1.25" Prism Comparison

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Shorty Barlow
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1.25" Prism Comparison

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Post by Shorty Barlow »


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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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-MC). The 2456005K has a very similar retail price to the Takahashi TKA00547, but in my opinion has a far superior housing. For a considerable amount of time I always perceived the Takahashi to be the slightly better prism, albeit there isn’t much between them. Although now I’m not so sure. Either way, considering its build quality, the Baader is very good value for what it costs.

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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.

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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 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.

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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.
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John Baars Netherlands
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Re: 1.25" Prism Comparison

#2

Post by John Baars »


Thanks for the display of your 1.25" prisms.
When I upgraded to better 1,25" or 2" diagonals or prisms, I always ended up with the best one during observation with each telescope.
The lesser gods as an expensive dust cap during storage. :D
Which one do you actually use most? But more important: Why?
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
Amateur astronomer since 1970.
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Shorty Barlow
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Re: 1.25" Prism Comparison

#3

Post by Shorty Barlow »


John Baars wrote: Wed Sep 07, 2022 10:51 am Thanks for the display of your 1.25" prisms.
When I upgraded to better 1,25" or 2" diagonals or prisms, I always ended up with the best one during observation with each telescope.
The lesser gods as an expensive dust cap during storage. :D
Which one do you actually use most? But more important: Why?

You're welcome. There's a good question lol. I think I basically upgraded the Celestron over a period of years. I have several scopes, so I tend to match the prism to the scope. The Tak' is lightweight and I usually use it for planetary with my Evostar 80ED DS Pro. With small Tak' eyepieces it gives less vibrational return problems at high magnifications. I use a 2" dielectric for rich field and DSO's on the same scope. The Baader T2 is predominantly used in my Maksutovs. The helical gives me a fine focus which the Mak's lack. The BBHS is usually used in my 102mm Altair Starwave and 60 EDF. I can use it in them with a twistlock as they have dual-speed focusers.

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The reasons are for practicality, convenience, and compatibility as much as performance.
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Bigzmey United States of America
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Re: 1.25" Prism Comparison

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Post by Bigzmey »


A nice collection of diagonals SB! Over the years I have developed the same approach, matching diagonals to the scopes and type of targets. So, all my scopes have assigned diagonals and rarely leave them.
Scopes: Stellarvue: SV102ED; Celestron: 9.25" EdgeHD, 8" SCT, 150ST, Onyx 80ED; iOptron: Hankmeister 6" Mak; SW: 7" Mak; Meade: 80ST.
Mounts: SW: SkyTee2, AzGTi; iOptron: AZMP; ES: Twilight I; Bresser: EXOS2; UA: MicroStar.
Binos: APM: 100-90 APO; Canon: IS 15x50; Orion: Binoviewer, LG II 15x70, WV 10x50, Nikon: AE 16x50, 10x50, 8x40.
EPs: Pentax: XWs & XFs; TeleVue: Delites, Panoptic & Plossls; ES: 68, 62; Vixen: SLVs; Baader: BCOs, Aspherics, Mark IV.
Diagonals: Baader: BBHS mirror, Zeiss Spec T2 prism, Clicklock dielectric; TeleVue: Evebrite dielectric; AltairAstro: 2" prism.
Filters: Lumicon: DeepSky, UHC, OIII, H-beta; Baader: Moon & SkyGlow, Contrast Booster, UHC-S, 6-color set; Astronomik: UHC.

Observing: DSOs: 3106 (Completed: Messier, Herschel 1, 2, 3. In progress: H2,500: 2180, S110: 77). Doubles: 2382, Comets: 34, Asteroids: 255
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Shorty Barlow
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Re: 1.25" Prism Comparison

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Post by Shorty Barlow »


Bigzmey wrote: Wed Sep 07, 2022 7:26 pm A nice collection of diagonals SB! Over the years I have developed the same approach, matching diagonals to the scopes and type of targets. So, all my scopes have assigned diagonals and rarely leave them.

Thanks Bigz. I have a couple of BBHS mirrors as well. I do like the prisms for planetary though. I have several grab and go bags so it simplifies things if certain diagonals are combined with certain scopes. Of course, I'll probably swap them all around in a few months lol.
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