barkis

You're wise to treat the rear of the disc first.

I had partially figured a 12" mirror a number of years ago, and I wasn't happy with the condition of the back side. It had suffered a few abrasions during the grinding and polishing stages, and I foolishly decided to clean it up, just using 400 grit, which leaves a nice very fine ground surface. Of course the disc back wasn't perfectly flat, so it took a fair number of operations to get it uniformly fine ground. The shock came when I placed the mirror on the test stand, and found it was all over the place. It had not even the resemblance of a sphere,

let alone a figure of revolution.

I was disgusted with myself, as if I had applied the two pennyworth of grey matter I do possess,. I would have left the back alone. After all, it was a fairly thin disc of plate glass.

The removal of glass that had occurred, was obviously sufficient to release some stress in the glass, resulting in the complete waste of time of all the figuring work I'd done previously.

That disc is still in a cupboard in my workshop. I lost the will to recover it.

Anyway, sorry to hear about your Car problems. Always a bad, and usually expensive time, and Christmas too makes it worse I guess.

Ron.

I love to see people taking on this challenge, and you have embarked on a very big one here.

You certainly have the right attitude, which is a big plus in itself

It's difficult to gauge the thickness of the blank, about 40mm perhaps?

Also, does it have a pre generated curve, which would be a big help in reducing the amount of coarse grit grinding?

How fast are you making it? f3.5 - f4.5 perhaps,? I'm only guessing.

My largest mirror was a 14" f5, but I did the grinding with a full size blank. I only  used sub diameter laps during figuring stages.

I wish you well, you will have a mighty big photon collector, and I will follow your progress with extreme interest.

Best Wishes .

Ron.

:D:D. Very amusing. I enjoyed reading that, although I believe you are more capable than you imply. Anyone who can relate a tale like that, is no duffer.I too dabbled in stepper motors years ago, but in the end I resorted to RS Components for a work of art they sold.

I made a camera platform with it as the drive, for the 1999 Solar Eclipse, and I and a couple of friends motored all the way to Devon. Unfortunately, cloud dominated the event. Anyway, it was an adventure we all enjoyed.

I actually dug it out of the dark cupboard it was locked in, just a couple of weeks ago. Connected it to my Power Tank, and it still works.

Cheers .

Ron:)