Some more info to help out:
Cleanliness is ULTRA important in taking flatness measurements with an optic flat. One piece of dust from the air will interfere with the air gap and therefore the interference fringes.
Same with any surface roughness on the optic flat or the surface (hence why you need a polished surface.)
If the inteference fringes (rings) are close together (like in most of your photos), this is 'usually' an indication that there is some dust inbetween the surfaces, OR the surface is not very flat to begin with. The farther apart the rings, the more of the surface inbetween the rings is the same level (flatter.)
True, the straightness (or curvature) of the lines is indicative of the flatness, BUT you can get false readings from one piece of dust.
I go through a lot of cans of air at work checking flatness (monochromatic light source, optics flats.) You can also get optic flats that are flat to different degrees. The ones I use at work are either 1/20 wavelength or 1/10 wavelength.
Pefectly straight lines mean perfectly flat. The more curved, the more out of flat the measured surface is.
Bill, I have a really informative handout that I can scan or send or fax to you on measuring flatness using optic flats, maybe by tomorrow night (I have it at work.) I will probably scan it for best quality and email it to you.
Let me find some pix and post them here in a little bit.
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