1.25" Prism Comparison

[Nightspore]

 

Above, left to right top row: Baader T2 2456095 Zeiss specification, Baader T2 2456005K. Left to right bottom row: Celestron 94115-A, Takahashi TKA00547.

 

The Celestron 94115-A prism diagonal weighs 150g with about 29mm of clear aperture. The aluminium eyepiece holder is 32mm tall and features two set screws but no compression ring. The housing is resin or polymer with a metal baseplate. I have two of these diagonals.

 

 

The older one has a chromed-brass nosepiece. The second, with an aluminium nosepiece, was bundled with my Celestron SCT. The newer aluminium noses are generally more compatible with filters. Although I personally prefer the brass nosepieces.

 

 

The prism itself is multi-coated. As a whole these diagonals are pretty good. Unfortunately they are often denigrated by some. It is a distinct possibility that they are confused with some of the resin-bodied mirror diagonals often bundled with entry level Synta scopes. These are cheaply made diagonals with what appears to be recycled Bakelite housings equipped with old shaving travel mirrors. In my experience the 94115-A has a fairly bright image with little or no scatter and is a perfectly usable prism diagonal. Its main downfall is the housing body itself. With a heavy enough eyepiece the metal threaded nosepiece or eyepiece holder could pull out of the resin housing threads. This diagonal is the cheapest of the four. I believe I originally paid less than thirty quid for mine several years ago.

 

The Takahashi TKA00547 prism diagonal weighs in at a paltry 130g and I make about 29mm of clear aperture. The housing is probably pressed aluminium or light gauge metal. This is small even for a 1.25” diagonal. It has a resin base plate of 60 x 42mm. It is so small in fact that I actually have back-focus problems with my 72mm Evostar ED DS Pro. It requires an extension on the 1.25” adapter in the focuser to actually rectify this. 

 

 

The entire diagonal is probably much stronger than it looks although I personally wouldn’t trust it with anything really heavy. I have been reliably informed that it holds binoviewers securely. There is an aluminium nosepiece and it features a twist-lock eyepiece holder. Like a blast from the past it is actually supplied without dust caps (I added my own).

 

 

Optically this is a superb prism, with a noticeable performance improvement compared with the Celestron. It gives a bright, defined image with excellent colour separation that belies its competitive retail price. In fact, this is one of my favourite diagonals for planetary observation, and it is no slouch for rich field/DSO viewing either. Most people who have used this prism praise it very highly. Unfortunately the housing design undermines its superb optical quality. The straight nosepiece is baffled with no undercut but also lacks a filter thread. Which brings me to the twist-lock.

 

 

This features a plastic collet mechanism. It holds eyepieces without undercuts perfectly well and is relatively easy to utilise. Many eyepieces with a barrel undercut have difficulties however. Often they will be held safely in the eyepiece holder, but will be loose enough to be rotated around their respective axes. I have two of these diagonals; and one of them holds my Tele Vue DeLites perfectly well while the other doesn’t. So there may be some build variation. The resin base plate is another weakness in my opinion. I have concerns about its durability over time. I don’t see why Takahashi couldn’t have supplied it with a metal one.  The otherwise excellent TKA00547 essentially belongs to an earlier era before heavy widefield eyepieces and undercuts existed. 

 

The Baader T2 2456005K weighs around just under 200g with a claimed 32mm of clear aperture by Baader Planetarium. This diagonal is usually supplied with a nosepiece and a helical focuser (T2 M42 x 0.75). The nosepiece has a shallow undercut. Some variants have a three set screw non-focusing eyepiece holder or are sold plain without any nosepiece or eyepiece holder at all. The plain body variant sans nosepiece and eyepiece holder can be up to twenty pounds cheaper. The main housing body and base plate are well constructed and probably made of cast aluminium.

 

 

The T2 nosepiece is baffled and has a filter thread. The helical focuser sets this apart from other diagonals. The helical allows incredibly precise focusing. It can also be locked and used as a conventional eyepiece holder. For me a helical focuser is basically a sine qua non for telescopes with single speed focusers. The prism has a high transmission and multi-coated surfaces (HT-MC) according to Baader. The 2456005K has a very similar retail price to the Takahashi TKA00547, but in my opinion has a far superior housing. However, although it performs very well optically, I wasn’t as impressed as with the Takahashi prism which seems to have an edge with slightly brighter overall transmission and definition. Either way this diagonal is very good value for money. 

 

The Baader T2 2456095 Zeiss specification, BBHS (Broad Band Hard Silver) coated, BaK4 diagonal is the most expensive of the four. I believe the housing is constructed of aluminium. It is usually sold without a nosepiece or eyepiece holder and weighs 170g.

 

 

This can increase to 225g with the nosepiece, helical focuser and a 7.5mm spacer added. The spacer can be necessary if long eyepiece or Barlow barrels are used with the helical mechanism. Longer barrels can make contact with the safety shoulder causing friction when rotated. According to Baader the T2 2456095 has a 34mm inner diameter/clear aperture and the BBHS coatings ‘have a much wider spectral window’ than conventional dielectric or aluminium coatings. This may actually be true, the prism has excellent transmission that at least rivals the Takahashi, if not exceeds it.

