Focal length flattening
A problem! After 6 hours of total grinding, I measured my mirror’s focal length and found it was 37". That’s weird because before it was 39" - maybe my hands were applying pressure in the center of the tool? Lower focal length means more zoomed out (and brighter) telescope views, but makes the final stage of parabolizing harder. If focal length / mirror diameter is less than 4, you also get “coma” that distorts the stars towards the edges of the view.
So… should I move forwards with my fine #320 grit, accepting my shorter focal length? Or should I lose progress and start using a coarser #220 grit to speed up glass removal and make the curve shallower, so there’s less coma and parabolizing is easier?