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
The older one has a chromed-brass nosepiece. The second, with an aluminium nosepiece, was bundled with my Celestron
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
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/
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
The T2 nosepiece is baffled and has a filter thread. The helical focuser sets this apart from most 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. According to Baader the prism has a high transmission and multi-coated surfaces (HT-
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 stop causing friction when rotated. According to Baader the T2 2456095 has a 34mm inner diameter/clear
The brightness, colour definition, separation, and overall chromatic richness are quite remarkable and I’ve not witnessed any scatter 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 finest views of Mars while using this prism diagonal. Admittedly it is an expensive diagonal compared to the other three. At least a hundred pounds (Sterling) more than the plain T2 2456005K. Adding an eyepiece holder and a nosepiece will be an extra cost.