Certainly not behind revolving optical instrument (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Certainly not behind revolving optical instrument NO (=certainly not) + LATE (=behind (schedule)); “revolving” indicates reversal; in physics, an etalon is a device consisting of two reflecting glass plates, employed for measuring small differences in the wavelength of light using the interference it produces