Hill's Space

12" polishing update: Oblate no longer?

 |  1 min  |  120 words

My mirror on may 7, showing an oblate spheroid: the center is flat, while there is a dip in the mirror surface at around 70% of the way from the center to the edge.
Mirror after a bit of parabolizing W strokes. The lines look like they're bulging outwards, a sign of a developing central hole. The zone at 80% diameter is still curved inwards.
The mirror on may 10. The central hole is more obvious, and the outer zone is looking straighter but still has that slight curve to it that reveals a depression in the mirror surface. The very edge bows inwards, revealing a turned up edge.
Five or so 30 minute long sessions of long parabolizing W strokes have finally turned my center hill into a center hole. The dip at around 80% to 90% diameter remains, while the 90% to 100% diameter edge zone is turned up.

I made two changes to get better pitch contact: I am filling a plastic container with hot water and pressing the pitch to the glass underwater, and I am only polishing for 30 minutes at a time instead of going for a full hour. I think it’s working!

Hopefully now that it’s no longer an oblate spheroid, the standard 1/3 strokes will work properly and allow me to fix that bumpy outer zone and its turned up edge.

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