Zeta Reticulan

That could work. Where are your filters in the light train? I find filters in the diagonal dew faster than in the eyepiece. There are dew straps for eyepieces.

It sounds bad. It's been very hot here of late. I was surprised to find no dew at all in early morning sessions. So global warming has an upside. There are downsides however ...

Yeah, I know in some parts of the world humidity is a huge problem for astronomy. It's not so bad where I am. September can be 'dewey'. I find that my reflex finders dew first. The only recourse is taking a cloth out to periodically dry them. Or ...

You're welcome, and thanks. The 2" TV definitely appears to have a non-metal base plate. Unless it's just some form of plastic veneer. It could be that it just seems like it is plastic. I have a couple of the 1.25" enhanced aluminium TV's. They're only a few years old though. Apparently they are better for planetary as they have less scatter than a dielectric. TBH I'm not that sold on dielectrics as a whole. I think Bill Paolini once described them as "a question to an answer". Mind you, that was also said about the laser lol. Yes, basically the views are similar through all of them. The differences are ergonomic and in design in the main.

I usually keep any filters (in dedicated plastic cases) in the pockets of the cargo trousers I normally wear for observing. I don't take this many out at one time, of course. I choose what is applicable for the session. TS Optics sell various cases in both 2" and 1.25".

Above, left to right top row: Tele Vue Everbrite, Baader Planetarium ClickLock. Left to right bottom row: Sky-Watcher, Altair Astro Posilock. At 390g the ‘Sky-Watcher’ 2” dielectric diagonal is the ‘no frills’ plainest and lightest of the four dielectric diagonals featured here. There is a good chance that this dielectric is manufactured by Long Perng. The main body appears to be an unpainted anodised silver coloured metal plate, probably consisting of pressed aluminium. The two plain black metal side plates are each secured to the main housing with three Allen screws. My version of this diagonal was actually bundled with my 80ED DS Pro Evostar. The nosepiece features a slight flare, is well baffled and includes a filter thread. I make about 44~45mm of clear aperture. I’m guessing the nosepiece is threaded into the body but mine is too tight to loosen and I don’t want to force it to discover how it is actually attached. The metal eyepiece holder is almost certainly brazed into the main housing body and features a brass compression ring. The diagonal feels solid and well made. Its dielectric mirror gives a bright image with no defects or aberrations that I can detect. In my experience light scatter is not a real issue and no better or worse than my other dielectrics. As these diagonals are often bundled with Synta-made telescopes they don’t have a particularly sparkling reputation and many people seek to upgrade them as soon as possible. Although in my experience there is nothing wrong with the diagonal and it performs admirably. As a decent basic 2” dielectric diagonal it compares well with anything else on the market. My only real concern is with the eyepiece holder itself. The compression ring isn’t particularly problematic but I am always wary of brazed holders. There is always the concern that they have the necessary strength to hold heavy eyepieces securely. Having said that I believe it would take a lot of brute force to physically detach the holder from its housing. I’m pretty sure mine came supplied with a 1.25” adapter. As I rarely used the original adapter I’m not sure of its present whereabouts. Although it was perfectly usable. The adapter I regularly use with it now was originally supplied with my Altair 60 EDF ED doublet. There are no compatibility issues with the Altair adapter. The Sky-Watcher diagonal is still used a fair bit, although now predominantly with my modified ST80's. The Altair Astro Premium ‘Posilock’ dielectric is in many respects a very similar design to the Sky-Watcher diagonal. It comes supplied with a 1.25” adapter. The main housing body is most probably cast anodised aluminium equipped with carbon fibre side plates. The aesthetically pleasing black and silver twill weave plates are attached to the main housing with three screws. This appears to be virtually identical to the mounting of the side plates on the Sky-Watcher diagonal body. The three Allen screws used in both diagonals are possibly the same size and thread. The nosepiece also features a similar slight flare, internal baffling, and a filter thread. The baffling appears more matte black than the Sky-Watcher nosepiece which is a tad more gloss. The Altair nosepiece definitely threads off from its respective housing. The positive lock mechanism is probably responsible for the diagonal’s 525g overall weight (not including the 1.25” adapter). Making it a good 100g heavier than its Baader ClickLock diagonal equivalent. The Altair positive lock mechanism features two pillar holders rather than a collet and is identical to the mechanism shown below in an APM Amici. In use the positive lock is precise and secure. For extra safety the metal screw at the top of the