questions for glass pushers

I'm working on my first mirror (a 10" f5.5ish), and I've been on the figuring stage for what seems like an eternity. I think I have developed a pretty good feel for how to use the strokes to target specific parts of the glass, but there are two techniques that have thus far eluded me.

1) I can't even count the number of times I have ended up with a mirror that is 1/4 wave overcorrected. If I had a reliable way to target the middle zones to de-correct the mirror I would have been done a long time ago. The problem is that if I do a spherizing stroke (1/3 diameter center-over-center) the spherizing action starts at the center of the mirror and spreads outward. This means that I have to go all the way back to a sphere and start over and I loose almost a quarter inch off my focal length after every iteration (I have lost almost three inches off my focal length since I started polishing, although most of that was from fixing two badly turned edges.) Is there a better way to target the middle zones? (I would like to stay with a full sized lap as I end up with astigmatism every time I use a sub-diameter lap.)

2) I don't know a reliable way to target the edge zones. Frequently I will get a mirror that is nicely figured out to the 85 zone, but the outer 1/2" is still spherical. Usually my outer zone correction ends up happening by accident. This would be tragic if this "accident" happened during polishing, but during a 5 minute figuring session the amount of glass removed from the edge is usually just about right. However, if I have two "accidents" then suddenly I am staring at the 1/4" wave overcorrected situation again. So I guess I guess this is a two part question. First, how do I intentionally remove glass from the 95 zone of the mirror in a controlled fashion, and secondly what can I do to ensure that this is the only time I remove glass from there? (since fixing the previously mentioned TDE's my edge has been very nice. The glass I've been accidently removing from the edge has been resembled nice smooth correction rather than a sharp TDE.)

Thanks in advance,
Jeremy

Hi, Jeremy.

Regarding 1), have you tried alternating MOT/TOT to minimize FL change? Should still bring you to a sphere. Also, someone suggested using rouge to slow down the polishing action while figuring. It also may result in a better polish (less micro-ripple). But it is messy and stains everything in which it comes in contact, so be careful with it.

Regarding 2), I usually end up with a too-short radius outer zone when figuring with a full-size lap. Is that what you mean when you say that you have a still-spherical outer zone? I haven't come up with a solution either. I'd be interested in hearing others' solution to that. I ended up parabolizing my 8-inch using a 4" lap, after getting a sphere with the full-sized lap.

FWIW, knowing the trouble I had with my "easy" to figure 8-inch f/8, I can only imagine trying to do a 10-inch f/5 with a full size lap. 8O

Hope this helps, if only a little.

Doug

Sometimes trying something entirely different works. Have you tried a star lap on the full sized tool? You don't need to mutilate the lap. You heat the pitch, for example with a blow drier, not much but enough for a star made of paper to imprint a little of its shape when pressed with the tool. If the back of the mirror is clear you can see the effect when you figure MOT. It disappears fairly quickly but you need to re-heat the lap and re-press well before going on to other figuring steps, to eliminate the bias from your pitch. When it works, it tends to be a little more predictable from center to edge. You can try changing the shape of the star, for example, the tapering of the tips at the edges, to vary the center vs the edge.
A reverse star lap can reduce the overcorrection without having to go back to a sphere.
To parabolize, since you are deepening the pitch with paper you actually want press a "mirror image" of the star on the lap by cutting a star out of a piece of 10" circular paper, putting the star itself aside, and centering what's left, the "reverse outer star" on the lap. This deepens the pitch more on the edge than in the center (loss of contact on the edge). If you want to bring an overcorrected figure back uphill to a sphere, do the opposite and press the star itself on the heated lap. This gives you better edge contact in a graduated way. You use a standard spherical polishing stroke MOT but the effect spread out more evenly between center and edge. I've moved a litte side to side with the stroke but don't know if that's better.
The concept is in Ingalls old Amateur Telescope Making book (updated version) chapter called "Working Uphill" page 91.
The "accidental/unpredicatable" frustrations I've experienced in the past improved by letting the mirror sit longer on the stand before retesting, and it usually makes an appearance when trying to work better than 1/4 wave. Try measuring twice with the sitting period you are using now, compared to a good 12 hours. They should be close.
Also, once you get down to 1/4 wave tolerances try shorter figuring sessions or use zirconium oxide instead of cerium. Once this precise it may be better to time the session based on number of strokes in each position and working only for a number of complete turns of the mirror on tool to avoid astigmatism. I've also overworked elevated zones, wondering why they won't disappear, only to find it took some more time (and ?cooling) for the real result to show up..usually overdone. Somewhere in there I had it right but just kept on going.