Stub Mandrel

> This excellent mirror, now located in the Centre of the Universe, was replaced by a ceramic mirror that was affected less by temperature changes. I'm curious as to why such a mirror would be 'archived'. I would have thought a university somewhere would snap it up - half the cost of a big observatory must be the mirror! <edit> Ah! I see it is still (just) in use - once the second largest in the world, glass made in Germany, ground three times in Pittsburgh (partly due to the 'mystery scratch' incident!) https://en.wikipedia.org/wiki/Dominion_Astrophysical_Observatory#Centre_of_the_Universe

The professional mirror makers features in an old video posted recently were using polishing machines made entirely out of wood! The laps appeared to be wooden too. One interesting bit shows them using two laps of different sizes at different diameters.

It's easiest to clamp a calliper if you haven't got a drill that will touch it. For the modest forces here you could even epoxy it in place, then when you are done, peel it off. If you want an accurate but cheap way to move anything, get a long M6 screw ('bolt') and force it into the central hole of a plastic 'fixit block'. The block can be attached to anything and the screw used to move it in and out quite accurately. The pitch of the screw is 1mm so a knob or dial attached to it with 40 divisions will move it almost exactly 1 thou per division or 50 divisions will move it 0.02mm. You can use an allen key superglued or epoxied into a cap head screw and the key acts as a 'pointer' that is easily aligned with a printed off dial 50-100mm in diameter. The reason for using the fixit block instead of a nut is you will get zero backlash.

It might be worth using a cheap digital calliper instead of a micrometer head to measure movement.

Is there a difference in that most cameras have a much bigger aperture than the eye's pupil?

Looks like a quarter to a third of the 'hill' gone to my eyes? I'm already getting nervous, don't know what you must be like!

It might be the file type. Try converting to JPG if it's something else.

From the sublime to the ... ? Seems to work quite well with standard issue Skywatcher EPs instead of the supplied ones. It was good enough to get me to want a bigger scope but that's a newt...

That video would have made a fine 'interlude' back in the day.

Exciting times!