Mirror vs prism diagonal for fast refractor

[azrabella]

Interested in a diagonal for my f/5 richest field refractor and am not sure of the issues with using a mirror vs prism diagonal. Some say that for fast scopes a mirror is best, while others say that faster than about f/7 though, the prism will induce spherochromatism. Does this apply more to a doublet (non apo) refractor, or is the difference negligible at relatively low power? I intend using the scope for comet patrol and ideally require orientation to be as per binocular view.

[vlaiv]

It has more to do with beam angle than with objective, so fast scopes + prism = some color, regardless of type of objective (ed, apo, ....).

If you want correct image orientation over other things, then erecting prism is the way to go.

These tend to be expensive in high quality/astronomical variants. At low powers I don't think that color nor lesser quality diagonal will be much of a problem though.

[Ben the Ignorant]

Sky-Watcher sells all types of refractors, achromats, semi-apos, apos, doublets and triplets, but Sky-Watcher sells no prism diagonals, only mirrors, so...

[Bruce Leeroy]

An interesting post here, maybe best to avoid the amici style prism?

 

[Moonshane]

I'd think for the stated purpose, the type of diagonal would make little if any difference and if you want a correct, upright image there's no option other than to use a prism.

[Klitwo]

Mirrors require maintenance over time....whereas prisms are more-or-less stand alone and require less attention.  If you're looking for perfection in reflection in terms of coatings....then expect to pay for it with mirrors.  Both mirrors or prisms used in star diagonals have thier attributes but in the end....astronomical discoveries are made with both or none at all. 

However...those who desire perfection in thier astronomical viewing will perhaps appreciate the following link and the information it contains in thier search for the so-called perfect star diagonal that will meet thier astronomical needs and expectations....>

http://www.baader-planetarium.de/news/mirror-Prism-Dielectric-Diagonal-Comparison_2014-03-06_v2.pdf

Klitwo

Edited January 16, 2019 by Klitwo

[Louis D]

8 hours ago, Klitwo said:

Mirrors require maintenance over time....whereas prisms are more-or-less stand alone and require less attention.  If you're looking for perfection in reflection in terms of coatings....then expect to pay for it with mirrors.  Both mirrors or prisms used in star diagonals have thier attributes but in the end....astronomical discoveries are made with both or none at all. 

However...those who desire perfection in thier astronomical viewing will perhaps appreciate the following link and the information it contains in thier search for the so-called perfect star diagonal that will meet thier astronomical needs and expectations....>

http://www.baader-planetarium.de/news/mirror-Prism-Dielectric-Diagonal-Comparison_2014-03-06_v2.pdf

Klitwo

And a more recent follow-on article by Bill P. about the Baader Broadband Hardened Silver (BBHS) diagonal versus a dielectric and prism diagonal.

[Klitwo]

5 hours ago, Louis D said:

And a more recent follow-on article by Bill P. about the Baader Broadband Hardened Silver (BBHS) diagonal versus a dielectric and prism diagonal.

With favorable reviews...the BAADER BBHS 2" CLICKLOCK SILVERED MIRROR DIAGONAL lists for a "deep pockets" price of $764USD....but probably could be had after shopping around a little for a modest sales price approaching $500USD....if you could find any that were available that is....>

https://www.eyepiecesetc.com/BAADER_BBHS_2_CLICKLOCK_SILVER_DIAGONAL_2456115_p/1403010.htm

Klitwo

Edited January 16, 2019 by Klitwo

[Alan64]

This erect-image diagonal has a 29mm aperture...

https://uk.telescope.com/Orion-125-Pentaprism-Diagonal/p/130876.uts

It's a five-sided prism, a pentaprism diagonal, and for correctly-oriented views.  In addition, there would be no Amici-line seen when viewing brighter objects; although you would very rarely encounter that, if at all.

Given its generous aperture, and per the observational agenda, that one would be best; for a correctly-oriented view.  It is only a 1.25", however.  For comet-hunting, I would use a 2" diagonal, and 2" oculars.

