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A diagonal is used to allow you to mount an eyepiece at a 90° angle to the rear cell of the scope. Catadioptric scopes (SCTs and Maksutofs) are usually not mounted very high up off the ground - they are heavy enough that it’s difficult to hoist them up very high when mounting them, especially onto an equatorial wedge.  So a diagonal usually makes for a more comfortable viewing position - you can be seated on an observing chair looking down into an eyepiece through a diagonal, rather than sitting on the ground and looking up into the scope.

There are two types of diagonals made for telescopes - one type uses a mirror and the other uses a prism.  The advantages and disadvantages of each are as follows:

Mirror (Reflective) Diagonals
These diagonals (often called Star diagonals) use a mirror set at a 45° angle inside the diagonal (so that you view the scope’s image at a 90° angle to the rear cell). Mirror diagonals produce an image in your eyepiece that is correctly oriented vertically, but is reversed left-to-right horizontally.  Until you become accustomed to this reversal, it causes some confusion when you are comparing what you see in your eyepiece to a star chart.  On the other hand, the major advantage to mirror diagonals is that they produce very little light loss in the optical path as long as the mirror is manufactured to be very highly reflective.

Prism Diagonals
A prism diagonal uses a prism rather than a mirror to bend the light path, as do the prisms used in binoculars or the pentaprism used at the top of Single-Lens-Reflex cameras.  The advantage to a prism diagonal is that if it is designed to be an “image erecting” or “correct image” prism it provides a view in your eyepiece that is correctly oriented, both horizontally and vertically. This means that what you see in your eyepiece is the same as what you see when looking at the sky, or a star chart or lunar map.  The disadvantage to prism diagonals can be that because the light path bounces around through a piece of glass, inexpensive prism diagonals reduce the total amount of light transmitted. So unless you purchase a very good one (and there are very good ones available - see below) you can lose 10% or more (depending on the prism design) of the light your scope collects.  (Also, prism diagonals may introduce chromatic aberration when used with short focal-length scopes although this isn’t a problem with most SCTs, which have long focal lengths.)  Because of their light loss, inexpensive prism diagonals are commonly used when you want to use your scope for terrestrial viewing (as a “spotting scope” for bird watching or similar applications) because in typical terrestrial situations there is plenty of light available and a 10% light loss isn’t a problem.

Most amateur astronomers recommend a highly-reflective diagonal for deep-sky observing with a scope rather than the factory-provided diagonal provided with most (i.e. affordable) SCTs. Experienced observers tend to use a 2” diagonal since they use at least some 2” eyepieces, and 2” diagonals usually include a reducer that allows you to insert 1.25” eyepieces so you can switch eyepieces without changing the diagonal. 

Reflective Diagonals
The most well-respected 2” reflective diagonals have traditionally been made by TeleVue and Astrophysics.  TeleVue makes an “Everbrite” dielectric-coated (rather than a simple aluminized mirror) 2” diagonal with 99% reflectivity ($275 street price). Astrophysics makes a “MaxBright” 2” diagonal (shown here on the right) that also uses a dielectric coating mirror, and claims a reflectivity better than 99%, with a listed price of $290.  One major advantage to a dielectric coating is claimed to be that over time a simple aluminized mirror’s reflectivity will degrade, and a dielectric-coated mirror will not (or at least, not very much).  Also, dielectric coating diagonals are more resistant to wear when cleaned - this is important because diagonals, like eyepieces, are exposed to atmospheric dust when you use them. But dielectric -coated diagonals are pricey (and don’t include an adapter to mate to the rear-cell threads of an SCT, which requires more expenditure for an adapter - see below), and there are less expensive alternatives. The classic aluminized 2” diagonal has been the Televue with 95% reflectivity, which has a $210 street price.  However William Optics, a company that is establishing a good reputation for making high-quality astronomy equipment at very reasonable prices, sells a 2” aluminized diagonal with a reflectivity of 97% for a street price of $100, or $130 with an SCT-threaded adapter (shown here on the left).  They also have introduced a 2” dielectric diagonal with a 99% reflectivity for $200.

Note that the vacuum deposition process that is used to produce dielectric surfaces can result in the surface not retaining its “flatness” across the entire surface of a 2” diagonal. Baader has a dielectric diagonal, called the “2” Maxbright”, which avoids this problem with a manufacturing process in which both sides of the mirror are coated.  (The rear-side coating is not needed for viewing, but it reduces distortions in the mirror.)  In appearance this diagonal looks like the Baader Amici prism diagonal shown immediately below, and like that diagonal the T-2 Maxbright is part of a diagonal system (the “T-2” system) with interchangeable barrels and eyepiece holders or camera mounts; it has a 98.5% reflectivity.  It is not currently available in the US but Baader’s US distributor, Alpine Astronomical, should be able to order one (its price in Germany is 330 Euros, which at current exchange rates is about US$380).  Mike Swanson has a thorough review of this excellent diagonal on the Equipment Reviews page of his NexStar Resource Site so I won’t go into more details here, but this is a top-quality diagonal worthy of your consideration.

Correct-Image Prism Diagonals
Baader makes a correct-image Amici prism diagonal with 96% transmissivity.  The basic diagonal is shown on the right; it looks strange only because it is the basis of a “diagonal system” where you order the diagonal body and add either a 2” or 1.25” barrel and eyepiece holder depending on what configuration you want; with a 2” barrel and a 2” eyepiece holder it sells for about $300. In the U.S it is available from Alpine Astronomical.  Note that this diagonal is not quite a 2” diagonal - it is a bit smaller and will vignette the view in a long focal length (very wide angle) eyepiece.

