Star diagonals are a necessary part of any Cassegrain or refracting telescope with an eyepiece at the back end. Without a star diagonal, viewing objects can be uncomfortable, and you literally might not be able to use your telescope as the eyepiece may not be physically accessible or able to reach focus without a diagonal.
Diagonals are often an overlooked part of a telescope, but they are a tremendously important optical accessory for achieving sharp views. Diagonals with poorly polished optical surfaces can induce scattering and warp the image with aberrations like astigmatism, and cheap diagonals with low reflectivity can dim the image through your telescope.
Most telescopes that need one come with a star diagonal supplied, but it may be of low quality or only in 1.25” format. A few telescopes have built-in “flip mirror” diagonals, which can often be of low quality. If you’re concerned about them affecting the sharpness of your views, it’s usually possible to adapt a diagonal to the back end port, but this may cause clearance issues on a mount.
Prism or Mirror?
Diagonals use either a mirror or a prism to bend light at a 90-degree angle. Astronomical prism star diagonals use a triangular prism and should not be confused with Amici “erecting prism” and other designs that can be used for terrestrial observing. Amici prisms produce annoying diffraction spikes and other effects on astronomical objects, and cheap ones are often supplied with beginner telescopes. Few are suitable for astronomy.
Prism and mirror diagonals can both achieve excellent light transmission; even the cheaper units sold today usually have 93% or better transmission, and all of the units we recommend are 98% efficient or better. The biggest difference in performance between the two types is that prisms scatter light less than mirrors and thus can produce a slightly sharper image on bright targets, such as planets. However, in refracting telescopes with fast focal ratios of f/6 to f/7 or below, prisms can add additional chromatic aberration to the view. Prism diagonals are also rarely available in 2” format, and those that are tend to be heavy and expensive.
While a niche concern, those using night vision eyepieces may want a prism diagonal because prisms transmit the near-infrared light picked up by night vision devices significantly better than aluminum mirrors. However, silver dielectric mirrors are available for this purpose as well. Mirror diagonals can also go out of collimation if dropped, though this is rare and user adjustment is usually possible to fix the problem.
Lastly, the prism diagonals’ solid prism optics essentially “seal off” a telescope tube when they are installed in your focuser. This makes them a little more rugged and ideal for use in dusty and dirty environments, especially if you are using a more expensive instrument.
2” or 1.25”?
While using a 2” diagonal with any scope that can physically accept it would be nice, in reality, this isn’t always beneficial. Some telescopes may be unable to both clear their fork mounts with proper balance or simply be unable to balance on their mount entirely with a heavy 2” diagonal and eyepieces. The added weight alone may also be too much for smaller and less-than-steady mounts. Also, smaller catadioptric telescopes with apertures less than 8″ may be able to physically fit a 2″ diagonal, but they may have baffle tubes that limit the usable field of view to a 1.25″ format eyepiece without causing vignetting.
Eyepiece Grips
Most good star diagonals use either a compression ring or a rotating-lock adapter to grip your eyepieces. The reasoning for this is that a tripod-mounted telescope, especially when on an equatorial mount, can have the eyepiece at angles where it could fall out due to an improper grip, whereas Dobsonian telescopes usually have gravity to help as a backup should you screw something less-than-tightly. Set screws can also cause cosmetic damage from digging into your eyepiece.
Twist-lock diagonal designs are usually a little more secure than brass compression rings, but are usually more expensive and slower to interchange eyepieces with.
Best Mirror Star Diagonal: Our Recommendations
If you’re using a 2” star diagonal, it’s likely to be a mirror diagonal. Dielectric mirrors have extremely durable coatings with many protective layers to minimize scatter and make them nearly impossible to scratch or chemically damage. Most mirror diagonals without dielectric coatings are likely to be cheap and of low quality.
If you’re limited to a 1.25” format, consider a prism diagonal instead of a mirror, as they are on average cheaper for the same or better performance in most telescopes, as mentioned above.
