Stable instrument sensitivity can be checked by comparing with stellar background at the same regions in the sky. This is fundamentally a test for the absence of long-term drifts in photomultiplier sensitivity, but also in filter transmission and bandwidth.
Independent confirmation of spectral stability (especially, filter bandwidth) can be obtained from Neon spectra that are occasionaly recorded during maintenance trips to the observing site.
Because of the high mechanical resolution of the wavelength scan (the spectral range of our instrument is covered by about 5000 motor steps, and the mechanical backlash of the drive mechanism is only 0.11 +- 0.14 steps, according to a recent empirical determination), drift diagnostics is possible well before substantial drift errors capable of affecting scientific data can occur. Wavelength stability information is monitored continuously so that long-term stability of the instrument can be assured.
Based on this experience, we call the tilting filter technique "intrinsically stable". This good performance record can be explained by...
* Absence of Humidity: the interference filter is protected in the hermetically sealed filter chamber, in dry atmosphere (in spite of external ambient humidity operating range 0 - 100 %, including condensing!).
* Use of Aged Filters: filters have not been used until several years after purchase, when early drift (if at all present) has died down.
* Thermal Cycling: filter temperature is not stabilized, so that it follows the diurnal cycle. This continuous "annealing" (with a diurnal range often exceeding 30deg C and annual extremes of -18deg C to +40deg C) is certainly beneficial for filter stability.
The "notoriously unstable" behaviour of interference filters experienced by other investigators is therefore avoided. And, last not least, ...
* Single filter approach: Since the temperature retrieval depends on the characteristics of a single filter, we avoid problems that might arise from different changes of different filters used to sample the same airglow band, when even small and difficult-to-detect changes of only one filter might have strong impact on rotational temperature.