If you use a refractor, SCT, or MCT, more than likely you are using a telescope diagonal. While there are a lot of people who blow off the idea of upgrading their diagonal saying it is “just a mirror”, they must have never used a really bad, or really good, diagonal.
Diagonals do a lot of things, including their primary purpose, which is to use a mirror to reflect the image at 45 or 90 degrees. This allows you to look through your telescope with less strain on your back and neck. In order to do this well they need to have a very good mirror, to be solid so they do not flex when used such as with a heavy eyepiece, to hold the eyepiece securely, and to eliminate reflections inside the housing so that the image remains crisp and contrasty.
Of all these things, the only thing anyone seems to fret about is the mirror. In the vast majority of telescope diagonals, there are two basic types of mirrors; standard and dielectric.
Standard telescope diagonal mirrors usually have a reflectivity of about 80-90% and can be coated metal, glass, or plastic.
Dielectric mirrors are generally at least 99% reflective, if not higher (some can reach 99.99% or higher). This is created by using various methods to deposit extremely thin layers of optical materials over a base, generally made of glass. Using this method not only allows one to very precisely control the reflectivity percentage, but also the wavelengths of light that are reflected, and how precisely that light is reflected.
In other words, the better the diagonal, the more light, and better quality light, will be reflected. Telescope diagonals are not all created equally.
That being said, you can gain quite a bit from upgrading that cheap diagonal that came with your beginner telescope but will gain little from a midrange dielectric model to the best telescope diagonal on the market. If however, you are attempting to squeeze every single photon out that you can, this is an excellent upgrade because it improves every telescope and every eyepiece used with it. Of course, better eyepieces help too!
Let’s take a look at the telescope diagonal reviews
1.25″ Telescope Diagonal
These are the diagonals you should consider if your telescope will only accept 1.25″ diagonals. If you just have 1.25″ eyepieces, but your telescope is capable of using 2″ eyepieces, I would highly recommend you look at the 2″ telescope diagonals instead.
Looking for something basic but better than the cheap one that came with your telescope and broke almost immediately? This SVBONY SV221 90 Degree Diagonal Mirror is just the ticket. While the body is plastic, it is more durable ABS instead of whatever they make the cheap ones out of that seem to break on first use.
This diagonal also uses a brass compression ring to prevent scratches to your eyepieces and provide better grip.
I personally really like the fact that they use a brass compression ring instead of a set screw, as they tend to hold better and prevent scratches to the eyepiece barrel, and it has a nice solid feel to it.
Being dielectric means it reflects more light than a standard mirror. While this may not sound like a big deal, it is when you are trying to see very faint nebulae or want the maximum contrast on planets.
When you want to up your game, the Celestron Dielectric Star Diagonal with Twist Lock is the next logical step. With this telescope diagonal, you get an impressive build quality, premium optical glass, 58 layers of dielectric coatings, and an amazing twist lock holding system for your eyepieces that can not scratch your eyepieces.
The diagonal also includes a nice barrel stop to prevent your eyepieces or accessories on your eyepieces (like filters) from coming into contact with the mirrors and ruining them.
Talking about build quality, the mechanical parts of this diagonal are machined black anodized aluminum, making this an amazing diagonal to hold while providing light weight and exceptional rigidity. Truly one to keep for the rest of your life.
While other manufacturer’s talk about reflectivity, light baffles, and build quality, no one ever mentions the mirror base, the part of the spectrum reflected, or the flatness of the mirror, except TeleVue. When you want to see the absolute best image possible through your telescope with no excuses, this is the one you should reach for.
2″ Telescope Diagonals
These are the diagonals you should look at if your telescope can accept 2″ eyepieces, regardless of whether you have 2″ eyepieces or not.
This is a nice, solid unit which provides far superior reflectivity (93%) than non-dielectric models (80-89%). They also use high quality mirrors and heavy aluminum alloys to make it rock stable. Overall a good choice for an entry level 2″ telescope diagonal.
They have also thought of the little things with a matte black interior, black velvet inserts on the sides, M48x0.75 filter threads on the nose, brass compression ring, and an included 1.25″ to 2″ adapter for your 1.25″ eyepieces.
For just a few dollars more than the cheapest 2″ telescope diagonal on this list, this is absolutely worth the money.
In other words, it is just an almost identical version of the 1.25″ model, with a higher price tag. Still, if you want a diagonal providing excellent views with excellent features that will last you a lifetime and protect your eyepieces, this is an excellent choice.
This is the big daddy of them all, the Tele Vue 2″ 90-deg Everbrite Star Diagonal with 1.25″ High-Hat Eyepiece Adapter # DDP-8004. If you need every single photon, then you need this. Just like their 1.25″ version shown above, their description reads “Whiter and brighter images”. Simply put, that is the benefit of the Everbrite mirror coating. This exotic, dielectric, nonmetallic coating is 99% reflective across the full visual spectrum, resulting in the “whitest,” most natural image available. The coating is painstakingly applied to Pyrex with a surface accuracy of a 1/10 wave flatness.
I know a couple of people online who have one of these, I have never actually seen anyone who uses one in real life. While I have absolutely no doubt they are the best of the best (everything else TeleVue I have used is), I have never been able to tell the difference between one of these and the Orion 8727 telescope diagonal I use. Maybe my eyesight isn’t good enough, maybe my telescope isn’t good enough, who knows.
Conclusions
Telescope diagonals are important parts of your ability to see objects and get the most out of your telescope. They can not only improve your views, but your enjoyment of the hobby as well. Nothing is worse than having equipment that breaks or only gives you a fraction of what you should have. They also improve the views of every single eyepiece you use.
That doesn’t mean you have to spend a fortune to get good views. A simple dielectric model with fit most people just fine, then you can upgrade to a higher-end model when you are more advanced and have a better telescope. I would never suggest you spend $300 on a diagonal for a $299 telescope with $20 eyepieces. Still, upgrading your little plastic diagonal that came with a beginner telescope is a worthwhile investment.
I hope this article will help you pick out the best telescope diagonal for your needs.