2” Mirror Star Diagonals
1. Best In Class 2″ Mirror Star Diagonal: Baader BBHS 2” Star Diagonal
The Baader BBHS 2” Star Diagonal uses a silver-based dielectric mirror, which is slightly superior to standard aluminum mirror diagonals but just as durable. The main benefits are that there is slightly less scatter and slightly more even color transmission. This makes the view through the eyepiece slightly more vivid. The mirror is made of Sitall, a low-expansion ceramic design, for the smoothest possible polish and the least possible affectation from temperature. The silver-coated mirror is also better for use with night-vision devices. The Baader diagonals can screw directly onto the back of a Schmidt-Cassegrain telescope with a minimum length for clearance purposes or fit in any 2” focuser, and this diagonal uses the Baader ClickLock system to securely center and grip your eyepieces.
2. Baader MaxBright 2” Star Diagonal
The Baader MaxBright 2” Star Diagonal has an extremely high-quality dielectric aluminum-coated Pyrex mirror and uses the Baader ClickLock system. However, you do need a separate 2” visual back or other adapter system to use this diagonal in a Schmidt-Cassegrain telescope.
3. Tele-Vue Everbrite 2” Star Diagonal
The Tele-Vue Everbrite 2” Star Diagonal is another great choice for a quality dielectric diagonal and includes 1.25” and SCT adapters by default. The standard single-screw compression ring grips are, however, not quite as secure as a twist-lock or ClickLock system.
4. Best Value 2″ Mirror Star Diagonal: Apertura 2″ Twist Lock
The Apertura 2” Twist Lock Carbon Dielectric Mirror Star Diagonal uses cheaper BK7 glass for its mirror, but it still provides sharp views and includes a twist-lock system and 1.25” adapters. The default version fits into a 2” focuser directly, but an SCT version of this diagonal is also available. The carbon fiber body is little more than an aesthetic choice, however.
5. Celestron 2″ Dielectric Star Diagonal with Twist-Lock
The Celestron 2” Star Diagonal with Twist Lock is similar in design and quality to the Apertura unit, though more expensive. It also uses a purely twist-lock system with no screws, which makes changing out eyepieces slightly faster, and comes with both SCT and 2” barrel adapters by default.
Tip: This is often available on Amazon for a discounted price of $199.
6. Tele Vue 2″ Enhanced Aluminum Diagonal
The cheaper Tele-Vue 2” Enhanced Aluminum Star Diagonal has only 96% transmission compared to the Everbrite diagonal but has the same high quality standards and smooth Pyrex mirror; if you want a cheaper but possibly less sharp diagonal with better transmission, however, the choice is yours.
7. Takahashi 1/10th Wave 2″ Diagonal
The Takahashi 2” Star Diagonal’s huge body uses an elliptical mirror like that of a Newtonian reflecting telescope, with theoretically more room for adjustment and less possibility of issues due to temperature-induced expansion and contraction. This diagonal is made to excellent quality control standards. However, it only uses a single thumb screw to grip your eyepieces, is quite bulky and may have clearance problems on Schmidt-Cassegrain telescopes, and you don’t get a ton of bang for your buck.
1.25” Mirror Star Diagonals
1. Best Performance 1.25″ Mirror Diagonal: Tele Vue 1.25″ 90 Degree Everbrite
The Tele-Vue 1.25” Everbrite Mirror Star Diagonal uses a high-quality dielectric Pyrex mirror with 99% transmission, just like the larger 2” Everbrite. It also has a compression ring to grip your eyepieces.
2. Best Value 1.25″ Mirror Diagonal: Apertura 1.25″ Carbon
The Apertura 1.25” Carbon Dielectric Mirror Star Diagonal lacks the roto-locking eyepiece grip of its larger cousin but otherwise is similar in design and quality standards; the carbon fiber body and tapered eyepiece holder are really only aesthetic choices. This diagonal probably provides the best value for your money of all the 1.25” dielectric diagonals, though a prism unit may make more sense.