eyepiece holder can be rotated and tightened/loosened along its own axis as a form of safety lock. I make the clear aperture to be the same as the Sky-Watcher at about 44~45mm. In use this is a perfectly decent dielectric and to be honest I can’t tell any real difference with my other 2” dielectrics. Apart from the carbon fibre sides feeling peculiarly sharp to the touch, my only real gripe is its excessive weight. It is only 75g lighter than a 2” APM Amici prism diagonal equipped with the same type of positive lock eyepiece holder. Mirror diagonals are usually a fair bit lighter than their prism equivalents. This weight is quite noticeable on lightweight mounts combined with small aperture short tube refractors. Oddly, these Altair diagonals often retail at exactly the same price as the 2” 'Sky-Watcher' dielectric. After sitting in a cupboard for a couple of years my Altair dielectric is now regularly used with my modified ST102 on a Vixen Porta II mount and a Vixen APP-TL130 tripod combination. The Baader Planetarium ClickLock #2956100 weighs in at a reasonable 425g. It is supplied without a 1.25” adapter. Baader states that it has 46.6mm of clear aperture, and I believe them. The housing body is white in appearance and feels comfortable to the hand in a tactile sense. Overall it is very well constructed with a high build quality. The oversized 1/10 wave mirror is held in place with a metal base plate separated from the main housing by what appears to be a red rubber gasket. The plate is held in with four screws. The removable nosepiece sports safety kerfs and uniquely both ends of the nose contain M48 filter threads. Furthermore, the rubber gripped click lock eyepiece holder can also be removed from the housing. Like most Baader products this diagonal can be customised somewhat to individual tastes and applications. Unlike the ‘posilock’ type the Baader locking eyepiece holder utilises a brass compression ring. Using a compression ring rather than the ‘pillar’ type of retainers is its only apparent Achilles’ heel. I say this as I once got a 19mm Celestron Luminos eyepiece well and truly stuck in this diagonal. Requiring both eyepiece and diagonal to be partially disassembled to completely extricate them from each other. The culprit undoubtedly being the undercut on the Luminos barrel that had become enmeshed in the compression ring. The Baader ClickLock diagonal holds every other 2” eyepiece I own perfectly well. Although it has to be stated that most of them don’t have undercuts. This is an aesthetic and ergonomically satisfying diagonal to use and performs very well in every telescope I own that has a 2" focuser. The click lock mechanism itself is smooth and secure in use. Although there is no safety mechanism like the one featured on the ‘posilock’ type. Out of the four featured diagonals this is my personal favourite. It was mainly used in my 60mm Altair EDF. Although recently I've started using it again with my 80ED DS Pro Evostar (which I originally bought it for). The Tele Vue DDP-8004 Everbrite diagonal weighs in at 455g (without the adapter). I make the Everbrite to have a clear aperture of 46mm. This is the most expensive diagonal reviewed here. It costs about two thirds more than the Altair and Sky-Watcher dielectrics and up to a third more expensive than the Baader ClickLock. However, there is also a slightly less expensive version on the market that utilises a mirror constructed of enhanced aluminium. The build quality is very high, like all Tele Vue products. That, and the fact that it is almost certainly manufactured outside of mainland China, probably accounts for its high retail price. Apparently the dielectric coating is applied to Pyrex with a 1/10 wave flatness. The housing is finished in matte black and well baffled. Unusually it has a body machined from a solid block of aluminium with the mirror being held in place by a base plate. The solid block design is to ensure that the nosepiece or eyepiece holder cannot accidentally be unthreaded in use. It also possibly contributes to its relatively light weight. Without its adapter it is only around 30g heavier than the Baader ClickLock diagonal. Both the adapter and the eyepiece holder itself feature brass compression rings. The rings and the screws for them are high quality and the long thumb screws are very smooth to operate. Neither of the thumb screws are captive. The overall ergonomics of the design are well thought out and the diagonal is a pleasure to use. Unfortunately the nose features a relatively deep undercut. I originally bought the Everbrite for my Altair 60 EDF. The undercut has given me problems when used in the focuser of the 60mm ED doublet which features a three screw compression ring. Smooth or flared nosepieces don’t get caught on the 60 EDF’s compression ring. Another slight point of concern with the 2” Everbrite is that the base plate appears to be made of a plastic polymer or resin material. This is unlike the 1.25” Everbrite and Enhanced Aluminium models which have metal base plates. Whether this plastic is employed for reasons of weight or economy is not known. It is slightly disconcerting on a diagonal costing around three hundred and fifty quid though.

