Hi all,
I am just going back to work on my 25"f4.7 mirror, after a long lay off. I now have two "pine butt" benches,(side by side), for working the 22" and 25" mirrors. Will be doing a few minutes figuring on each, day by day. Currently using a 17" lap (TOT) on the 22", usually 10 or 15 mins a session (one or two sessions a day). The present aim for this optic, is to take the correction from the inner 50% of radius out to the circumference, and deal with a TUE (last 22" mirror had TDE). Also smooth out some of the roughness left from using 5" to 11" laps which were used to deepen center/remove central hill.
As I said before I have just pulled the 25" from the shed (yesterday). Had to set about, and conform my 14" lap to the mirror, using "hot presses". To hot press, I place the lap face down in a tub of hot water, (at water cylinder temperature (65-70degC), for about 90-120sec. At this time I also use warm (45-50degC) water to warm the mirror itself, (sloshing water onto the mirror, as it rests on the pine log bench). Warmed lap then pressed onto warmed mirror, (with a micro cloth in between-no Cerox), pressure applied, 12.5kg, and some considerable body weight for about 75-90sec. This procedure I had to repeat four times, with three channel recuts, (used an electric soldering bolt for the cuts), before getting the 100% contact needed between mirror and lap. Two hours later in the cool of the evening, (7degC) was able to put in two 20min sessions of figuring. In this case I need to fix an oblate condition, so used a 'w' parabolizing stroke with a bias to the left side of center to avoid "zones" forming.
Stephen.