WO 2inch erecting prism - webWilliam Optics makes a true 2” correct-image prism diagonal - it is shown here on the left.  It’s transmissivity is not published but William Optics told me it has a 95% transmissivity.  That’s slightly less than the Baader but this diagonal is also a little less expensive, at about $250. It is well made and worthy of your consideration.

Markus Ludes sells an Intes Micro 1.25” erect image prism diagonal on his APM site for $145; it has a good reputation although Markus doesn’t have transmissivity data for that diagonal.

University Optics sells a 2” prism diagonal for $210.  It is NOT a correct-image (amici prism) diagonal, but rather a plain 45° prism, so it will still produce a left-right-reversed image like a reflective diagonal. However, it is multicoated and provides a 95% transmissivity according to UO.  It would have the advantage of being easier to clean than a reflective diagonal since any dust would collect on the outside surfaces which are easier to reach than the angled mirror of a reflective diagonal.  The disadvantage is, of course, that for the same price the Williams Optics dielectric diagonal has a 99% reflectivity.  Note that for some reason the UO 2” prism diagonal is not shown on their web site, but is contained in their printed catalog.

The high price of these diagonals is related to the quality of the mirror or glass surfaces and coatings, and also to the care taken in assuring that the optical alignment of the unit is as good as possible.  (Otherwise you would find that as you rotate the diagonal to achieve a comfortable viewing position, objects shift location in your eyepiece and worse, a poorly-aligned diagonal can actually impact the overall collimation of the optical system.)  It’s best to get the highest-reflectivity/transmissivity diagonal you can - over the life of your scope the cost will be small and your deep-sky viewing ability will be as good as the scope can deliver. (This is one of the main reasons that most SCT users eventually replace the lower-reflectivity factory-provided 1.25” diagonal.)

Our eyes have a logarithmic response to light, so theoretically we can’t really detect the difference between a 95% reflectivity diagonal and one with 99% reflectivity. But experienced observers claim that they can see a difference, and in any event the fact that a dielectric coating is resistant to degradation over time is a compelling reason to invest in one if you get a mirror diagonal. After all, the price of a really good diagonal is small compared to the price of a scope and the incremental price difference of a dielectric -coated mirror will be insignificant over the life of the scope (assuming you can afford to buy one).  All that said, I do want to point out that I personally use the Baader 2” prism diagonal even though its 96% transmissivity is less than the 99%+ reflectivity of the best dielectric mirror diagonals. It is expensive but I really do enjoy seeing the same orientation in my scope as I see in the sky or on star charts. However, because it is not physically as large as a good 2” mirror diagonal it will vignette your view in a very wide -field eyepiece. I haven’t personally found this to be a major problem - there are many deep-sky objects that are just too large to see in a long focal length scope like an SCT, regardless of the eyepiece you use, and to view them well you’ll eventually need to mount an inexpensive rich-field refractor on top of your scope anyway.

A Few Notes

1. Most diagonals do not include a mount for the rear cell of an SCT - they are intended to be placed in the 2” focuser of the refracting scopes these companies sell.  TeleVue does sell a threaded adapter to mount a 2” diagonal on an SCT for $45 and you would presumably need to use that one for the Astrophysics or Baader diagonal as well since these companies don’t sell such an adapter at all. But if you choose to use a rear-cell-mounted electric (or manual) Crayford focuser you wouldn’t need the adapter - the diagonal will fit directly in the focuser. (See the Focus Options page here for a discussion of a rear-cell-mounted focuser.) Another option is the “Virtual View Adapter” sold by Starizona - in the photo on the right it is the darker object between the OTA and the diagonal.  The Virtual View has an internal tension system that will hold a diagonal and eyepiece at any angle, eliminating the necessity of loosening thumbscrews to rotate the diagonal.  It sells for $130 and is a very convenient accessory for visual observing. 

2. William Optics has a 45° prism diagonal for 2” eyepieces, that was featured in the “Product Showcase” page of the August, 2002 issue of Sky & Telescope.  It is claimed to have excellent optical alignment and has a retail price of $200.  Their web site doesn’t quote the diagonal’s transmissivity but William Optics told me it is 89%. However, 45° isn’t a particularly helpful diagonal angle for astronomical viewing with an SCT, and when coupled with a 10% lower light transmission than the best mirror diagonals, this unit seems to be the answer to an astronomical-observing question no one is asking. Nevertheless, 45° prism diagonals are useful for a wide-field refractor atop your scope or for terrestrial observing, but for these applications you wouldn’t need to spend the extra money for a 2” diagonal - a 1.25” 45° diagonal would work fine. And in fact the 1.25” 45° diagonal sold by Williams Optics for $100 (shown here on the right) would be a good choice - it also has an 89% transmissivity which is higher than the less-expensive models commonly sold. I use one on my rich-field “ST-80” scope and it does provide a brighter view.  (Note that although the Williams Optics 1.25” 45° prism diagonal is threaded to accept 1.25” filters, those threads are very tight and some filters will only barely screw in. While this is not a show-stopper, it is surprising in a diagonal that is relatively expensive compared to its competition.)

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