Today I performed a little experiment. I observed the sun at the same altitude and barometric pressure, but at the different temperatures. I observed the sun from outside at my backyard at 767nm Hg and -5C and from inside (+25C) through a window with good flat glasses surfaces. My estimation of the air refractive index change is from 1.000263 (inside) to 1,000275 (outside).Bob Yoesle wrote:
So for etalon tuning, item one (1) is air pressure tuning as implemented by Lunt, which effects a change in refractive index n. Item two (2) is where “rich-view” mechanical pressure tuning is used by Coronado to change the actual gap thickness t. For both filter makes, tilt tuning is utilized to change the incidence angle θ for front mounted etalons. Rear mounted etalons use temperature, which affects both the gap thickness t, and the refractive index n.
The CWL shifted red when inside (lower refractive index).
So, we can conclude that with the refractive index goes smaller, the etalon's CWL shifts red.
The only factor of uncertainty is the etalon's spacers thickness change. With it's increasing the CWL should shift red-ward. So, I am not sure what is the reason of this CWL shift - the air refractive index change or the etalon spacers thickness change.
As some peoples realized, the Rich View tuner does not change the pressure on the etalon plates and does not change the etalon gap. It just changes the range where one can tilt the etalon. The same is with PST etalons.