Edited January 16, 2019 by Alan64

[Klitwo]

1 hour ago, Alan64 said:

This erect-image diagonal has a 29mm aperture...

https://uk.telescope.com/Orion-125-Pentaprism-Diagonal/p/130876.uts

It's a five-sided prism, a pentaprism diagonal, and for correctly-oriented views.  In addition, there would be no Amici-line seen when viewing brighter objects; although you would very rarely encounter that, if at all.

Given its generous aperture, and per the observational agenda, that one would be best; for a correctly-oriented view.  It is only a 1.25", however.  For comet-hunting, I would use a 2" diagonal, and 2" oculars.

Speaking of comet hunting brings to mind Mr. William Bradfield of Australia and his homemade 6-inch 100 plus year old Dallmeyer camera lens refractor that was equipped only with an old war surplus 26x Erfle eyepiece mounted on his barn yard rickety looking wooden adjustable height tripod.  However strange looking his 6-inch refractor and wooden tripod setup appeared to the modern day comet hunter....Mr. Bradfield was able to discover "visually" an extrodinary long list of comets that bear his name and his name only.  And interesting enough...he did so "without" the use of a star diagonal....>

https://www.skyandtelescope.com/astronomy-news/bill-bradfield-comet-hunter-extraordinaire-1927-2014/

http://i.picasion.com/pic88/b11d367439789ef9e6c7fc3806230a38.gif

P.S.  Even with all of the modern comet hunting refractors of today that sport the ED glass, top-of-the-line star diagonals and high-end primary mirrors and secondaries that yield nearly 99% reflectivity...including those that are equipped with the latest CCD technology.....it's a good bet that Mr. Bradfield's remarkable list of visual comet discoveries in the modern era will no doubt "not" be surpassed and will be around for a long long time. Whereas most if not all of us would be happy with just "one" comet discovery bearing our name in our life time....Mr. Bradfield had 18.....

Klitwo

Edited January 16, 2019 by Klitwo

[Alan64]

2 hours ago, Klitwo said:

Speaking of comet hunting brings to mind Mr. William Bradfield of Australia and his homemade 6-inch 100 plus year old Dallmeyer camera lens refractor that was equipped only with an old war surplus 26x Erfle eyepiece mounted on his barn yard rickety looking wooden adjustable height tripod.  However strange looking his 6-inch refractor and wooden tripod setup appeared to the modern day comet hunter....Mr. Bradfield was able to discover "visually" an extrodinary long list of comets that bear his name and his name only.  And interesting enough...he did so "without" the use of a star diagonal....>

https://www.skyandtelescope.com/astronomy-news/bill-bradfield-comet-hunter-extraordinaire-1927-2014/

http://i.picasion.com/pic88/b11d367439789ef9e6c7fc3806230a38.gif

P.S.  Even with all of the modern comet hunting refractors of today that sport the ED glass, top-of-the-line star diagonals and high-end primary mirrors and secondaries that yield nearly 99% reflectivity...including those that are equipped with the latest CCD technology.....it's a good bet that Mr. Bradfield's remarkable list of visual comet discoveries in the modern era will no doubt "not" be surpassed and will be around for a long long time. Whereas most if not all of us would be happy with just "one" comet discovery bearing our name in our life time....Mr. Bradfield had 18.....

Klitwo

Indeed, the addition of a diagonal of any sort can never be an improvement, observationally; only in an ergonomic sense.  However, the use of diagonals has become so commonplace that their omission is practically unthinkable for the vast majority of amateurs; and that is regrettable, particularly when combining a mirrored diagonal with a refractor, thereby transforming the refractor into a catadioptric.

[mikeDnight]

1 hour ago, Alan64 said:

Indeed, the addition of a diagonal of any sort can never be an improvement, observationally; only in an ergonomic sense.  However, the use of diagonals has become so commonplace that their omission is practically unthinkable for the vast majority of amateurs; and that is regrettable, particularly when combining a mirrored diagonal with a refractor, thereby transforming the refractor into a catadioptric.

