Thanks George, I will “re-correct” the text back to the original. The reason I thought it might have been a typo is that I recalled - and then confirmed - that the SM series of Tucson etalons made in the early 2000's, and the Isle of Man etalons currently made by Solarscope, have a specification for thermal stability stated to be < 1 Angstrom / 200 degrees C.
So good find - and great info!
Quote: Coronado’s first solar filters basically took a prominence filter, which was an ERF plus a PROM15T (prominence, 15mm, maybe T for tilting?) and inserted an etalon behind the ERF. The prominence filter by itself showed no solar disk details... Over time I guess they switched to a blocking filter designed for the etalon. I don’t know, but I suspect that the new blocking filters were wider: still narrow enough to avoid other etalon orders but wide enough to avoid having to tune it with temperature variation (and perhaps cheaper to produce).
Yes, the Coronado blog confirms the PROM15T used a 2A bandpass prominence filter as described, while the later blocking filters used a 6A bandpass blocker, which apparently is well within the range needed for an etalon with a FSR (i.e. free spectral range - the distance between the side-band harmonic peaks) of 10A.