I never made a flat mirror yet because I couldn't test it. If I can get a collimated light source going it should be easier. The flat in the pictures is not done polishing but with a window glass on it it seems 5 waves concave and convex near the edge. I never seen fringes in the water test two previous tries, probably the course threaded bolts and not getting it level. I focused my eyes on the bulb reflection instead of the water and the fringes were seen. Also I could see them by focusing on the water. It took about half an hour to see the fringes, I used the reflection of an LED merging together, the glass under water reflection's about 20 times dimmer than the water one. Not much fun and not a useful test either. Needs a better setup.