[Stefan Buda]

Originally Posted by Imme

Looks great....what kind of time are you talking to go 1/4 to 1/10?

The closer one gets to the desired precision, say 1/10 lambda, the shorter the figuring spells become between testing. The problem is that after a short figuring run, that can get down to a few seconds, one has to wash and dry the optic before placing it on the test plate and then wait at least 15 minutes for the temperatures to equalize for a reliable reading.
After that the optic goes back onto the polishing lap where one also has to wait for the temperatures to equalize. So one hour goes by for a couple of short figuring spells. And many are required unless you have some supernatural ability to predict the behaviour of the polishing lap.

Originally Posted by Atmos

Excellent work Stefan, great to see this project slowly coming together.

Thanks Colin.

Originally Posted by Lognic04

Wow, you seem to be very fast at this process?!

I hope to become faster with more practice.

Originally Posted by multiweb

Hi Stefan, what are we looking at? Parallel lines? Can you make the difference between 1/4 and 1/10 just looking at the patterns?

Yes the spacing between the interference fringes represent 1 lambda on the wave front regardless of the relative tilt between the two surfaces. In other words they show the size of the air gap between the test plate and the optic. If the two surfaces have the same shape, the fringes are straight.
It is very easy to see a deviation of 1/4 spacing, especially if one spread out the fringes. I kept the fringes rather tight for the pictures as it was easier to photograph them.