A good diagonal won't degrade the image though, as if they did, no one would use them. Also, turning a refractor into a catadioptric by the use of a diagonal is like saying a reflector becomes a refractor by the use of an eyepiece. The mirror or prism only redirects the light, it doesn't play an integral part in forming the image as a corrector plate or meniscus lens would in a true cat'. 

It's generally felt that mirrors work best in fast F ratio scopes and that may be true, but a friend of mine uses a Tak prism in his Sky 90 without any noticeable degradation in image quality. You could perhaps find an old circle T prism, which are excellent, and try it out. Some cheap prisms though can be atrocious. I had to atomise at least two Celestron prisms because they delivered putrid images. But cheap mirrors can be equally as bad.

Edited January 16, 2019 by mikeDnight

[iPeace]

1 hour ago, mikeDnight said:

a friend of mine uses a Tak prism in his Sky 90 without any noticeable degradation in image quality

I use a Tak prism in my Borg 71FL, which, if I'm not mistaken, has the same focal ratio of 5.6. The views are better to my eyes than with a mirror diagonal - one labelled 'everbrite dielectric' from some obscure firm named 'Tele Vue'.

That said, I'll probably keep at least one good mirror handy to swap in whenever doubts should descend.

[alanjgreen]

6 hours ago, Alan64 said:

Indeed, the addition of a diagonal of any sort can never be an improvement, observationally; only in an ergonomic sense.  However, the use of diagonals has become so commonplace that their omission is practically unthinkable for the vast majority of amateurs; and that is regrettable, particularly when combining a mirrored diagonal with a refractor, thereby transforming the refractor into a catadioptric.

Actually, SCTs use a special “magnifying” mirror for the secondary. Celestron’s SCT secondary mirror magnifies the image by x5 as well as reflecting the image. That is where the long focal length of SCT come from.

I use a Zeiss Prism diagonal and really like the clarity of the image in my refractor. It is also less prone to reflections generated by bright objects from outside the fov.

Alan

[Alan64]

2 hours ago, mikeDnight said:

A good diagonal won't degrade the image though, as if they did, no one would use them. Also, turning a refractor into a catadioptric by the use of a diagonal is like saying a reflector becomes a refractor by the use of an eyepiece. The mirror or prism only redirects the light, it doesn't play an integral part in forming the image as a corrector plate or meniscus lens would in a true cat'. 

It's generally felt that mirrors work best in fast F ratio scopes and that may be true, but a friend of mine uses a Tak prism in his Sky 90 without any noticeable degradation in image quality. You could perhaps find an old circle T prism, which are excellent, and try it out. Some cheap prisms though can be atrocious. I had to atomise at least two Celestron prisms because they delivered putrid images. But cheap mirrors can be equally as bad.

People use diagonals regardless if they are good or bad because it's far more ergonomic.  Straight-through observing is a lost practice for the vast majority, save for the Japanese(at last count).

The secondary-mirror of a Newtonian is that telescope's "diagonal".  Have a look at this "refractor"...

https://explorescientificusa.com/products/products-package-deal-bresser-comet-edition-102mm-refractor-kit-variant-2221347844

That is a catadioptric.  

In so far as a diagonal not degrading an image, think again.  The more you add to the light-path, the greater the chance for degradations; light-scattering in the case of a mirrored diagonal.  Then, there's the miscollimation of either a mirror or prism with which to contend.

When one does place a diagonal into a refractor, you still can't use the telescope until you add an eyepiece, therefore the diagonal, when integrated, becomes one with the refractor, a component of the telescope's light path, and just as the secondary-mirror of a Newtonian; therefore, again, a catadioptric when using a mirrored diagonal.  I suppose, rather, that you might hold that a Newtonian becomes a catadioptric when an eyepiece is inserted; most definitely not a refractor by any stretch of the imagination; however your point is moot, as eyepieces are not an integral part of a telescope's light-path, rather the receivers of the light-path instead.  Observe what happens when the tip of the light-cone of an objective nears an eyepiece...