Probably. Is it that much of a problem? The only 2" EP I use in it is a 30mm SuperView. In an ST80. Although the diagonal itself was originally bundled with my 80ED DS Pro. Edit: It gives a 6mm exit pupil (13.3x) which is about right for rich field IME. I don't recall any vignetting. My other 2" dielectrics are an Altair twistlock, Baader Maxbright and a TV Everbrite. The Baader has an oversize mirror.

I think it's an achromatic doublet. It might be worth learning how to set-up an EQ mount. Have fun!

These are probably the best value for money. At 390g it's the lightest 2" diagonal I own.

I predominantly use my ST102 with 2" eyepieces. EP's vary a bit: 36 & 31mm Baader Aspherics, 27mm Panoptic and 18mm ES (19mm Luminos shown below). If I want higher magnification I usually use a zoom.

Nice choice. I was thinking about the 11mm for one of my refractors. DeLites have a really nice field, it's hard to believe it's only 62 degrees sometimes. The contrast is as good as an orthoscopic IMO.

You're welcome. The DeLites are basically replacements for the Radian series I believe. When people ask me if they are really that good, I say yes.

I don't use my 150mm (GSO) Newtonian much. I like Vixen SLV's for their orthoscopic-like contrast and overall ergonomics. I actually bought the SVLs for it. TMB clones are always good. If you want to splash the cash DeLites are very nice.

