• A word to the wise: SAFETY PRECAUTIONS
• How a Ribbed Mirror Deforms: Quilt Pattern
• How a Ribbed Mirror Deforms: Ridge Pattern
• Correcting Quilt Pattern
• Correcting Ridge Pattern
• Turned Edge in Solid Mirrors (a heretical proposal)

A word to the wise: SAFETY PRECAUTIONS

Thermal Shock. You probbly won't be injured, poisoned, or killed, but Ribbed mirrors made of plate glass are sensitive to thermal shock. There is almost no thermal path between the front and back plates, so there is no way for the temperature to equalize and no way for the thermal stress to be relieved. Therefore, take extreme precautions to maintain all parts of the structure at the same temperature-- especially when pressing the optical surface onto a warm pitch lap.

One way to avoid thermal shock when building the pitch lap is to use a separate glass blank (a simple disk of 1/4" plate glass is OK) to form the pitch during the pouring and channelizing process. If you grind this glass on the same tool as the mirror blank, it will have a curve that is close enough to that of the mirror to allow final adjustment by cold pressing alone. I normally ship a slumped auxilliary disk with ribbed mirror blanks so that fine grinding is all that is required to make this accessory.

You can avoid thermal shock to the ribbed mirror when warm-pressing the lap by doing it under water. Submerge the pitch lap and the mirror separated in water in the sink at room temperature. Then slowly raise the temperature of this bath. Pressing should be done under water. Allow the water to cool to room temperature before removing the glass and the lap. (Yes, I had a mirror crack when it cooled while resting on a warm pitch lap. The BACK of the mirror cooled, but not the front!) This procedure is a nuisance, but will avoid cracking the ribbed mirror with thermal shock. Normally, cold pressing is sufficient for maintaining intimate contact between mirror and pitch lap.

Reducing stress due to thermal shock is a subject of ongoing investigation at Star Fleet Engineering.

There are two kinds of print through that occur when polishing ribbed mirrors. Both are easily corrected.

Ronchi test setup for viewing print through

• How Quilt Deformation Happens

  Quilt deformation is caused by pressure on the surface being polished. This pressure is sometimes enough to depress the surface between the ribs. The glass over the ribs is then polished down to the level of the depressed area. When the pressure is relieved, the area between the ribs springs back, but the area over the ribs does not, leaving grooves over the ribs. This results in a quilt pattern with grooves over the ribs.

• How Ridge Deformation Happens

  Ridge deformation is the opposite of quilt. If you could reverse the pressure load, and apply suction all over the mirror while polishing, you could raise the area between the ribs, polish that area flat, and when the suction would be released, you would have raised ridges over the ribs. Another possibility would be to apply pressure from below the mirror in a fixture similar to that used in producing a Schmidt corrector plate. At the 2001 Optics Workshop, Dean Kettleson disclosed this is exactly what is done during the polishing of the largest mirrors (8 meter diameter). At the same conference, Alan Adler discussed the use of puller pads of foam rubber that are bonded to the back of the mirror for warping a sphere into a parabola. Such complication is not needed; there is another way...

  If you apply heat to the surface being polished, that surface will rise as the glass expands. (this will not happen with zerodur) However, the ribs act like heat sinks, and the glass will rise LESS over the ribs. The source of heat is polishing friction. Once the heat causes the area between the ribs to rise a little, there will be even greater friction and greater heat over the raised area, causing it to raise even further. This thermal deformation gets flattened by the polishing action, and when released by the cooling glass leaves ridges over the ribs.

• Correcting Quilt

  Correction of quilt print though is done by a polishing stroke that generates a lot of heat, but with very little pressure. That is done with light pressure and a very rapid polishing stroke. Some say that such a stroke creates "lemon peel", however I have not had that problem. In any case, lemon peel is very shallow and is quickly and easily removed with ordinary polishing strokes. Another possibility that I have not yet tried is to heat the front surface of the glass with hot air. That would probably be best for large optics where generation of heat by friction is not practical.

• Correcting Ridge

  Correction of ridge print through is done by a polishing stroke that generates very little heat, but applies heavy pressure to the glass. This is done by bearing down with plenty of force, while moving the pitch lap very slowly.

  Equal heat flow thermal model

  Thermal effects may be helpful in correcting turned down edge. If heat is made to flow uniformly into the mirror surface, and escape from the back, there will be some additional heat loss at the edge of the mirror. That means that the edge will cooler and will be turned down during polishing. Since the edge is turned down, the polishing tool is not working it, and the REST of the mirror will be taken down. After polishing, the mirror comes to thermal equilibrium, and the warm central area will shrink more than the edge. Hopefully, you can find a happy medium between these two extreme polishing strokes that leaves your glass free of all print through. Thermal effects in glass can be investgated with FlexPDE software; a 300 node version adequate for simple heat flow modeling can be freely downloaded from the internet.

• Correcting Turned Down Edge in Mirrors?

  The above suggests the possibility that turned edges in solid mirrors may be swiftly corrected by warming the mirror and pitch lap in warm water, placing the mirror on the pitch lap, and allowing only the edge of the mirror to cool before polishing. (Do not do this with a ribbed mirror, or the thermal stress may crack it!) Alternately, a rapid and vigorous polishing stroke might provide the heat to expand all but the edge of the glass. Some say that this causes a "lemon skin" or "potatoe peel" texture, but such a defect should be easily corrected by normal polishing strokes after the turned down edge is fixed. Feedback from anybody who tries this would be appreciated.

Home