Top-rated Star Diagonals I've Experimented With and Suggest for Your Telescope
Star diagonals are an integral component of Cassegrain and refracting telescopes, enabling comfortable observation by directing light towards the eyepiece located at the rear end. Their absence might render the telescope inaccessible or focus-incompatible, causing inconvenience.
Inexplicably, diagonals are often disregarded. Poorly polished optical surfaces can distort images with aberrations like astigmatism, while low reflectivity can dim the observable image. To maintain sharp views, it's advisable to use an improved diagonal when concern arises regarding its impact on image clarity. However, mount clearance must always be checked prior to adaptation.
The majority of telescopes equipped with star diagonals offer poorly performing or smaller 1.25" dimensions models. Some telescopes have built-in "flip mirror" diagonals, which can be equally disappointing.
Prism vs. Mirror Diagonals: Choosing the Right One
Diagonals utilize either a mirror or a prism to bend light at a 90-degree angle, yet they differ significantly. In my experience, prisms and mirror diagonals can both achieve exceptional light transmission, with even cheap units possessing at least 93% or superior transmission. The key performance differences are outlined below.
- Prisms scatter light less than mirrors, potentially contributing to sharper planetary images. However, prisms might introduce additional chromatic aberration in refracting telescopes with fast focal ratios (f/6 to f/7 or faster).
- Mirror diagonals may warp if dropped but can usually be realigned. I favor prism diagonals due to the sealed optics that secure the telescope tube when installed in a focuser, making them more durable for use in dusty environments, particularly expensive instruments.
- Night vision eyepieces are better served by prism diagonals, as prisms transmit near-infrared light far superior to aluminum mirrors. Nevertheless, silver-dielectric mirrors are also available for this purpose.
- Prism diagonals are seldom available in the 2" format, and those that exist are often heavy and pricey.
Astronomical prism star diagonals use a triangular prism and should not be confused with Amici prisms, suitable for terrestrial observing only. Cheap Amici prisms may be supplied with beginner telescopes but are not suitable for astronomy.
Size Consideration: 2" or 1.25"?
While a 2" diagonal with any compatible scope would undoubtedly be advantageous, this may not always be beneficial in practice. The added weight of a 2" diagonal and eyepieces can overload some mounts, causing them to lose balance. Furthermore, even smaller catadioptric telescopes with apertures under 8'' tend to have baffle tubes that restrict the usable field of view to 1.25" format eyepieces, thereby preventing vignetting.
Different Eyepiece Grips to Consider
Most high-quality star diagonals use either a compression ring or a rotating-lock adapter to secure eyepieces. This is to avoid potential dropouts when the telescope is mounted on a tripod or equatorial mount at an angle, and to prevent damage from set screws digging into the eyepieces. Twist-lock diagonal designs offer more security but are usually more expensive and slower for eyepiece interchangeability.
Recommended Mirror Star Diagonals
If you're using a 2" star diagonal, it's likely to be a mirror diagonal. Dielectric mirrors boast highly durable coatings with numerous protective layers to minimize scatter and are nearly impervious to scratching or chemical damage. Most inferior mirror diagonals, lacking dielectric coatings, are cheap and of poor quality.
For 1.25" format scopes, consider a prism diagonal instead of a mirror, as they generally offer better value and performance in most telescopes.
2" Mirror Star Diagonals
- Baader BBHS 2": This premium diagonal uses a silver-based dielectric mirror, offering slightly less scatter and more even color transmission. The Sitall low-expansion ceramic mirror ensures the smoothest possible polish and minimal temperature-induced impact.
- Baader MaxBright 2": With excellent dielectric aluminum-coated Pyrex mirror and Baader ClickLock system, this diagonal requires a separate 2" visual back or adaptor to function with Schmidt-Cassegrain telescopes.
- Apertura 2" Twist Lock: This affordable diagonal uses a less-expensive BK7 glass mirror and features a twist-lock system and 1.25" adapters. A Schmidt-Cassegrain variant is also available.
- Celestron 2" Dielectric Star Diagonal with Twist-Lock: Similar to the Apertura unit in design and quality, though more costly. Its exclusive twist-lock system facilitates faster eyepiece interchangeability and comes standard with both SCT and 2" barrel adapters.
1.25" Mirror Star Diagonals
- Tele Vue 1.25" Everbrite: This exceptional dielectric diagonal boasts a high-quality Pyrex mirror with 99% transmission, much like its larger 2" Everbrite counterpart. It employs a compression ring to grip eyepieces.
- Apertura 1.25" Carbon: Similar to its bigger sibling in design and quality standards, except it swaps the rotating-lock for a carbon fiber body and tapered eyepiece holder—primarily aesthetic choices.
- Celestron 1.25" Dielectric: Nearly identical to the Apertura dielectric diagonal, but uses a twist-locking mechanism rather than a compression ring for eyepiece grasp.
- SVBONY SV188P: Inexpensive and no-nonsense option boasting scatter-minimizing BK7 mirror, although not subject to the same quality control standards as more expensive diagonals.
- SVBONY 1.25" Zenith: More budget-friendly choice compatible with standard compression rings and boasts 99% reflectivity. Be aware of some reports of red glare on bright targets due to unpainted red housing.
Selecting the correct star diagonal for your telescope can significantly enhance the observing experience by ensuring proper focus, reduced light loss, and optimal image quality. Consider your telescope's specifications, desired field of view, and personal preferences when making a decision.
- Star diagonals, whether prism or mirror, are essential for telescopes, as they direct light towards the eyepiece and enable comfortable observation.
- Picking the right star diagonal is crucial, as poor optical surfaces can distort images with aberrations like astigmatism, while low reflectivity can dim the observable image.
- Prism and mirror diagonals have unique properties, with prisms scattering light less and potentially contributing to sharper planetary images but introducing additional chromatic aberration in refracting telescopes with fast focal ratios.
- Mirror diagonals might warp if dropped but can usually be realigned, while prism diagonals are more durable for use in dusty environments due to their sealed optics.
- Night vision eyepieces are better served by prism diagonals, as prisms transmit near-infrared light far superior to aluminum mirrors.
- When choosing a star diagonal, consider factors like mount clearance, eyepiece grips, and the telescope's specifications and desired field of view. A relevant diagonal can significantly enhance the observing experience by ensuring proper focus, reduced light loss, and optimal image quality.
