Comment by joebig

I don't think optical flats operate based on moiré gratings. The former is an interference/diffraction effect while the latter, is you underpinned earlier, is a sort of spatial heterodyning effect. For instance, you can generate moiré rulings (which effectively an optical illusion) with two angularly misaligned combs, but the comb spacing is order of magnitude larger than optical wavelengths, leading one to conclude it's not a wave interference phenomenon.