This diagonal is a part of Orion Optics (UK) set of "Ultra Grade" optics. It is a 100mm Newtonian diagonal (100x141x15.5mm Pyrex substrate), and it comes with a Kendrick XL diagonal heater.
I know Orion Optics has somewhat of a mixed reputation locally - I was skeptical too, so I retested the optics myself.
This flat was tested in ASV's recently acquired Twyman-Green interferometer (donated by CSIRO), and it came out with excellent figure (close to 1/27 wave rms wavefront, which is 1/54 wave surface RMS error), see below. I've also tested it in front of a good quality APO at 45 degrees (very tough test) and it shows no signs of astigmatism. Secondary was made in March this year and it has Orion's excellent HiLux coatings.
N.B. Twyman-Green reference elements are uncoated (but of very high quality) so resulting interferogram is of rather low contrast and my standard software for analysis (DFTF) couldn't pick all of the lines reliably. I had to resort to a bit older package (OpenFringe) to trace and measure the fringes.
The diagonal and heater would cost nearly 600$ to get it into Oz. My price is $400 for the whole lot.
Reason for selling - I want smaller diagonal for my Newtonian as I will concentrate on planetary imaging.
