The easiest way to grind with different diameter discs is to have the small one at least slightly larger than half the diameter of the primary. That way, the strokes will overlap somewhere on the large disc. I used an oval stroke to increase the depth on the primary, and a round overhanging stroke to lessen it. (secondary on top) You have to get a feel for doing it, and a sharpie test is a big help in seeing where the progress is going. There will also be much more wear on the secondary disc. Be careful about the long radii, and confusing the back side of the secondary with the desired face. I've been in the middle of that, and have inadvertently "swapped sides" more than once. Write "back" on the reverse side to prevent that.
Reason for the oval stroke deepening the curve is that it misses the edges in part of the stroke, resulting in it raising this area.
Also, contrary to belief, in working unequal discs, I generally rotate the disc slightly in the SAME direction I walk around the stand. This prevents repetition when you do the opposite. I have often wondered why so many sources recommend the opposite direction rotation. I find it's easier to get smoother spherical surfaces this way.
You can be assured of fairly accurate spheres when the radii of the 2 discs do not change much during fine grinding. This is accomplished as you get closer to the required radius, and you find a "neutral" stroke condition to accomplish that. If you play around long enough, you will find that you can tweak the curve with 2-3 micron abrasive.
Mark Harry writes:
When trying to maintain the curve, or lengthen it using the wide circular stroke, you must also occasionally go over the center to maintain roundness, or it will resemble a long stroked curve generated shape.(not really spherical)
When generating through the series of grits, generally you swap off early, before reaching the target curve. You have to develop an idea on how far you can go before swapping to a finer grit. This is arrived at by measuring what each wet can do to the curve, but you must also make sure the edge of the primary is thoroughly ground. The secondary is actually thoroughly ground in a matter of seconds, literally. Don't forget to bevel adequately!
Mark Harry writes concerning the turned down edge I started to create:
If you want to get rid of the edge problem, use a polisher about 7/8 size of the secondary mirror, much easier to make, and thin the compound to almost all water. Make sure just a trace of compound is there, and use random oval/w strokes, really mix it up. It will be suprising how far you can stroke it and have no edge problems with the right temperature. In fact, with polisher on top, it will accentuate work at the edge zone and get rid of the hook without hardly changing the radius.
Mark Harry writes concerning the pitch lap:
Polisher grooves in the sub dia. polisher should be kept to nothing but deep scratches. The more solid the polisher's surface is, the less change will be made, and the ROC can be held very accurately. If they fill in, just scratch more in. Also, place a dimple right in the center of the polisher with the corner of a razor blade, or the pitch won't have anywhere to go, and will punch a hole in the center of the secondary. has to be there for small lens work, especially shallow radius.
One word of warning, it is probably ok to cold press over-night. Once I cold pressed one of my mirrors for 2 to 3 weeks and noticed that a foggy pitch lap pattern appeared on the surface of the glass! There is some glass etching going on if left for extreemly long periods.