They say all things come to those who wait. I don’t know who ‘they’ are, but they may actually have a good argument. I still required a relatively lightweight, uncomplicated 3x Barlow for my ST80 grab and go kit bag. My Tele Vue 3x is a little weighty for the ST80’s aftermarket GSO focuser, especially combined with a zoom. The TV was the first 3x Barlow I ever owned and (rightly or wrongly) I tend to compare all subsequent 3x Barlows with it. The incredibly shiny SvBony 3x Barlow looked promising and actually performed well. I wasn’t entirely confident its seemingly delicate and lightweight housing would take the leverage strain of a zoom. Furthermore its lack of internal blacking was similarly disconcerting. The GSO ED 3x initially appeared to have potential and fulfilled every criteria except the most important one. Its visual performance was woefully lacking and delivered a kaleidoscopic rainbow of false colour. The appropriately named Orion Tri-Mag 3x Barlow at £44.99 was only £6.59 more expensive than the unsatisfactory and inadequate (notwithstanding oddly psychedelic) GSO 3x ED Barlow. According to Orion Telescopes & Binoculars the ‘Tri-Mag’ is a multicoated two element lens with a clear aperture of 24mm. Orion also claim that it is 127mm tall. It isn’t though, and I make it 138mm, with about 53mm of that being the length of the barrel plus the element. The element contains a filter thread. The barrel itself is smooth chromed-brass without an undercut. The rest of the housing is machined aluminium. The element itself appears identical to the Orion 2x 'Shorty' Barlow. Like the ‘Shorty’ it is internally baffled and there is no compression ring. There are two screws allocated to hold the eyepiece. Daylight trials seemed promising with clear, bright, and contrasted views. I briefly experimented with nylon screws to hold eyepieces. Eventually I returned to the original metal screws. I got first light with the Tri-Mag on the 2nd of January. The target was the Orion Nebula and the eyepiece placed in the Orion Barlow was an Orion 7~21mm zoom. Which I considered apposite. Combined with the Tri-Mag the zoom gave a magnification range between about 57x to 171x. I could easily make out the four main Trapezium stars at about 100x. After a while I removed the Tri-Mag and used the zoom on its own. After a further period of rich field observing with 30mm and 15mm GSO SuperViews I decided to view more doubles with the zoom/Barlow combination. First up was the Pole star. At around 150x I could only just about see the companion, Polaris B, eighteen arc seconds away from the primary star. Although this was undoubtedly due to the poor transparency. After it rained on me for a while the sky cleared somewhat. I turned to Iota Cassiopeiae. Although seeing the individual stars that constitute the triple system required some concentration. I switched back to the 30mm SuperView for a while, fairly convinced the overall humidity, poor transparency and dodgy seeing were the main culprits for any observing difficulties. Eventually I decided to give the Orion zoom/Barlow combination a third try. Now the doubles split much more clearly with superb acuity, excellent contrast and colour separation. The clear, well defined, small blue point of Polaris B was easily perceived without a hint of any aberrations, chromatic or otherwise. On the 5th of January I had a second session with the Tri-Mag. The seeing was a little better and Iota Cassiopeiae was much easier to split. I observed more doubles including Eta Cassiopeiae and Gamma Andromedae. When M42 was high enough for me to observe comfortably I viewed the nebula at a variety of magnifications. I experimented with different eyepieces inserted into the Barlow. A 15mm GSO SuperView gave a panoramic 80x (1mm exit pupil) and I couldn’t see any vignetting or edge distortion. A 10mm Plossl gave a sharp and contrasted view of the Trapezium stars at 120x. In my opinion the Orion Tri-Mag 3x Barlow is definitely worth its retail price. Especially considering it is only a few quid more than the optically challenged GSO ED 3x Barlow. I’d say the Orion’s performance gives the Tele Vue 3x a run for its money. Obviously the overall build quality of the Tri-Mag isn’t the same as the Tele Vue 3x. Having said that though the Orion Barlow is solidly built and has a chromed-brass barrel. Its two piece element seems exceptionally good and may even contain ED glass. It certainly wouldn’t surprise me if it did. I waited a long time to find a suitable 3x Barlow for my ST80. Finally it came.

