Prism diagonal & relative fast refractors

[YKSE]

Not so often there's recommendation about diagonals from a telescope maker:

CFF telescopes optical designer Pal Gyulai recommended prism diagonal to their telescopes in their website (http://cfftelescopes.eu/optics/color-correction-lenses/), depite the their refractors are relatively fast:

"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.

Considering that APO telescopes tuned for “Photographic” color correction can be used both for photography and for visual “work” (with prism diagonal), we believe this tuning of color correction is optimal (Zeiss tuned the color correction of their APQ series of apochromats very similarly) and it shall be used as the ‘default option’ for our apochromat objectives."

[Piero]

Hi Yong, 

I just read the text in your link, and this is what I've understood from that. CFF offers two types of lens:

1) Visual-tuned lenses. "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."

2) Photographic-tuned lenses. "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." 

 

Therefore, it seems to me that they advise prism diagonal only when one selects the installation of photographic-tuned lenses and uses the OTA visually.

I was not aware of the optimisation of lenses for photographic and visual use by CFF. It makes me wonder what the other brands optimise their lenses for (assuming they do!).. 

[YKSE]

Piero,

Yes, I did read the whole page, therefore quote the relevant text, maybe I've miss-interpretated something in these text?

"Considering that APO telescopes tuned for “Photographic” color correction can be used both for photography and for visual “work” (with prism diagonal), we believe this tuning of color correction is optimal (Zeiss tuned the color correction of their APQ series of apochromats very similarly) and it shall be used as the ‘default option’ for our apochromat objectives."

[Piero]

I find this topic intriguing. Just found this interesting Q/A here http://www.baader-planetarium.com/en/accessories/optical-accessories/prism-and-mirror-star-diagonals/baader-2"-bbhs-r-prism-star-diagonal-prism-with-2"-clicklock-clamp.html  

Q:  "Do the Baader prism diagonals with BBHS coating work well with refractors faster than f/7 or f/8? I am wondering if chromatic aberration (or spherochromatism) would be a problem as the light propagates through the glass prism."

A: "We do not recommend a prism for refracting telescopes unless the optical calculation of the objective lens calls for using a prism instead of a mirror.

For 100 years - all Carl Zeiss Refractor telescopes were designed to compensate for the added glasspath of a prism. 

And throughout our 25 years of offering this prism we repeatedly found refractor telescopes - especially of very short focal length - where the airy disc went colorless only when adding a prism into the beam of light - as was the case with the "long gone" Astro Physics Traveler.

So please check carefully with your existing mirror star diagonal if the first ring of the airy disc appears reddish when observing a bright star - or better when observing an artificial star at high magnification. if this is found to be the case then a prism will bring a noticeable improvement. 
If the first diffraction ring appears white already than a prism would not lead to an improvement but will induce a color error." 

 

It would be great to have more details on how our refractors are optimised. Good to hear that CFF does this! 

[John]

I was vaguely aware of some of this stuff when choosing a diagonal for my Tak FC-100DL (F/9). I was happy to go for a prism (Baader / Zeiss T2) for that scope but I might have hesitated if I'd had the F/7.4 version of the FC-100.

On my LZOS 130 F/9.2 triplet I use an Astro Physics Maxbright mirror diagonal.

I've switched the above diagonals between the scopes mentioned and in all honest can't see any optical differences between them in either scope.

I've not tried the Baader prism in either my F/7.5 ED120 or my Vixen ED102 F/6.5 as yet. I might not bother !

It's good to see a scope manufacturer making comments on suitable accessories though. I guess they will not want to have their scopes blamed for issues that are being produced by other componants in the optical train !

 

[mikeDnight]

Having used both the FC100DL and FC100DC side by side, both with prisms, (the DL with a Baader prism & the DC with a Tak prism),  I'd have no hesitation in saying "there isn't the slightest visually observable difference in colour correction between the two." Both scopes show zero CA on the lunar limb when in focus! They are the most colour free doublets I've ever used, so may be the prism has been taken into account during the design stage. It would seem strange if Taks didn't work well with prism diagonals, as they don't offer a 1.25" mirror option for use with their scopes, and their 2" mirror diagonals are scary money. ?

