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Opinions on prism/mirror diagonals


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I recently purchases a SvBony f5.5 Achro refractor and I have been very pleased with the results. I have not seen the requirement to purchase any of the Baader correction filters.

When I first obtained the scope I bought a StellaMira erecting prism -https://www.firstlightoptics.com/diagonals/stellamira-125-90-erecting-prism-diagonal.html because I was going on holiday and wanted to undertake some terrestrial viewing. The quality that I obtain was very good including many delights of Sagittarius.

Although the StellaMira diagonal is very good and I have viewed many DSOs recently (some with a TeleVue Nebustar filter) M11, M16, M17,M20, M21, M13, M92, M2, M15, Helix Nebula etc to give you an idea. I am wondering whether I could improve things a bit more by obtaining a normal star diagonal.

I have looked at this Tak prism - https://www.firstlightoptics.com/diagonals/takahashi-125-diagonal.html or a quality dielectric diagonal - https://www.firstlightoptics.com/diagonals/stellamira-1-25-90-di-electric-diagonal.html

I have never used a star diagonal prism and I am wondering which might be best - hence the question.

 

 

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In 1.25-inch size, the Tak prism diagonal is hard to beat.

Optically it is excellent. Some people comment negatively about its construction, but an advantage of its lightweight construction is that it is, well, light weight 🤔

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I have a Tak prism that I like, but I use it with an F12 Mak.
There is some thought that fast scopes are better with a dielectric than a prism, less CA.

https://stargazerslounge.com/topic/328670-mirror-vs-prism-diagonal-for-fast-refractor/

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

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I've got the William Optics 45 degree 1.25" erecting prism and it's great at low powers but starts to get soft as the magnification increases and doesn't let me get close to pushing the limits of my little 60mm scope. Above about 50x I can see a very noticeable difference in sharpness and micro contrast when using my StellaMira dielectric diagonal.

Prisms tend to have shorter optical path lengths which can enable them to be used with a wider range of scopes and should have less scatter, but as Zermelo points out they might not be quite as good in faster scopes. I've also got the Baader 32mm T-2 prism which is superb. It was a toss-up between that and the Takahashi prism and I went with the Baader because of the flexibility of the T-2 system but would have been just as happy with the Tak model which I gather is optically superb.

You can get astro quality erecting prisms; Baader do a couple, but they come with an astronomical price tag (see what I did there 😅).

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I use the excellent Baader T2 Zeiss prism with my Tak 100 but that is F/9. Not sure that prisms are quite such a good option for faster refractors :icon_scratch:

Personally I stick to mirror diagonals for my faster refractors. The Stellamira 1.25" looks the same as the diagonals sold under a number of brandings including William Optics. They are nicely made and work very well. The only exception is that the design uses a rather thick ring internally where the eyepiece holder joins the body. This can vignette the field of view when used with eyepieces approaching the maximum field stop size for the 1.25" fitting eg: 32mm plossls, 24mm 68's etc.

 

Edited by John
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Following in from my previous post, some refractors are specifically designed for use with prisms and are better corrected than if they're used with a mirror diagonal. Zeiss did it with the APQ and I've read that it's also true of the little Tak FS-60 but I suspect that there are relatively few scopes designed this way and probably none outside of a handful of high end models.

Ideally you'd test the different types side by side to see if there's a difference, but as the safer bet I'd go for a mirror diagonal with an f5.5 achromat.

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52 minutes ago, Mark at Beaufort said:

There have been some excellent replies. This is not my main scope and I think I will go with dielectic diagonal. 

 

Ah, but which one? StellaMira?

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In microscopy the higher magnifications produced by shorter focal length objectives ( steeper cones of light ) require an adjustment to correct for the thickness of the cover slips.  A cover slip, which is a plane sheet of glass, introduces spherical aberration in the converging/diverging optical path. A correction for this aberration is achieved with an adjustable ring on the higher power objectives which alters the spacing of the objective's elements. Applying this to telescopes, a prism is effectively a plane sheet of glass ( albeit folded ) in the converging beam from the objective. It must, therefore, introduce some spherical aberration to the image. This aberration will be worse with smaller F/ ratios ( steeper cone of light ) and will always show up more at higher powers. Hence the comments in the posts above.

I very much doubt that any telescope manufacturer will design their objectives to correct for the use of a prism diagonal as when used without a prism there would then be spherical aberration of opposite sign present in the image.