 

Note where the tip of the light-cone ends, and just as it exits the body of the diagonal.

Eyepieces are, again, merely receivers, and therefore cannot transform any telescope into another.

This is my 80mm f/6 achromat, but with a 2" mirrored diagonal.  Configured so, it is in a catadioptric mode of operation...

If I take the mirrored diagonal out, and put a prism diagonal in its place, the telescope then reverts back to being a refractor; closer in any event, as some regard the "total internal reflection" of a prism diagonal as just that, a reflection.  However, there is no light-scattering, aluminised coating involved.  In a straight-through configuration, without a diagonal at all, the refractor is at its very best, in form, function, and performance. 

Incidentally, in the case of a barlow, if inserted before the diagonal it becomes a part of the telescope's light-path.  If the barlow is inserted after the diagonal, it becomes a part of the receiver, the eyepiece.

2" mirrored diagonals are more popular not because they're so very wonderful, but simply because they're cheaper to make, and purchase.  I do not look upon mirrors as a cure-all, and as so very many others do.

[vlaiv]

25 minutes ago, Alan64 said:

People use diagonals regardless if they are good or bad because it's far more ergonomic.  Straight-through observing is a lost practice for the vast majority, save for the Japanese(at last count).

The secondary-mirror of a Newtonian is that telescope's "diagonal".  Have a look at this "refractor"...

https://explorescientificusa.com/products/products-package-deal-bresser-comet-edition-102mm-refractor-kit-variant-2221347844

That is a catadioptric.  

In so far as a diagonal not degrading an image, think again.  The more you add to the light-path, the greater the chance for degradations; light-scattering in the case of a mirrored diagonal.  Then, there's the miscollimation of either a mirror or prism with which to contend.

When one does place a diagonal into a refractor, you still can't use the telescope until you add an eyepiece, therefore the diagonal, when integrated, becomes one with the refractor, a component of the telescope's light path, and just as the secondary-mirror of a Newtonian; therefore, again, a catadioptric when using a mirrored diagonal.  I suppose, rather, that you might hold that a Newtonian becomes a catadioptric when an eyepiece is inserted; most definitely not a refractor by any stretch of the imagination; however your point is moot, as eyepieces are not an integral part of a telescope's light-path, rather the receivers of the light-path instead.  Observe what happens when the tip of the light-cone of an objective nears an eyepiece...

 

Note where the tip of the light-cone ends, and just as it exits the body of the diagonal.

Eyepieces are, again, merely receivers, and therefore cannot transform any telescope into another.

This is my 80mm f/6 achromat, but with a 2" mirrored diagonal.  Configured so, it is in a catadioptric mode of operation...

If I take the mirrored diagonal out, and put a prism diagonal in its place, the telescope then reverts back to being a refractor; closer in any event, as some regard the "total internal reflection" of a prism diagonal as just that, a reflection.  However, there is no light-scattering, aluminised coating involved.  In a straight-through configuration, without a diagonal at all, the refractor is at its very best, in form, function, and performance. 

Incidentally, in the case of a barlow, if inserted before the diagonal it becomes a part of the telescope's light-path.  If the barlow is inserted after the diagonal, it becomes a part of the receiver, the eyepiece.

2" mirrored diagonals are more popular not because they're so very wonderful, but simply because they're cheaper to make, and purchase.  I do not look upon mirrors as a cure-all, and as so very many others do.

If you go by the definition: Catadioptric telescope is one combining reflective and refractive elements to form the image at focal plane of the telescope - then you could say that refractor + diagonal is catadioptric telescope.

Then there is different definition: Catadioptric telescope is one using both reflective and refractive elements to shape light into forming an image at focal plane - then flat diagonal is nothing more but device to position focal plane without altering the shape of light beam. It is not altering the waveform in any way (not deliberately / by design at least).

