Hi Grinder/Polishers,
Have been doing more critical testing of my 22" mirror. On re-appraisal, what I have is a paraboloidal figure, (conic -1), across the central 17.5" of the mirror diameter, between 17.5" and 17.75", to mirrors edge, the surface curvature more approximates a hyperbole (conic -1.5). ( The curves (ronchi test 133 lpi at the focus), do not actually "hook" at all.)
I have carried out visual tests with the uncoated mirror in the scope using aperture masks of various apertures from 17.5" to 20.5", and at full aperture 22.2". Test objects Moon, and Saturn (with its moons). Interestingly the images focused up identically sharp regardless of aperture employed. In fact I preferred full aperture, as images were brighter ( I could clearly see at least four of Saturns moons at full aperture, whereas only Titan could be seen at the 17.5" setting). Had stunning views of the 2/3 gibbous moon, (full aperture), with detail sharp, with strong contrast terminator to edge. Features in the fully lit moonscape,which normaly tend to be some what washed out, stood out in sharp relief.
When viewing stellar/DSO objects the uncoated 22" gave less spectacular views than the 4.5" Skywatcher Newt finder. The finder using a 20mm 70deg eyepiece for 25x, with a 2.8deg field of view, compared to the 22" at 100x with 0.7deg field. (So actual area of the FOV being 1/16 the size of the finders(so fewer stars)). Therefore not a great jaw dropper on DSOs.
Still working the second of the 22"ers. Intending to leave one of these mirrors uncoated, for exclusive use on moon and planets. One advantage with an uncoated optic, is that there is no coat to deteriorate, and when the surface gets dirty/dusty, then a good easy washup, with a window cloth, and clean water will do the job.
Stephen.
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