Attendees: EdK, JerryW, TomT, DmitriiZ, TimC, TomW, ChrisU
EdK-brought in 1968 pyrex 12.5” f/5 mirror blank (also w/ couder disks http://stellafane.org/tm/atm/test/mask.html) that had been polished and TomW set up his tester (1/4-80 threads on adjuster!) for Ronchi test and resultant patterns in and at focal length. TomW's verdict was center of mirror was not quite deep enough for perfection, but edges look good. Foucault knife edge test did not show the problem in the center as well. TomW recommends 6” tool to work on center to deepen it. TomW will bring in a calculated Ronchi pattern chart from the focal length of 127” to use by EdK for finishing the mirror.
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| TomW says edges good |
TimC-continued work on donated 8” mirror blank grinding to f/5 with 80 grit still. Many “wets” or “charges” of the grit was used. When the measurement of the sagittal, depth, is satisfied the smaller grits will be used.
TomW-brought in another crisp, this time rubarb, apple, peach, strawberry…delicious again.
ChrisU-has been looking for mirror blanks prefinished on the internet; JerryW recommended Cloudynights.com for its classifieds. TomW showed him the tester and pin sticks describing the percent of light gathered at each pin distance across a mirror.
JerryW-has been working on his 10” Cassegrain, 14” Dahl-Kirkham, and 18” Newtonian at home…all have the grinding stage done. He is currently grinding the 6” spherical secondary for the Dahl-Kirkham.
See all of tonight's photos and videos of mirror here: http://www.flickr.com/photos/27241501@N03/sets/72157635163404065/
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