Thanks everyone, and I hope I haven't led you down a rabbit hole. I'm just relating what I have observed with my tilt-tuned etalons which have a CWL close to being on-band and needing very little tilt to get on-band. YMMV...
I usually do imaging. What I use to determine if I’m on band or not with my single pressure tuned etalon is to check the histogram level. The image gets dimmer and more contrasty when approaching on-band, so the histogram moves to the left. If I tune past the on-band position, the histogram will start to move to the right again. Thus, on-band is achieved with the lowest histogram level for a correctly exposed image.
I wonder, how the approach I described above correlates with your visual approach? Not disagreeing at all (I don’t consider myself an expert in any sense anyway), just would like to understand the whole picture.
Well, first, I'm no expert either. It's what I have observed and used numerous times, but it does seem a bit at odd's with your histogram method. But from your description, and my limited experiments with using the histogram for focusing, I might postulate the histogram going from bright to dark may be similar to focusing. It might be as shown below:
Here one can see the solar spectrum features (this is a conceptual diagram only BTW ;-) The H alpha photosphere absorption line contains the H alpha emission line of the chromosphere. The etalon produces narrow etalon peaks, all but one of which is blocked by the blocking filter. This etalon peak wavelength is tunable via tilting or changing the (gas/mechanical) pressure.
My guess is that when the pressure tuned etalon shows a bright image, the etalon peak is somewhere between A and B (or similarly the opposite side of the emission line). This is between the blocking filter passband limit and the H alpha absorption line (mostly if not all photosphere). As the etalon gets closer to being on band, the histogram becomes darker between B and C as you enter the absorbtion line. If you're averaging back and forth between position B-C on one side of the etalon peak and the same relative positions on the other side of the H alpha emission line, you should end up at position D being on-band. But again this is just theory until someone can verify this scenario.
The other possibility is that in starting out in the wings of the etalon, you have more parasitic continuum leaking through in the B to D region, and it isn't until you get on-band that the continuum is reduced or eliminated and only the chromosphere at D is seen, which is dimmer than the parasitic continuum leakage.
What I think is happening with my tilt-tuned etalon - which has a far-less ability to tune off-band - is that when I observe the essentially collimated light from the Sun via the etalon peak, I'm somewhere in the region of the filter transmission profile "wings," B-Cish to D, and that with the proper tilt, the image becomes brightest on-band at D.
That's my theory anyway. Hopefully some of you can verify what you find and how it fits with this postulation.
I suppose with a double stack, one could do each separately, then put them together.
Or would it be better to tune the first, and then tune the second with them stacked?
I think it's usually easier to tune the primary etalon first, then add the secondary DS etalon, and go from there with the tilt and/or pressure tuning. The DS etalon will usually require some tilting, which changes the tuning it would have had if tuned separately. OTOH, once you tilt the secondary etalon to remove ghost reflections, you can remove it and - being sure to not change the tilt setting - tune it separately, and then reinstall it. This should result in the optimum DS tuning.
Edited by BYoesle, 25 January 2021 - 11:41 AM.