I own a 2x GSO Barlow. It is lightweight, simple, and I like its ergonomics. It performs well with my modified ST80 and a Celestron zoom. I have a 5x GSO Barlow (as Revelation Astro) as well. Although it can suffer from some chromatic aberration. It doesn’t see much use nowadays as it was replaced by a 5x TV Powermate. I desired a basic 3x lightweight Barlow. Among various criteria it would require good ergonomics. Basically I’d need to like the overall feel of it, it wouldn’t be so long as to poke the mirror out of the diagonal, and it would need to hold the zoom safely and securely. It would also be required to perform well in an inexpensive short tube achromat. Lastly, it had to be relatively economically priced. So, probably an impossible aspiration then? The GSO 3x ED (2-element) Barlow at £38.40 seemed to fulfil all of these requirements. Around 125mm tall with a barrel of about 53mm including the element. I measured 22mm of clear aperture. I make the Barlow about 140 grams in weight, although admittedly my scales are not particularly accurate. The housing is nicely sturdy and constructed from aluminium. The ED element and barrel both thread out of each other. The aluminium barrel is smooth with no undercut. It is competently blackened and baffled internally. The multi-coated element contains a filter thread in its own housing. There is no compression ring although the rather large standard GSO thumb screw is reassuringly knurled. Daylight trials seemed impressive and delivered a sharp, bright and clear image. On the 28th of April I ventured out with the ST80 on a night of poor seeing. After viewing some open clusters at low magnifications I decided to split some doubles. I placed the 3x GSO in the 2” dielectric adapter and then put the Celestron zoom into that. This now gave me a range of exactly 50x to 150x. Initial target was α Bootis (Arcturus), easy to find as it ‘follows the Bear’. This allowed me to gain focus at around 70x so I could proceed by star-hopping to the next port of call, ε Bootis aka Izar. The orange primary star at a magnitude of 2.5 is easily seen with the naked eye. The secondary has a magnitude of 4.8. and is basically nestling just inside the first diffraction ring. I have split this double with smaller apertures and it isn’t usually particularly difficult in my experience. However, I seemed to be having some difficulty this time. I could see a lot of what appeared to be chromatic aberration. I wondered if the poor seeing was a factor. I expect a certain amount of false colour with a short tube achromat. Although it has to be said that the ST80 doesn’t usually suffer terribly from this. At most there is only a hint of the ‘Jimi Hendrix Experience’, more often referred to as ‘purple or cyan hazing’ around the target object. This was definite and obvious colour either side of the target object, so much so that it was a hindrance to observation. At times I could actually see a rainbow-like spectrum of colour! I decided to try δ Boo as it is in the vicinity. The CA was a bit less, but still there. I decided on brighter doubles as a comparison. These included α Gem, γ Leo and later ꞵ 1 Cygni. The false colour was especially noticeable on brighter stars. Finally I tried ε Lyr (Double Double). I needed at least 150x to see all four stars in the conditions. I could still see the aberration. It wasn’t so noticeable on fainter doubles such as 24 Comae Berenices. I compared the GSO 3x and 2x at roughly the same magnifications on a few targets. The fairly intrusive chromatic aberration was definitely being produced by the 3x Barlow. Maybe I should have expected a DIY spectrometer for a 3x Barlow under forty quid. Although my twenty quid SvBony 3x Barlow doesn’t act in this manner. I must say I am quite disappointed in the 3x GSO. It could be that my sample is a bit of a turkey, but I have since discovered a similar complaint about the same Barlow on the Internet. In all honesty I can’t recommend this Barlow lens. Even the element interface thread is an odd size and doesn't seem to fit anything else I have. I thought a relatively well made light, basic 3x Barlow with an ED element would be just what I needed. The GSO 3x unfortunately doesn’t make the grade.

Nice edit, Spock.

This should work, providing the barrel length is 22mm. Otherwise the magnification will be over 2x.

Cheers. I knew I'd posted it in the wrong section. I just didn't realise that I knew that I knew.

You're welcome. To be honest, I couldn't detect any scatter with the SvBony, particularly compared to the GSO. I'm not saying it isn't there. I just didn't see it in my tests. SvBony also have a T-thread version. As far as I can tell the element is identical. But definitely blackened inside. It is about £10 more though. In my opinion, this BST is one of the best shorties on the market. I paid over 80 quid for mine years ago because it has 'Omegon' written on it. At the time I had no idea it was originally Barsta. I have the 3x version as well. Although it doesn't say Omegon on it lol. I find that I can often use longer Barlows in diagonals. If the size of the eyepiece used isn't too large or heavy it shouldn't be too detrimental to balance. The TV 2x works well. I often use it with a 19mm Panoptic in a a short 60mm doublet. There is no vignetting at all, which can happen sometimes with wide angle EP combination in shorties. The Vixen DeLuxe 2x is similar. Although it's primarily designed for short focal length scopes. It also has no compression ring. I must have originally posted this in the wrong section. My brain isn't always totally on the ball these days. I've literally had brain damage (stroke). I'm lucky if I know what day it is lol.

