After removing about 4mm from the center of the blank, and 2.5mm from the edge using #60 grit, and about 40 hours of smoothing with #120 grit, and a further 15 hours of smoothing with #220 grit, Joe measures the radius of curvature and inspects the surface. Time to move on to #320 grit.
Don, Joe, and some students from the telescope making class help to move the 275 lb blank for cleaning and preparation for further smoothing with #320 grit.
Moved! Now cleaning of everything begins. Mirror, tools, stands, counters, etc etc.
Now tipping the mirror up for cleaning. Note the hollow spaces in the mirror where the ceramic cores used to be (see Mirror Fabrication for pictures of how the blank was made).
Smoothing work continued through 25um, 15um, 9um, 5um, and finally 3um. The surface was in excellent shape and ready for polish. In all, we removed about 4 mm of glass from the edge and 5.5 mm of glass from the center of the faceplate. To our amazement, the mirror polished out in only 20-manhours using polishing pads. The figure (the shape of the surface) was highly oblate (more curved at the edge than the center), the opposite of what we desired. Here is a picture of Don smoothing the mirror as James Ross (a helper and avid telescope maker), Manishi, and Joe coach.
Once the mirror is polished, we can test it and begin shaping the mirror by microscopic (but crucial) amounts towards our final goal - a prolate ellipse...