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 will 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). 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. 

[+Bino-viewer]

Nice review, thanks for posting.

[Nightspore]

7 hours ago, Bino-viewer said:

Nice review, thanks for posting.

 

Thanks, and you're welcome. It's only in the past two or three years that I've really paid much attention to my 1.25" prism diagonals. For a long time I just used the bog-standard Celestron, or a GSO Amici for the Moon. Although I swapped the GSO for a Williams Optics helical Amici five or six years ago (then a Baader Zeiss Amici eventually). I was lacking a good 1.25" prism in my light train. I had high hopes for the Tak' as it is a really nice prism at a good price. I'm sure they could improve the 1960's retro housing if they tried. 

Edited June 23, 2021 by Nightspore