 

 

The brightness, colour definition, separation, and overall chromatic richness are quite remarkable and I’ve not witnessed any ghosting at all. It is often stated that silver coatings improve viewing at the red end of the spectrum. Red stars do indeed seem to benefit from this when observed using this prism. I had some of the best views of Mars I’ve ever had with a refractor while using this prism diagonal. Admittedly it is an expensive diagonal compared to the other three. At least a hundred pounds more than the plain T2 2456005K. Adding an eyepiece holder and a nosepiece will be an extra cost. However, it does combine the performance of the Takahashi prism with the build quality of the Baader T2 housing. 

 

Edited June 21, 2021 by Nightspore

[Marmot]

It is now apparent that you not only have a room full of eyepieces but a cupboard where you store your collection of diagonals! However, this is a good thing as people like myself get to benefit from your wide experience of equipment.

Thanks for another informative review and please don’t stop submitting them as they are not only interesting reading, but they are genuinely helpful to someone like myself who is in the early stages of acquiring a collection of the gear that works best for me.

[Nightspore]

1 hour ago, Marmot said:

It is now apparent that you not only have a room full of eyepieces but a cupboard where you store your collection of diagonals! However, this is a good thing as people like myself get to benefit from your wide experience of equipment.

Thanks for another informative review and please don’t stop submitting them as they are not only interesting reading, but they are genuinely helpful to someone like myself who is in the early stages of acquiring a collection of the gear that works best for me.

 

Maybe I'll do the mirror diagonals next. Although to be honest I can't really tell any difference optically between most of them lol. 

[Marmot]

9 hours ago, Nightspore said:

 

Maybe I'll do the mirror diagonals next. Although to be honest I can't really tell any difference optically between most of them lol. 

 

Well whatever you chose to review next, you can be guaranteed I will read it. My one and only diagonal (apart from what came with the scope) is a Skywatcher dielectric mirror, so although I don’t have a prism, I still enjoyed your review. Getting an alternative diagonal, such as a prism, will just have to wait at the moment as I am still trying to get a collection of eyepieces that suit me. Just obtaining the eyepieces is taking time as I seem to be buying a few and selling on the ones that don’t suit me and keeping those that do, But it’s fun trying out different EPs and I do now feel that the collection is nearing completion. One or two have been as a result of your reviews😉

[Nightspore]

5 hours ago, Marmot said:

 

Well whatever you chose to review next, you can be guaranteed I will read it. My one and only diagonal (apart from what came with the scope) is a Skywatcher dielectric mirror, so although I don’t have a prism, I still enjoyed your review. Getting an alternative diagonal, such as a prism, will just have to wait at the moment as I am still trying to get a collection of eyepieces that suit me. Just obtaining the eyepieces is taking time as I seem to be buying a few and selling on the ones that don’t suit me and keeping those that do, But it’s fun trying out different EPs and I do now feel that the collection is nearing completion. One or two have been as a result of your reviews😉

 

The Sky-Watcher 2" dielectric is a perfectly good diagonal. Mine came bundled with my 80ED Evostar. It still gets used. On some forums *cough* Cloudy Nights *cough* they seem to be obsessed with performance and pointless bickering. Especially with diagonals. They seem to constantly argue about reflectivity percentages, intrinsic photon emission expulsion control parameters or some other such BS. 

 

I use certain diagonals for a variety of reasons. Some are purely practical. My 2" Everbrite doesn't play well with the compression ring on my 60 EDF for instance. The Baader Maxbright works perfectly with it, but its clicklock doesn't work too well with my 19mm Luminos. A lot of diagonals I use for particular scopes are chosen for their light weight. I tend to use Amici prisms for lunar viewing.  The only 2" prisms I own are Amicis.  

 

I think you have to find what suits you and your particular scope and gear. Unfortunately that's usually trial and error. 

[MarkAR]

On 6/22/2021 at 5:24 PM, Nightspore said:

*cough* Cloudy Nights *cough* they seem to be obsessed with performance and pointless bickering. Especially with diagonals. They seem to constantly argue about reflectivity percentages, intrinsic photon emission expulsion control parameters or some other such BS. 

🤫 Thought about it and still can't fathom the last part. 

I think with some things it's a case of.......does it fit? does it work? and have I dropped and broken it.

 

Keep up the good reviews 👍

[Nightspore]

2 hours ago, MarkAR said:

🤫 Thought about it and still can't fathom the last part. 

I think with some things it's a case of.......does it fit? does it work? and have I dropped and broken it.

 

Keep up the good reviews 👍

 

Thanks. OK, I admit I made the reflectivity percentages, intrinsic photon emission expulsion control parameters bit up lol. But that's a lot like how some of them discuss things on CN. 

 

I can tell the difference between various prism diagonals. Which I assume is down to the quality of the glass used, coatings, and general build material quality.

 

I'm not so sure about dielectric mirrors though. They all seem the same to me in actual performance. If you pay 300 quid for a dielectric I assume you're paying for the overall housing build quality.

