For the third time in the past several years, I'm attempting to refigure an 8" f/8 mirror. Every time I have tried I've run into the same problem. No matter what I do, no matter what stokes I use, I end up with a hill in the middle of the mirror. Sometimes broad, sometimes narrow, but always there, and always too large to ignore (I'll be using a 1" minor axis diagonal).
The mirror is completely polished out to the edge. I'm hand polishing on a channeled full sized lap (on Pyrex tool), mirror on top. Gugolz #73 pitch. Facets are 3/4", spaced 1/4". I use netting to microfacet the surface of the lap for better contact/conformity. Polishing with cerium oxide. The basement where I'm working is usually between 62 and 64 degrees F., with between 70 and 75% relative humidity.
I don't think that lack of contact is the problem. I always start the session by pressing. Earlier in the project I would leave the mirror on the lap overnight and wrap them with wet towels to prevent drying. Now I've been starting out by hot pressing, and commencing work before all the heat has gone out of the mirror/tool. In either case, I would polish for short periods, 5 minutes at most, then cold press for 5+ minutes before polishing some more, to maintain contact. There's no slipping and grabbing - the feel is good and even. No ripple nor dog biscuit either.
1/3 W or center over center strokes always leave the mirror severely oblate spheroid with turned up (yes, up) edge. Longer (1/2 dia. or longer) W or center over center strokes tend to flatten the edges, but I still end up with a broad hill about 4" in diameter. Really long parabolizing strokes with lots of overhang side to side tend to leave the center of the mirror too shallow, and with a smaller (maybe 1.5 - 2") hill in the middle.
Getting to a sphere has been all but impossible. Use of a lot of overhang, and doing most of the polishing in the overhang position reduces but does not eliminate the hill. To get it to go away completely I have to go so far over to the edge that I invariably dig a ragged hole.
Another curiosity is that no matter how much long strokes I use, I have never turned the edge. I always have a good diffraction ring all the way around. In fact, too much short strokes (1/3 - 1/2 dia) always leave the edge turned up.
It's almost as if the only part of the lap that is working is the extreme edge. I can control the width of the hill by varying the amount of overhang side to side that I use. Using more overhang will leave a narrower hill - less overhang leaves a broader hill.
I theorize that my pitch is too hard? That would explain why I have not turned the edge. Is there anything else that could explain why I'm getting the hill? In all the books I have (Texereau, Thompson, Howard, Ingalls, Berry), none of them explain what might create a central hill, only how to treat it (which never seems to work as described).
The mirror is completely polished out to the edge. I'm hand polishing on a channeled full sized lap (on Pyrex tool), mirror on top. Gugolz #73 pitch. Facets are 3/4", spaced 1/4". I use netting to microfacet the surface of the lap for better contact/conformity. Polishing with cerium oxide. The basement where I'm working is usually between 62 and 64 degrees F., with between 70 and 75% relative humidity.
I don't think that lack of contact is the problem. I always start the session by pressing. Earlier in the project I would leave the mirror on the lap overnight and wrap them with wet towels to prevent drying. Now I've been starting out by hot pressing, and commencing work before all the heat has gone out of the mirror/tool. In either case, I would polish for short periods, 5 minutes at most, then cold press for 5+ minutes before polishing some more, to maintain contact. There's no slipping and grabbing - the feel is good and even. No ripple nor dog biscuit either.
1/3 W or center over center strokes always leave the mirror severely oblate spheroid with turned up (yes, up) edge. Longer (1/2 dia. or longer) W or center over center strokes tend to flatten the edges, but I still end up with a broad hill about 4" in diameter. Really long parabolizing strokes with lots of overhang side to side tend to leave the center of the mirror too shallow, and with a smaller (maybe 1.5 - 2") hill in the middle.
Getting to a sphere has been all but impossible. Use of a lot of overhang, and doing most of the polishing in the overhang position reduces but does not eliminate the hill. To get it to go away completely I have to go so far over to the edge that I invariably dig a ragged hole.
Another curiosity is that no matter how much long strokes I use, I have never turned the edge. I always have a good diffraction ring all the way around. In fact, too much short strokes (1/3 - 1/2 dia) always leave the edge turned up.
It's almost as if the only part of the lap that is working is the extreme edge. I can control the width of the hill by varying the amount of overhang side to side that I use. Using more overhang will leave a narrower hill - less overhang leaves a broader hill.
I theorize that my pitch is too hard? That would explain why I have not turned the edge. Is there anything else that could explain why I'm getting the hill? In all the books I have (Texereau, Thompson, Howard, Ingalls, Berry), none of them explain what might create a central hill, only how to treat it (which never seems to work as described).