3. Second Best Value 1.25″ Mirror Diagonal: Celestron 1.25″ Dielectric
The Celestron 1.25″ Dielectric Star Diagonal with Twist-Lock is much like the Apertura dielectric diagonal, with the same BK7 mirror and 99% reflectivity, but uses a twist-locking mechanism to grip your eyepieces.
Tip: This is often available on Amazon for a discounted price, but the price is still higher than Apertura’s.
4. Best Tele-Vue 1.25″ Mirror Diagonal: Tele-Vue 1.25” Enhanced Aluminum
The Tele-Vue 1.25” Enhanced Aluminum Star Diagonal is quite a bit cheaper than the Everbrite and uses a high-quality Pyrex mirror, but it does have 3% lower transmission. However, it’s great for those who must have a Tele-Vue unit but are on a budget.
5. Best Budget 1.25″ Mirror Diagonal: SVBONY SV188P
The SVBONY SV188P Dielectric Mirror Star Diagonal is similar to the Apertura unit but isn’t subject to quite the same quality control standards; the mirror is still BK7 and usually works fine but may not be quite as smooth and sharp. Some users also report a failure to paint the inside of the anodized red housing of the diagonal black from the factory, leading to reddish glare on bright targets.
6. Cheapest 1.25″ Mirror Diagonal: SVBONY 1.25 inches 90 Degree Zenith
The SVBONY 1.25” Aluminum Star Diagonal doesn’t have as much in the way of transmission, durability, or quality control standards compared to a more expensive diagonal. However, it doesn’t use any plastic parts, features a compression ring, and still achieves sharp images. There are far worse budget options.
Best Prism Star Diagonals: Our Recommendations
2” Prism Star Diagonals
Best Performance 2″ Prism Diagonal: Baader 2” BBHS
The Baader 2” BBHS Prism Star Diagonal is a bit of a niche product given that the silvered Sitall diagonal from Baader is nearly identical in performance, although it has slightly more scatter. However, both are great choices.
Best Amici 2″ Diagonal: Baader 2″ Astro Amici
The Baader 2” Astro Amici is one of the few Amici prisms we would actually recommend if you must have a corrected left-right, up-down system. It has extremely sharp optics on account of the prism being made by Zeiss and Baader’s high-quality coatings. However, the price is a bit of a shocker, and it really has little in the way of advantages over the standard BBHS prism or mirror diagonals.
1.25” Prism Star Diagonals
1. Best 1.25″ Prism Diagonal: Baader T2 Prism
The Baader T2 Prism Star Diagonal is probably the best 1.25” prism diagonal around and uses interchangeable threads, which can allow you to secure the diagonal to a telescope with other accessories for maximum clearance and as little spacing as possible. The T2 Prism also has a fine-focus adjustment built into its compression ring eyepiece holder, which is an extremely convenient feature for high-power planetary viewing. The optics are, of course, top-notch too, with maximum light transmission and minimal scatter.
2. Takahashi 1.25” Prism
The Takahashi 1.25″ Prism Star Diagonal is yet another excellent choice for a 1.25″ prism, with maximum transmission and minimal scatter and a quick-release rotating collet to securely grip your eyepieces.
Best Value 1.25″ Prism Diagonal: Celestron 94115-A Prism
The Celestron #94115-A Prism Star Diagonal is the same diagonal that comes with many of Celestron’s mid-range and expensive telescopes when the cheap plastic Amici prism is not included. The 94115-A has some plastic innards but uses a metal body, and the prism is of excellent quality, especially for the price. The only issue is that it uses thumb screws to grip your eyepieces, and these screws are so short and fat that they can prevent you from inserting wider eyepieces all the way; this can thankfully be remedied with a trip to the hardware store.
Tip: This is often available on Amazon for a slightly discounted price when compared to HighPoint.
Awesome concensus!
While the amici diagonal from Baader is showing amateurs the sky as it should be, correct, this diagonal is best for correct view deep sky objects. For bright objects it basically cannot be used. The best correct views of bright objects requires a binocular telescope with superior optics.