[iPeace]

Not entirely sure I have a meaningful contribution to make here, but for what it's worth, this fairly clueless dedicated casual observer has used and continues to regularly use not only prisms, but amici prisms (Baader) with his f/7, f/6 (TeleVue) and f/5.6 (Borg) fracs.

I might not know what to look for, but I sure do notice soon enough if something's not to my liking (took a couple of attempts to get the right prisms). Haven't noticed anything wrong yet with the current gear.

Come to think of it, I also marginally preferred a Baader prism in the f/7.5 120ED.

I still appreciate a good mirror though and keep a small herd of dielectrics on the farm in case a bout of awareness breaks out.

[Moonshane]

I have not had any issues with my f7 or f7.5  fracs and Baader Zeiss prism. That said I  mainly chose it for T2 compatibility and shorter focal length. The BBHS might be ever so slightly better but at f7.5 I doubt I'd see it.

[Piero]

I use a bbhs mirror diagonal on my frac and have certainly no complaint!

 

[JeremyS]

Fascinating discussion. I never realised that prism diagonals could play such a critical role, but I always use my Tak refractors with the Tak prism diagonal anyway.

Jeremy

[alanjgreen]

Never had any issues with the Zeiss prism diagonal in my Borg89.

the Baader markv binos add more prisms still further and the views through it with a pair of delos or panoptics are great!

The zeiss is one of the best you can buy though.

 

[Peter Drew]

I'm not totally convinced by some of the manufacturers comments. Diagonal mirrors are commonly available (at a price!) with claimed 1/20 wave accuracy, I don't see that being inferior to the accuracy of a prism. Also, a mirror, being apochromatic, shouldn't impart CA to a refracting system evenn if it can't correct it whereas a prism can. A mirror can introduce more scatter than a prism but at the high end of the market this will be minimal.  

[John]

I think it was Bill Paolini's report that highlighted the potential benefits of the prism over the mirror in diagonals:

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

I have the Baader T2 Zeiss one, an Astro Physics Maxbright and a couple of Tele Vue Everbrights but their performance seems pretty equal to my eyes.

 

[Piero]

I've never used a prism and, somehow, I'd be curious to see whether there is an improvement compared to mirror diagonals.

On my TV60, I use a 1.25" TV Everbrite. To my eye the larger reduction in light scattering was achieved when my eyepieces were upgraded, rather than by the diagonal. This result was very clear (to my eye at least) when I used the Docter, Zeiss zoom, Nikon zoom, and Vixen HR. On a light scope like the TV60, heavy eyepieces can cause far more issues than a diagonal, because they can cause a marginal but detectable misalignment on the focuser, which is the weakest component on that telescope. This issue can be solved by using lighter eyepieces or tightening the focuser screws a bit more. 

To add more, I observed Jupiter, Saturn and the Moon in straight through mode with the my previous 5mm Vixen SLV and TV60 a few times. This mode should allow one to achieve the best possible view. To my eye the reduction in light scattering was so marginal that I struggle to detect it. There was no change in contrast or image sharpness. Colour was also comparable. I do keep my diagonals clean. 

 

I've never compared the 1.25" TV Everbrite against the 2" BBHS on the Tak-100. The reason why I opted for the BBHS was because a few CN members aside from W. Paolini, reported a colour rich view of the GRS. The BBHS mirror was also sold by Microglobe at a very good price compared to the prism version. 

[Doc]

Well I've asked Santa for the following so fingers crossed I will let you know what it's like.

http://www.apm-telescopes.de/en/optical-accessories/stardiagonal-mirror--prism/apm-2-inch-erect-image-prism-fast-lock-ultra-broadband-coating.html

 

[Paz]

On 24/12/2017 at 19:04, Doc said:

Well I've asked Santa for the following so fingers crossed I will let you know what it's like.

http://www.apm-telescopes.de/en/optical-accessories/stardiagonal-mirror--prism/apm-2-inch-erect-image-prism-fast-lock-ultra-broadband-coating.html

 

I'm interested in getting a good quality correct image diagonal and am looking forward to hearing about this one.

[Peter Drew]

Good quality erect image diagonals are available- at a price . I use one, only for lunar observation so that it matches my map, I don't need a 2" version as I also use binoviewers. I made my own unit with a high spec exWD prism.  

[Doc]

Take a look here:

https://stargazerslounge.com/topic/306212-apm-2-inch-erect-image-prism-with-fast-lock/