Most people when using term Catadioptric system - thinks second definition, so refractor + diagonal is not considered to be catadioptric telescope.

[Alan64]

29 minutes ago, alanjgreen said:

Actually, SCTs use a special “magnifying” mirror for the secondary. Celestron’s SCT secondary mirror magnifies the image by x5 as well as reflecting the image. That is where the long focal length of SCT come from.

I use a Zeiss Prism diagonal and really like the clarity of the image in my refractor. It is also less prone to reflections generated by bright objects from outside the fov.

Alan

A catadioptric is any telescope where refraction and reflection are combined; lenses and mirrors, whether a modified-Cassegrain(Schmidt and Maksutov), or a refractor with a mirrored diagonal.  If you must use a diagonal with a refractor, always endeavour to make that one a prism; to preserve the "feng shui".  A mirrored diagonal is suggested, however, for f/5 achromats.  In addition, combining a mirrored diagonal with a Schmidt- or Maksutov-Cassegrain is, in effect, adding a third mirror to the light-path, which may only serve to increase the incidence of light-scattering.  

[vlaiv]

3 minutes ago, Alan64 said:

A catadioptric is any telescope where refraction and reflection are combined; lenses and mirrors

Just add "to form the image" at the end of that sentence

Refractor is well capable of forming the image without use of diagonal (either mirror or prism).

If you want to be more specific, then there is divide in catadioptric telescopes in true catadioptrics and hybrid catadioptrics, have a look here:

https://www.telescope-optics.net/catadioptric.htm

"Strictly talking, catadioptric telescope is designed as a synergy of reflective and refractive elements, requiring both for its functioning. Such design is sometimes called true catadioptric. That separates it from hybrid catadioptric, usually a mirror telescope using refractive field-corrector, which can function without its refractive component."

But in all cases both reflective and refractive elements need to alter waveform and converge/diverge light beam to be considered integral part of telescope design.

Although Newtonian is considered two mirror system (it's got two mirrors, right?) - it is not really - it is single mirror system, and you can use such telescope without secondary mirror (for photographic purposes for example, much harder for visual as one's head would be in the way).

[Alan64]

24 minutes ago, vlaiv said:

If you go by the definition: Catadioptric telescope is one combining reflective and refractive elements to form the image at focal plane of the telescope - then you could say that refractor + diagonal is catadioptric telescope.

Then there is different definition: Catadioptric telescope is one using both reflective and refractive elements to shape light into forming an image at focal plane - then flat diagonal is nothing more but device to position focal plane without altering the shape of light beam. It is not altering the waveform in any way (not deliberately / by design at least).

Most people when using term Catadioptric system - thinks second definition, so refractor + diagonal is not considered to be catadioptric telescope.

Most people when using or hearing of the term "catadioptric" do indeed think of the second definition: Schmidt- and Maksutov-Cassegrains, therefore a refractor, with or without a mirrored diagonal, is most certainly not either one of those.  Although, the Maksutov-Cassegrain has been described as being "refractor-like", and is the only reflecting telescope so considered.  A Maksutov is also the only modified-Cassegrain that I would own and operate.  I would also like to have a classical Cassegrain.

[Alan64]

21 minutes ago, vlaiv said:

Just add "to form the image" at the end of that sentence

No, thank you.

[JTEC]

Bill Paolini’s review of different diagonals, including the silvered Baader is, imv, a must-read.  After using the Baader Zeiss spec 2 inch and 1.25 inch prisms, I sold the Astro-Physics dielectric that I’d bought thinking it would be The Best.  It clearly wasn’t in terms of contrast, transmission or scatter. It wasn’t bad, of course, just a shade behind the prisms. The smaller one works exceptionally well with a binoviewer.  I’m a fussy observer and I’ve never seen any residual colour when using the prisms at any power. Oddly enough, I sometimes suspected it with the heavily coated dielectric. I couldn’t explain this but speculated that the accumulated depth of the numerous coatings was perhaps sufficient to make some unwanted contribution to the image. Whether or not that’s right, I’m quite clear that the B-Z prisms give superior results. That said, that BBHS mirror does look extremely interesting and I’d love to give one a go ... but there comes a point when the spending has to stop ?

