Short strokes will spherize a mirror fairly quickly. I tend to make the guys shorten their stroke with each grit of the sequence and that keeps lets the spherical shape become more easy to attain. A stroke length of about 1/4 to 1/3 of the mirror diameter should do a good job. Bob May http://nav.to/bobmay bobmay@nethere.com NEW! http://bobmay.astronomy.net