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.