Best Telescope Diagonals for you in 2023

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 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 96% 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 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.

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.

If you are looking for a high quality basic telescope diagonal, look no further than the Orion 8763 1.25-Inch Prism Star Diagonal. This is a very solid replacement for a lost or broken diagonal that came with your first telescope, or a great upgrade if your telescope came with a plastic diagonal.

I personally don’t like the fact that they use a set screw instead of a brass compression ring as they tend to work loose and scratch the eyepiece barrel.

Another excellent telescope diagonal by Orion Telescopes is their Orion 8879 1.25-Inch Enhanced Mirror Star Diagonal which while not a dielectric mirror, does provide improved reflectivity at 97% with an excellent housing. Also note the brass compression ring which will hold your eyepiece more securely while not scratching its barrel. If I did not want to spring for a dielectric mirror telescope diagonal, this would be my choice.

In fact, you will see this diagonal, or one very similar, with most midrange telescope kits out there today.

I normally do not put this much of one manufacturer’s equipment in an article but Orion just happens to have one of the best selections of diagonals around. This Orion 1.25 Inch Twist-Tight Dielectric Mirror Star Diagonal is no slouch either with it’s 99% reflectivity, machined anti-glare baffles, and their wonderful Twist-Tight method of securing the eyepiece which ensures it is held not only securely, but also centered without scratching.

If you want 99% of the best view you can get in an extremely well built package, this is the diagonal to buy.

 

For those who want the best, this is it, the Televue 90° 1 1/4” DPC-1250 Everbrite diagonal. This is one you can invest in and use for a lifetime without every worrying about needing something better. Their description says “Whiter and brighter images. Simply put, that is the benefit 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.”

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 Tele Vue. 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.

Orion also has the best selection of 2″ diagonals around so this will look a lot like the 1.25″ models. This Orion 8850 2-Inch Refractor Telescope Mirror Diagonal is a solid basic model that will replace the one that came with your entry level telescope. Nothing special, just solid and functional.

I personally don’t like the fact that they use a set screw instead of a brass compression ring as they tend to work loose and scratch the eyepiece barrel.

This Orion 8727 2-Inch Dielectric Mirror Star Diagonal is the one I currently use and it has been an excellent performer for years. If you want a solid performer but don’t want to spend the money on a Tele Vue model, this one will do quite nice. With 99% reflectivity, an anodized aluminum housing, and machined baffles inside to reduce glare and increase contrast, you will be very pleased for years to come.

My only complaint with this one is that the part that slides into the focuser on my telescope is a little too smooth. While my focuser grabs it just fine, and holds it well, it goes from being held very securely to slippery as a goose with a slight release of the focuser set screw.

Orion’s top of the line 2″ Twist-Tight Dielectric Mirror Star Diagonal is basically the previously covered 8727 model with their new Twist-Tight method of securing the eyepiece. It includes the same anodized aluminum housing and the same machined baffles. While the Twist-Tight might seem like a fairly insignificant addition it makes taking the eyepieces in and out much easier in both very hot (sweaty slippery hands) and cold (wearing gloves) conditions. If you have expensive eyepieces, this can save you from dropping that eyepiece onto the ground.

It does not take but one time dropping that $100+ eyepiece onto concrete until you realize the $38 increase between this diagonal and the 8727 could be well worth it. I would have probably bought this one instead of the 8727 but when I made my purchase this one had not been released yet.

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 Tele Vue 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.

I hope this article will help you pick out the best telescope diagonal for your needs.