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Comes with a thick, precision, optical flat mirror with an enhanced 99% reflective dielectric coating. Compression ring holds eyepieces firmly and the insert tube has the safety groove and filter thread. All aluminum construction. Individually tested here for perfect alignment and freedom from astigmatism
Posted by Ralph E. Taggart on 10th Apr 2015
The finest telescope in the world is part of a system. The telescope is certainly the most critical component of the system, but the mount, drive, diagonal, lenses, and any filters are all part of the total system. There is an old saying that a chain is no stronger than its weakest link, and this certainly applies in astronomy. The humble mirror diagonal is a case in point. A good diagonal should be optically flat to avoid distortion in the optical chain, it should have high reflectivity (to avoid wasting what light your telescope collects, and the mirror should be mounted very accurately to maintain collimation. I inherited my first diagonal from the package that represented my first serious telescope. When I started using it with my first Stellarvue refractor, I noticed that, with the diagonal in place, collimation was a bit less than perfect - not characteristic of the instruments from Vic's shop. A few simple tests with a garden variety laser pointer quickly showed that the mirror was not optically flat, the angle of the mirror was not precisely 45 degrees across its surface, and that the mirror showed stress at all mounting points. The result of all this was the purchase of one of Vic's mirror diagonals. Back to my redneck optical lab and the mirror was as perfect as my testing could determine. Its performance with Stellarvue refractors was as perfect as one could ask for. I now use it with all my telescopes, including an SCT and an MCT. When using an instrument that requires routine collimation (such as an SCT), using a high quality diagonal assures high-quality alignment.
If you are shopping here, you are looking for quality. Don't short-change a fine telescope by using a second-rate diagonal!
Posted by Johnathan Dolby on 7th Apr 2015
The diagonal that came with my 5-inch SCT had some annoying flare lines when looking at bright stars or planets. When I removed the diagonal and used an eyepiece straight-thru, the lines disappeared. I wanted a better diagonal and I saw Stellarvue's 1.25-inch dielectric offered at a reasonable price. I don't claim to be an optician, but I am impressed. I could not detect any difference in image quality when using this diagonal versus using the eyepiece by itself. Image brightness and sharpness are unchanged and there are no annoying flares or lines. Also, the larger thumb screw is a nice touch, along with the compression-ring design, making it easier to use than my old diagonal. Insertion and removal of eyepieces is much smoother. A star diagonal really ought to go unnoticed, but I'm liking everything about this one, including its good looks.