I was pretty impressed with the SvBony 3x Barlow, considering its competitive retail price. This made me wonder about the 2x version of the same bargain basement model. So, for a grand total of £19.99p I ordered one. My first impression of the Barlow, after taking it out of the box, was that it felt like it could have come out of a Christmas cracker. It’s not heavy, lighter (barely 55 grams) in fact than its GSO equivalent, which is mostly made of aluminium. The SvBony has a chromed-brass barrel. The housing is very shiny, both outside and inside, which is not ideal. I was pretty convinced for a while that this is constructed from a lightweight polymer rather than a form of alloy. I don’t know. Kryptonite perhaps? It is very shiny. I now think it is light cast aluminium. SvBony’s marketing department claim: “Cemented doublet and FMC film enhanced light transmission and achromatism; Unique matting ink around the glass lens decrease the stray light” (sic) So I’m guessing the element edges have been blacked. The barrel is conventionally baffled, the element has no provision for a filter thread. From my measurements this ‘shorty’ Barlow is 75mm tall with a standard 22mm brass barrel. The element stands 16mm tall in its housing with at least 20mm of clear aperture. Like its taller 3x sibling, the element is coated, although not obviously. I’ve read erroneous reports that these Barlows are uncoated. This is patently untrue as even a cursory glance reveals a pale green hint of optical coating. It’s probably fairly basic notwithstanding claims of ‘fully multi coated’. Although it doesn’t appear detrimental or affect its efficacy in use. It features a brass compression ring and a plain brass screw. The screw isn’t the smoothest I’ve used but it held a Celestron zoom pretty securely. I certainly didn’t have any anxieties about it working loose. Many short Barlows have very small, almost undersized fastening screws. I don’t always trust heavier eyepieces with these. I directly compared the SV137 with a stock 2x GSO short Barlow that has only a slightly more expensive retail price. The GSO is basically a bog-standard two element Barlow. The comparison was on various double stars that could be viewed with my modified ST80 and a Celestron zoom giving magnifications between 33.3x and 100x. The seeing was below average. I honestly couldn’t see any discernible difference between the two Barlows. They both gave sharp, bright images, without any excess false colour or aberrations that I could see (I expect a certain amount of natural CA with the ST80). The ‘Christmas cracker’ SvBony should be at least as good as the GSO on most targets. The element is fairly well constructed but the housing has its issues in my opinion. It isn’t internally blackened, which possibly explains its competitive retail price. Optically I could perceive no difference to the similarly priced GSO unit.

Yeah, a lot of Baader stuff is like a Meccano set. I have considered the 2" BBHS mirror. The prism looks a tad heavy.

Thanks. The Zeiss Baader is probably the best 1.25" prism I've ever used.

Thanks. Yes, tell me which diagonal and what aspects you want me to measure.

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 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. 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 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 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 more than the plain T2 2456005K. Adding an eyepiece holder and a nosepiece will be an extra cost.

