The Celestron Diagonal Mirror is a basic accessory for taking the image from a Schmidt Cassegrain or Refractor telescope and redirecting it to be easier to see by eye. This particular diagonal has mixed success in attaching to telescopes. The difficulty in attachment is its only real limitation for use, though it is a big one.

Background

Diagonals are used with Refractors, Schmidt Cassegrains, Maksutovs, and other makes of telescopes with their light outlet at the back of the tube. Pointing a telescope with its outlet in the back at an object at high altitude leaves the back pointed at the ground, so the observer would have to climb under them to look through the eyepiece. A diagonal points the eyepiece 90 degrees from the axis of the telescope, so it is much easier to look through. A telescope with a focuser which is already on the side, such as a Newtonian, has no need for a diagonal.

There are two major types of diagonals- prism and mirror. Prism diagonals use a glass prism inside to bend the image. The worst difficulties with a prism diagonal are from the fact it has three optical surfaces, and imperfections in any of them will degrade the telescope's image. The other effect of a prism is it may only pass through 75-85% of the telescope's light. One version, the Amici prism, can produce upright images, these have bright glare when seen against a dark background at night, where these defects are hidden in bright daylight.

Mirror diagonals solve many of these problems since they only have one optical surface, and it can easily be over 95% reflective. At the same time, they are a one-trick technology- the mirror can produce an upright image reversed from right to left.

Any diagonal has to preserve the telescope's alignment with the eyepiece. If it moves the eyepiece out of the true optical path, the image becomes distorted and blurred. The final hazard for these devices is they add the length of their own optical path to the tail end of the telescope. So, for example, if they have a small diameter, they will vignette the image the way looking through a paper towel roll does.

Description and Use

The Celestron 2" mirror diagonal has a rugged aluminum housing and all-metallic construction. The size is considerably larger than a 1.25" diagonal. In fact, it is even bigger than a typical flip mirror used for switching between a T-threaded camera and 1.25" eyepiece. This model is a little unusual among 2" diagonals since it was designed to be adapted to either refractors with 2" diameter focusers or to Schmidt Cassegrain telescopes (SCTs) which have a threaded hard connection at the back. The diagonal does this by including a 2" sleeve (is is actually the same part as the sleeve on Celestron 2" E-Lux eyepieces) as well as a thread-in Schmidt Cassegrain screw-on connector. This feature unexpectedly turns out to be the most important limitation to this piece of equipment. I have broken out the results with three different telescopes to illustrate how this component works in practice.

Refractor with 2" Focuser:

Telescope used: Celestron 80ED f/7.5 refractor.

The diagonal has to use its 2" sleve for this application, and on the face of it, it appears to be an easy fit since the collar readily accepts 48mm filters, which are the standard size used with 2" eyepieces. However, the sleeve has a recess cut into it to cause the set screw in the focuser which comes so close to the edge of the sleeve it causes the diagonal to cant in the focuser so it is no longer aligned. The only correction I have been able to find for this has been to put aluminum air conditioning tape into the recess to make it flush with the rest of the outside of the sleeve. The set screws bite into the tape, so the part can't slide out, but this is far from an ideal solution. On other diagonals, the 2" sleeve has a conical cut in the set screw region to cause the set screws to pull the diagonal in flat against the focuser. Strangely enough, the 1.25" adapter Celestron includes with the telescope is built this way, but the diagonal does not include this feature.

Other than this problem, the diagonal does a good job of opening up the field of view for a 2" eyepiece. The diagonal will also accept 1.25" diameter eyepieces with a 1.25" adapter inserted in the top. However, the focuser is near the front stop when this diagonal is in, so when a barlow lens is added, the telescope cannot reach focus. So, for this appliacation, a smaller 1.25" diagonal must be used. With that said, this diagonal is usually left in this telescope with a 48mm Neodymium contrast booster filter left in it.

Small Schmidt Cassegrain with SCT mount:

Telescope used: Celestron C5+ f/10 or f/6.3 SCT.

The C5 is the smallest SCT Celestron makes, and it has the standard SCT thread-on interface of larger models. The telescope is about the size of a coffee can, so it is a prime choice for a portable telescope. However, attaching the 2" diagonal doesn't work well on this unit. The problem is the focusing knob is close to the threaded back. As a result, the large diagonal body will interfere with the focusing knob unless it is mounted with the focus knob to the side of the diagonal. This is only possible if the telescope is mounted upright, as it would be on a camera tripod. Astronomical mounts, like the NexStar 5i, the C5+, or mounts on a German equatorial require the diagonal to move from this position, and so the 2" diagonal will not work in these applications.

For what it is worth, the diagonal works well with a large field eyepiece, and it would be possible to overcome some of these limitations by using a 2" visual back and attaching the diagonal to it, but overall, the complexity of attaching to this telescope doesn't seem to equal the reward. So, I suggest staying with a 1.25" diagonal.

Larger Schmidt Cassegrain with SCT mount:

Telescope used: Celestron NexStar 8 GPS f/10 or f/6.3 SCT.

The 8" Schmidt Cassegrain is one of the most common serious amateur telescopes thanks to its compact size and ability to perform as a general purpose telescope. In this case, the diagonal easily attaches with the SCT collar and now can swing through a complete circle without hitting the focusing knob. The long back focus of the SCT means barlow lenses and other accessories which would not work in the refractor can drop into the 2" diagonal and still work.

The end result is the larger SCT is the best match for this diagonal, and is the reason why it is included as standard equipment with some of Celestron's largest Schmidt Cassegrain telescopes. When used with a 2" eyepiece, the SCT presents a truly impressive "Big picture" view. However, the SCT adapter does not allow for a 48mm filter to attach to the diagonal, so unless you are using one of the few filters which come with SCT thread to mount between the telescope and the diagonal, the filter will need to mount to individual eyepieces.

Conclusion

The Celestron 2" Mirror diagonal is a basic accessory for getting greater visual performance from mid to large sized telescopes. Unfortunately, the modular connections included with this diagonal handicap its ability to attach to telescopes. The 2" sleeve has to be modified to achieve a true fit. The SCT adapter still doesn't have a provision for 48mm filters, so these either need to attach to eyepieces or an SCT mount filter would need to be used.

All in all, this part suffers from several faults which would be trivial to correct. As a diagonal, it works reasonably well, but is clearly most useful for larger Schmidt cassegrain mountings.

Recommended: No