A further consideration is that a prism diagonal has three surfaces with which the light interacts giving three opportunities to add their own errors to the wavefront while a reflecting mirror only has one. The reflective surfaces of both the mirror and diagonal will add double their surface errors. The plane surfaces of the prism are transmitting the light and their individual errors affect the light much less than a reflective surface, but there are two of them.

 

Nigel

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1 hour ago, Astrobits said:

In microscopy the higher magnifications produced by shorter focal length objectives ( steeper cones of light ) require an adjustment to correct for the thickness of the cover slips.  A cover slip, which is a plane sheet of glass, introduces spherical aberration in the converging/diverging optical path. A correction for this aberration is achieved with an adjustable ring on the higher power objectives which alters the spacing of the objective's elements. Applying this to telescopes, a prism is effectively a plane sheet of glass ( albeit folded ) in the converging beam from the objective. It must, therefore, introduce some spherical aberration to the image. This aberration will be worse with smaller F/ ratios ( steeper cone of light ) and will always show up more at higher powers. Hence the comments in the posts above.

I very much doubt that any telescope manufacturer will design their objectives to correct for the use of a prism diagonal as when used without a prism there would then be spherical aberration of opposite sign present in the image.

A further consideration is that a prism diagonal has three surfaces with which the light interacts giving three opportunities to add their own errors to the wavefront while a reflecting mirror only has one. The reflective surfaces of both the mirror and diagonal will add double their surface errors. The plane surfaces of the prism are transmitting the light and their individual errors affect the light much less than a reflective surface, but there are two of them.

 

Nigel

Nigel what a fantastic reply and really explains the situation. This confirms my intention to get a dielectric diagonal.

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3 hours ago, Astrobits said:

In microscopy the higher magnifications produced by shorter focal length objectives ( steeper cones of light ) require an adjustment to correct for the thickness of the cover slips.  A cover slip, which is a plane sheet of glass, introduces spherical aberration in the converging/diverging optical path. A correction for this aberration is achieved with an adjustable ring on the higher power objectives which alters the spacing of the objective's elements. Applying this to telescopes, a prism is effectively a plane sheet of glass ( albeit folded ) in the converging beam from the objective. It must, therefore, introduce some spherical aberration to the image. This aberration will be worse with smaller F/ ratios ( steeper cone of light ) and will always show up more at higher powers. Hence the comments in the posts above.

I very much doubt that any telescope manufacturer will design their objectives to correct for the use of a prism diagonal as when used without a prism there would then be spherical aberration of opposite sign present in the image.

A further consideration is that a prism diagonal has three surfaces with which the light interacts giving three opportunities to add their own errors to the wavefront while a reflecting mirror only has one. The reflective surfaces of both the mirror and diagonal will add double their surface errors. The plane surfaces of the prism are transmitting the light and their individual errors affect the light much less than a reflective surface, but there are two of them.

 

Nigel

There are manufacturers that design objectives with a view to them being used with a prism diagonal, but it's not common. Here's an explanation from CFF about choosing a diagonal to go with their different refractor models:

The color correction of Visual-tuned lenses is designed to give peak Strehl ratio where human eye is the most sensitive.

For this reason, these lenses deliver best possible image sharpness and contrast for visual observations. Technically speaking, they are tuned to reach peak Strehl ratio in a limited range of wavelengths (430 nm – 700 nm).

Our recommendation is for these Visual tuned lenses to be used with mirror diagonals. This type of lens gives excellent photographic results when used with IR/UV filters.

The color correction of Photographic-tuned lenses is designed for covering a sensibly wider range of wavelengths. These lenses will be better corrected for near IR and violet wavelengths. Depending on the type of the photographic sensor used and its sensitivity range, images can be taken without the need of an IR/UV filter.

The acceptable corrected wavelength range is usually 400 nm – 1000 nm. As photographic sensors are sensitive over this range, this type of color tuning nicely matches the needs of photographers.

It has to be noted that Strehl ratio becomes slightly lower in red and blue colors and experienced planetary observers might detect the difference under excellent seeing conditions. To get optimal Visual performance from these lenses, some glass needs to be added in the light path, and in our case, this can be represented by a prism diagonal (1.25″ or 2″ size). As prism diagonals typically have better optical quality, better light transmission and less light scatter than mirror diagonals, we encourage all amateur astronomers to use them.

With the Zeiss APQ 100mm f/6.4 the effect of diagonal choice was quite significant and using a mirror rather than a prism meant it behaved more like a semi-app than a super-app as it was with a prism. Their scopes were apparently designed for use with a BK7 prism with an optical path length of 35-45mm. A prism should have lower scatter than a mirror and it should have better plane surfaces for a given quality of manufacturing but how much difference the average observer would see when using either with the average refractor is another matter.