[John]

This thread is in danger of becoming an elaborate discourse on the topic of diagonals which might not actually be of much practical help to the original poster

I think Moonshanes post hits the nail on the head with regard to the original question:

 

 

 

[Louis D]

On 15/01/2019 at 10:03, Moonshane said:

I'd think for the stated purpose, the type of diagonal would make little if any difference and if you want a correct, upright image there's no option other than to use a prism.

I think if you use two ordinary star diagonals at 90 degrees to each other and look into the eyepiece from the side rather than back of the scope, you'll also get an upright and non-reversed image.  This presumes sufficient backfocus, but if the scope supports binoviewers natively, this shouldn't be an issue.

[Klitwo]

There are so many choices for star diagonals (prisms or mirrors) out there on the market today that it's almost too confusing for the user or beginning novice to decide which one to choose...especially if you are budget minded.  No matter which top quality star diagonal you end up with...there is always going to be a better one out there thanks to new coatings technology.  However, if you're just interested in buying a star diagonal for general observing use on a f/5 Richest Field refractor....I would recommend reading the following information first to educate one self about star diagonals (their attributes...etc.) and then start focusing (pardon the pun) in on which commercial star diagonals (read the reviews) would be the best fit for your observing needs.....>

https://en.wikipedia.org/wiki/Star_diagonal

Klitwo  

[Alan64]

The hunting of comets requires a wide field-of-view, the widest possible, and at the lower if not the lowest of powers.  An f/5 refractor is ideal for that very thing.  Somewhat recently, I had my 80mm f/6 out observing Comet Wirtenan...

Over the course of time, fifteen to thirty minutes, I was able to notice the comet's movement in relation to other objects nearby, and quite close to the comet those were, thereby making the detection that much easier.  All in all, not a bad show, not at all.  A 200mm f/2.4 achromat would've been far better, of course, if such were possible; ever hopeful I am in that fantasy.  

The f/5 achromat in question, an "ST80" I believe, comes with a 1.25" focusser.  A 32mm Plossl would provide the lowest power(12.5x) and the widest view practical.  80mm f/5 achromats are routinely re-fitted by amateurs with 2" focussers, for this purpose and that.  This one from China appears to be viable, and with a 78mm threaded interface; naturally, a bit of DIY work is to be expected in its integration...

https://www.ebay.com/itm/New-2-Manual-Gear-Focusing-Focuser-for-Refraction-Astronomy-Telescope/323419115167?hash=item4b4d480e9f:g:PwsAAMXQDK1RtT2P:rk:27:pf:0&shqty=1#shId

I changed the country to "United Kingdom", then selected "Get Rates": £54, and with free shipping.  

However, it would be for the user to decide if the effort would be worth the while.  To wit, a view of the Moon with an 80mm f/5 achromat, a 1.25" 32mm 52° Plossl, and a 2" 32mm 70­° wide-angle ocular...

https://goo.gl/fCQkdG

Then, if the views through the achromat have been beyond satisfactory, that is, if the achromatic doublet is of an above-average figurement and polish, then it would most certainly be worth the while to upgrade it.  If the level of quality of the doublet is presently unknown, get the power up to 150x or greater, aim it at the Trapezium within Orion, for one, and note the thinness of the first-diffraction rings of the brightest stars.  You want them as razor-sharp as possible, and the atmospheric seeing is to be at least good, or better.

If said upgrade is not possible or practical, a 32mm Plossl does, after all, offer a reasonably wide field-of-view.  Although, I would not suggest spending a lot on a 1.25" mirrored star-diagonal, if you're going to stick with the 1.25" focusser.  You may, however, want better, a dielectric, and for the specialised purpose at hand.