 

I remember reading a thread on CN about orthoscopics. There was much argument/debate whether Takahashi, Hutech, Fujiyama, Edmund Optics, Circle T or BGO's were the best.

 

 

AFAIK all of them apart from Takahashi were made by Ohi (or used Ohi glass). I have compared my Tak' ortho' focal length equivalents with my Hutech, KK and Circle T. The only differences I can see are ergonomic and build.

 

 

 

[Marmot]

5 hours ago, Nightspore said:

 

Thanks. OK, I admit I made the reflectivity percentages, intrinsic photon emission expulsion control parameters bit up lol. But that's a lot like how some of them discuss things on CN. 

 

I can tell the difference between various prism diagonals. Which I assume is down to the quality of the glass used, coatings, and general build material quality.

 

I'm not so sure about dielectric mirrors though. They all seem the same to me in actual performance. If you pay 300 quid for a dielectric I assume you're paying for the overall housing build quality.

 

I remember reading a thread on CN about orthoscopics. There was much argument/debate whether Takahashi, Hutech, Fujiyama, Edmund Optics, Circle T or BGO's were the best.

 

 

AFAIK all of them apart from Takahashi were made by Ohi (or used Ohi glass). I have compared my Tak' ortho' focal length equivalents with my Hutech, KK and Circle T. The only differences I can see are ergonomic and build.

 

 

 

Phew....what a relief.....glad all that “photon emission intrinsic stuff” was made up, I was starting to think this was even more complicated than I first thought.!😀

on a serious note: your remarks are exactly why I am a member here (and was on the Shed): I enjoy the friendly exchanges and the manner in which the members comment on their gear and achievements in a way that’s objective and honest. As previously stated, keep up the good reviews.

[Nightspore]

7 hours ago, Marmot said:

Phew....what a relief.....glad all that “photon emission intrinsic stuff” was made up, I was starting to think this was even more complicated than I first thought.!😀

on a serious note: your remarks are exactly why I am a member here (and was on the Shed): I enjoy the friendly exchanges and the manner in which the members comment on their gear and achievements in a way that’s objective and honest. As previously stated, keep up the good reviews.

 

Sorry, I have an odd ironic sense of humour lol. I think marketing has a lot to answer for with astronomy equipment. Especially with dielectric mirrors. I don't doubt there will be some quality differences between different makes.

 

 

But I think a dielectric is a dielectric is a dielectric. I can't tell any difference in performance with mine. There might be but I'm not seeing it.  The differences are ergonomic or build quality. The TV Everbrite (foreground) is three times the price of the Sky-Watcher diagonal furthest away from the camera in the above picture. Is it three times better? Is it expensive because it is not manufactured on the Chinese mainland? Who knows?

[MarkAR]

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

 

Read this and thought, ok so it's got a dielectric mirror but what wavelengths is it reflecting.

It's possible to get an expensive dielectric mirror that is rubbish because it's not reflecting the wavelengths you require !

 

[Nightspore]

9 hours ago, MarkAR said:

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

 

Read this and thought, ok so it's got a dielectric mirror but what wavelengths is it reflecting.

It's possible to get an expensive dielectric mirror that is rubbish because it's not reflecting the wavelengths you require !

 

 

Maybe, but it's all a bit too anorak for me lol. 

 

RP Photonics Encyclopedia

 

"A Bragg mirror (also called distributed Bragg reflector) is a mirror structure which consists of an alternating sequence of layers of two different optical materials. The most frequently used design is that of a quarter-wave mirror, where each optical layer thickness corresponding to one quarter of the wavelength for which the mirror is designed. The latter condition holds for normal incidence; if the mirror is designed for larger angles of incidence, accordingly thicker layers are needed."

 

"A quarter-wave mirror, also called a Bragg mirror, is a dielectric mirror structure which consists of an alternating sequence of layers of two different optical materials, with each optical layer thickness corresponding to one-quarter of the wavelength for which the mirror is designed (here, the wavelength is taken to be the wavelength within the material, not the vacuum wavelength). For a given wavelength and number of layer pairs, a quarter-wave mirror is the structure which can result in the highest reflectivity. Quarter-wave stacks are also very basic building blocks of more complicated dielectric mirror structures." ~ op cit

 

I accept that dielectric mirrors are probably a few per cent more reflective than aluminium mirrors. I'm just not so sure the human eye can detect the difference. 

 

 

According to Baader Planetarium this uses an oversize 1/10 wave mirror. I believe them. I'm not sure exactly what it means though. I just like its clicklock and its housing lol. Plus it is a hundred grams lighter than my Altair posi-lock dielectric.

 

 

Visually, I see no difference between them. 

Edited June 25, 2021 by Nightspore

[MarkAR]

18 hours ago, Nightspore said:

Maybe, but it's all a bit too anorak for me lol. 

Me too. 🤣

[RTW]

Thank you very much for your review on prism diagonals. I’ve been researching online and your comparison post was both extremely helpful and presented in a way that answered all the questions I had. 

[Nightspore]

45 minutes ago, RTW said:

Thank you very much for your review on prism diagonals. I’ve been researching online and your comparison post was both extremely helpful and presented in a way that answered all the questions I had. 

 

You're welcome.