A Simple Newton Interferometer
To test flats we compare them to reference flats using interferometric fringes [see link of flats and secondary mirrors]. Here is how I made ours.
Between the glasses are three triangular shaped pieces of cellophane, which act as spacers. You can slide them outward to adjust the number of fringes. The reference flat ought to be suspended on three pieces of riser material that are directly below the cellophane spacers. This box is open in the front to allow the viewer to see in.
Here are two shots of the same secondary mirror about to be cut out from a quartz flat. At this point in the process the flat is about 1/9 wave P-V. The odd markings are remants from the Hg bulb and scratches on the reverse side of the blank. The optic surface is quite clean.