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Call me naive, but I go by what I see rather than all the clever theory and detailed analysis 🙂.

I use a Baader T2 prism with 32mm clear aperture and both 2" and 1.25" nosepieces, and I also use an Astro-Tech 2" Dielectric mirror diagonal in my FS128.

Both are excellent and give great views. I cannot usually see any meaningful difference between between either unit apart from the prism taking up much less inward focus than the 2" mirror unit..this matters when I am binoviewing as I can use my binoviewers natively with no barlow lens needed.

Occasionally, when viewing close doubles at high power I have had the impression that light scatter is slightly better controlled in the prism.

The Baader T2 prism, having 32mm clear aperture, will fully illuminate most 1.25" eyepieces with no vignetting. 

I chose Baader over Tak for the prism purely because of the impression of more robust, stronger construction, especially when loaded with a binoviewer and 2 eyepieces. The Tak is superb when using lightweight small eyepieces.

HTH,

Dave

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The point about some frac manufacturers optimising for the type of prism is presumably moot for Tak. Most Japanese Tak users use their fracs in straight through mode, which seems to be the standard in Japan.

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I use a Baader prism on my classical cassegrain and Scopetech 80 where they are ideally suited. 

My Made in Japan Scopetech 80 f/15 is indeed intended for straight through viewing, hence the long drawtube. 

6C94419A-F39A-4969-BE83-595D213B52BE.jpeg

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2 minutes ago, johninderby said:

I use a Baader prism on my classical cassegrain and Scopetech 80 where they are ideally suited. 

My Made in Japan Scopetech 80 f/15 is indeed intended for straight through viewing, hence the long drawtube. 

6C94419A-F39A-4969-BE83-595D213B52BE.jpeg

One thing I've sometimes wondered about refractors with thinnish diameter long drawtubes is, with a diagonal in use and the drawtube therefore racked inwards to take account of the diagonals light path length, is there any possibility that either the inner end of the drawtube or the baffles within it could clip the light cone from the objective ?

 

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1 hour ago, John said:

One thing I've sometimes wondered about refractors with thinnish diameter long drawtubes is, with a diagonal in use and the drawtube therefore racked inwards to take account of the diagonals light path length, is there any possibility that either the inner end of the drawtube or the baffles within it could clip the light cone from the objective ?

 

I suspect this is why Tak fracs tend to have limited travel drawtubes and rely on separate extension tubes that can be added or removed as needed.

(Of course some people moan about their limited travel 🤔😊)

Edited by JeremyS
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2 hours ago, John said:

One thing I've sometimes wondered about refractors with thinnish diameter long drawtubes is, with a diagonal in use and the drawtube therefore racked inwards to take account of the diagonals light path length, is there any possibility that either the inner end of the drawtube or the baffles within it could clip the light cone from the objective ?

 

Mark, sorry for the slight diversion from your original thread topic..

The answer is "yes", John.

On some of the old Japanese 76-80mm F15 refractors by Towa, Kenko, (some branded for retailers eg Prinz 660, Topic etc), this was known to happen when upgrading a scope originally intended for use with 0.965" eyepieces: these scopes have very long drawtubes, and in the case, for example, of the Topic 80mm F15 1200mm focal length scope, the long drawtubes contained at least one, sometimes two, inner baffles. 

An adapter became available which threaded into end of the chromed drawtubes (the original screw in adapter with 0.965" fitting being removed first), and this adapter allowed the insertion of a 1.25" diagonal or eyepiece instead.

It was noted, however, that the view using some 1.25" eyepieces was rather dim, and investigations showed that the inner baffle(s), which was optimised for 0.965" eyepieces, was actually narrowing the drawtubes' internal diameter, and thus starving the 1.25" of precious light.

Fortunately, the baffles could easily be removed with a sharp tap from a screwdriver and hammer, and this enabled the 1.25" to work properly, often transforming the performance of the old refractor, widening the fov, and allowing often good quality lenses to deliver of their best.

I did this myself on more than one occasion 😊.

These threads might be of interest if you have nothing else to do..🤭😂

https://www.cloudynights.com/topic/573766-towa-drawtube-modifications/

https://www.cloudynights.com/topic/482086-towa-965-focuser-extension-tube-modification/

(Scope image credit Cloudy Nights).