The right-angled Amici roof prism, also referred to as a direct vision or erecting prism, was invented by the Italian astronomer Giovanni Amici (1786-1863). Roof prisms contain a section where two of the roof faces meet at a 90° angle. Reflection from these faces produces an image that is laterally flipped. A conventional right-angled prism, as utilised in a star diagonal, will deviate the light path by 90° and simultaneously invert the image. Above: William Optics Amici diagonal body displaying the roof prism in its housing. These diagonals are most likely made by Kunming United Optics. The view through a telescope directly (without a prism) actually produces an upside-down image. The prism effectively uprights this image. However, the now upright image is mirror-reversed. The Amici prism not only deviates the light path and inverts the image, it concomitantly reverts the perceived image by flipping the reflection back to a natural view. Roof prisms are fine for diurnal viewing but have an inherent flaw for general use in astronomy. The roof section where the two faces meet needs to be manufactured to very high tolerances. Otherwise a noticeable diffraction spike may be revealed when used to view bright or highly contrasted nocturnal objects. Another purported effect of using an Amici, particularly at high magnifications, is phase interference and light scattering. Although these effects can be ameliorated somewhat with modern phase coatings. Old and new style William Optics Amici diagonals featuring a helical focusing eyepiece holder. I am not sure why the design was changed but it may have been to rectify a problem with the prism occasionally becoming unseated in its housing. Most of the one and a quarter inch 90° Amici diagonals on the market are probably intended for occasional daylight viewing with astronomical telescopes. They tend to fall into two categories: resin-bodied and metal-bodied. Above: 'Omegon' metal-bodied Long Perng Amici. The nosepieces and eyepiece holders of both sorts are usually metal and threaded into their respective housings. However, some of the metal diagonals feature brazed eyepiece holders. Interestingly; the nosepiece and eyepiece holder threads of virtually all of the Asian manufactured Amici diagonals are the same size. They are actually compatible with each other. Left to right: GSO, Orion, Antares and WO erecting diagonals. They all feature resin bodies except for the WO. They all have metal base plates. Only the WO has a compression ring. Another feature of these diagonals, regardless of their specific manufacturing origin, is that they only have around 22mm of clear aperture. In my experience only the Baader Zeiss specification Amici prisms have a larger aperture. A feasible problem with the resin-bodied housings is that heavy eyepieces may cause either the holder or nose to pull out of the resin body itself. Although I should imagine that it would take a great deal of force to achieve this. Background, left to right: 2" Baader Zeiss specification, 2" Baader (Long Perng). Foreground: 2" APM 'roof prism'. As far as I know there are only three variations of 90° two inch Amici diagonals on the market. A diagonal manufactured by Long Perng, often marketed under other brand names, an APM and a Baader ‘Zeiss specification’. The Long Perng has a brazed eyepiece holder. The APM and Baader both possess a form of twistlock. The Long Perng and Baader Zeiss specification eyepiece holders both feature compression rings, in contradistinction to the APM which deploys internal ferrules to lock the eyepiece in place. APM eyepiece holder showing internal ferrules. The APM features a 46mm clear aperture as well as the ‘Q-Lock’ quick clamp eyepiece holder. In use its overall optical performance compares favourably with the 2” Baader BBHS which has a 44mm clear aperture. Oddly, the APM nosepiece has no filter thread. The Baader is slightly heavier as well as being more expensive. The 2” Long Perng Amici variants perform no better than their 1.25” equivalents. Although this can be perfectly adequate for rich field and lunar viewing. There is no visible diffraction spike while lunar observing and most rich field or deep sky objects lack sufficient brightness to produce one. Above: 1.25" Baader Zeiss specification with T2 'CLICKLOCK' and helical eyepiece holders. I have used both 1.25” and 2” Amici prisms for several years, primarily for rich field with short tube refractors and lunar observing with catadioptric and refractor telescopes. The less expensive 1.25” resin-bodied types are perfectly good for this type of astronomy. I find that there is no discernible difference in visual quality between the majority of them and the diagonals featuring all metal housings. The exception being the Baader Amici diagonal, which is deliberately made to a Zeiss specification and the prism is of superlative manufacturing quality. Currently I tend to prefer conventional prisms or mirrors for general observing. With the predominant exception of dedicated lunar sessions in which I generally employ an Amici diagonal. This is usually the 1.25” Baader Zeiss specification model. I still utilise some of the less expensive Amici diagonals for sporadic Moon observing as they are comparatively lightweight and are easily carried in a small accessories bag. The Amici prism has both advantages and disadvantages for astronomical observing. The optical simplicity of the light path of a conventional prism or mirror outweighs most of the inherent disadvantage of returning a reversed image. There is less light loss with a conventional prism and this possibly contributes to better overall contrast. However, many of the comparatively inexpensive Amici prisms on the market today can give very enjoyable high magnification lunar images. These same diagonals can also work perfectly adequately for rich field and deep sky viewing.