni...-vs-mirrors-r79

 

And further  http://www.astrosurf...tardiagonal.htm


Edited by Andrew Brown, 20 September 2020 - 06:13 PM.

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#15 luxo II

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Posted 20 September 2020 - 10:25 PM

Shhhh.... don't tell the APO refractor brigade - especially Takaholics !!!

 

I suspect the modern fast APOs intended for imaging likewise have no compensation for a prism diagonal, and if using one for visual, have their polychromatic strehl similarly affected. If - at f/7 - it is effectively no better than 0.95 and quite possibly 0.90 the reaction from owners of Taks and AP's could be interesting to watch.

 

40 years ago the traditional reason not to use prisms with reflectors concerned spherical aberration - a prism in a converging beam of light introduces a small amount. The correction of premium refractors sometimes assumed a prism diagonal would be used and the objective compensated for it, by design.

 

But reflectors are generally fully corrected for spherical aberration as the basic assumption is that if a diagonal is used, it would be a mirror...


Edited by luxo II, 20 September 2020 - 10:57 PM.

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#16 Bill Barlow

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Posted 22 September 2020 - 09:26 AM

I’ve compared the Tak prism diagonal to a TV 1.25” Everbright many times in my refractors and the Tak is sharper.  
 

Bill


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#17 hoof

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Posted 24 September 2020 - 12:33 PM

Interesting tidbit:  Baader's high-end prism diagonals have silver coatings on the relfecting sides to ensure that all light is reflected, even if the critical angle issue would mean it passes through.  That should reduce the chromatic aberration induced by the prism on faster telescopes.

 

If you've ever wondered why they talk about the silvered sides in their prism descriptions, that is why.


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#18 TG

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Posted 24 September 2020 - 04:50 PM

Thanks for the tidbit @hoof. I knew about the flocked backing but not this. Makes perfect sense.

#19 luxo II

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Posted 24 September 2020 - 05:07 PM

... the Tak is sharper.


But in what scope ?

#20 charlesgeiger

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Posted 27 September 2020 - 09:57 PM

I agree that a quality 2" diagonal from most suppliers is up to the task.  One of the things so far not mentioned is the application of dielectric coatings onto a 1/10th wave mirror diagonal.  To get the high reflectance, many applications of coatings (at higher temperatures) are applied.  All these coatings can change the final figure.  So it has been said to check if the 1/10th wave rating is before or after these applications.  Most say the rating is given before the applications and that the flatness is much altered after the application.  So do diligence suggests one ask the sales rep if the wave rating is after the coating process.  If it is before, which apparently most all are, you may have at best a 1/4 wave mirror diagonal.  But for most applications not having a steep light cone to the diagonal, the flatness of the diagonal isn't of great concern and the central part of the diagonal is what is most important.

I have a 2" dielectric I purchased a few years ago for about $120.00 of great quality and rigidity.  I use it on a 80mm ED refractor and on a large SCT and it is excellent in all respects.  If you want to get a short optical path length for binoviewers using a short refractor or reflector, a Baader prism or mirror will give you that.  I cannot speak to the quality.  

Charlie




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