Dave

Towa 80mm F15.jpeg

Edited by F15Rules
Photo added, Text added
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11 hours ago, Mark at Beaufort said:

That is interesting Andrew. I assume that my inexpensive 90mm f5.5 Achro will not benefit buying a prism diagonal. Whereas a quality scope like a Tak might???

For an inexpensive fast achromat I think a mirror diagonal is the safest bet unless someone has tested a prism with that particular scope and found that a prism works better. Nobody is going to bother trying to optimise their objectives to work best with one or other outside of a handful of very high end models like the now discontinued and fantastically expensive Zeiss APQ apo refractors, so choice of diagonal is more likely to be based on overall quality, size, weight, optical path length and other more practical considerations. My reason for buying a Baader T-2 prism diagonal was based on the need to have a short enough optical path to allow my eyepieces to work with the limited range of back focus available and prisms tend to have shorter paths than mirrors.

Prisms should have less scattered light, but again the difference compared to a reasonably good mirror diagonal probably isn't worth worrying about if you're using it with an inexpensive scope.

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19 hours ago, Andrew_B said:

There are manufacturers that design objectives with a view to them being used with a prism diagonal, but it's not common. Here's an explanation from CFF about choosing a diagonal to go with their different refractor models:

Yes, there are optics intended for applications other than visual observing but these are sold and described as such. I was referring to telescopes as sold for "normal" ( i.e. visual ) use. I also note in your quoted extract that they only refer to chromatic performance of their photographic optic not the spherical aberration that I referred to. They also recommend that their "visual tuned" 'scopes should be used with mirror diagonals.

Regarding relative quality of prism and mirror diagonals, if they are made to the same optical standard ( i.e. all surfaces 1/10th wave ) then my comments apply. The prism will have more effect than the mirror. There are other factors affecting performance that are specific to each. Scatter from the mirror, homogeneity and strain in the prism glass and, if using an image erecting roof prism, the coatings to correct for out-of-phase condition, for example.

Nigel

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I have just swapped the stock mirror diagonal I got with my Starbase 80 f10 for the Tak 1.25 prism diagonal. I did a few comparisons on Jupiter and my conclusion is that the Tak prism reduced light scatter which seemed to enhance colour, contrast and detail. It wasn’t a huge difference but I could definitely tell.

Anecdotally, using the prism I could see Europa transit in front of Jupiter’s disc and continued to observe it in front of the planet which I have never seen before. I was too in the moment to swap the mirror diagonal in to compare so don’t know if it was the seeing or the prism.

I love the locking mechanism, the only EP it doesn’t play nicely with is the Delite but I was happy to leave it in the diagonal unsecured. No more thumb screws! Very happy with the purchase. 👍🏻

 

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On 07/10/2021 at 11:13, Astrobits said:

Yes, there are optics intended for applications other than visual observing but these are sold and described as such. I was referring to telescopes as sold for "normal" ( i.e. visual ) use. I also note in your quoted extract that they only refer to chromatic performance of their photographic optic not the spherical aberration that I referred to. They also recommend that their "visual tuned" 'scopes should be used with mirror diagonals.

Regarding relative quality of prism and mirror diagonals, if they are made to the same optical standard ( i.e. all surfaces 1/10th wave ) then my comments apply. The prism will have more effect than the mirror. There are other factors affecting performance that are specific to each. Scatter from the mirror, homogeneity and strain in the prism glass and, if using an image erecting roof prism, the coatings to correct for out-of-phase condition, for example.

Nigel

Comparison of optical standards isn't helped by manufacturers playing fast and loose with their specifications. Is that 1/10th wave mirror quoting the surface accuracy of the substrate or the finished product after the application of the dielectric coatings? If it's the former then the resulting mirror can be nowhere near 1/10th wave but this isn't always made clear to the buyer.

Ultimately there are enough factors affecting the quality of the final image that it's hard to judge that much from spec sheets and the best comparison is at the eyepiece.

 

Edit - Baader are one of very few manufacturers who quote surface quality of their dielectric mirrors after they've been coated.

Edited by Andrew_B
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I mainly use my Tak prism diagonal in my 127 MAK since its a 1.25 diagonal. The contrast and clarity is a huge step up from the mirror diagonal that came with the scope. In my Vixen ED80 I have used the Tak as well, but  the focuser racked almost all of the way out for anything to come into focus. It works though and I do notice a little less light scatter with the Tak compared to the 2 inch dielectric diagonal I normally use. Its minimal, but I do see it when viewing the planets. I mainly just use the 2 inch diagonal since the ED80 is my only scope that can use it and I can just keep it in the focuser along with the 1.25 inch adapter.

Edited by cupton
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