Here is the bad data I was talking about. Straight out of stacking, no other processing. You can see a bright but non symmetric ring around the limb... My fear is I won't be able to get it on band photographically but I could be wrong... I just hope the scope will be able to get on band.
Unfortunately from what I can see in you image, things currently don't look too great. What I have encountered with a small sample of MEADE Coronado etalons is that they are tuned too high above 656.28 nm. This is likely due to the spacers being too thick and the resulting large etalon gap. While being tuned a bit high is ideal for the secondary (DS) etalon so that tilting it removes both the ghost images and brings the filter on-band, MEADE apparently relies on being able to both tilt and compress the etalon gap to correct this - and apparently believes this gives them leeway in not meeting stricter etalon spacer production tolerances.
Ideally the CWL of the etalon should be just a bit high so that minimal tilting will bring the etalon on-band. This is a critical feature for tilt-tuned etalons and requires a good deal of time and attention. If the spacers are too thick, the etalon CWL will be too high, and excessive tilt will be needed to get on band (if it can get on-band). Therefore "banding" likely will be present as sections of the etalon will be off-band with respect to the tilt axis portions in which the acceptance angle of the filter is exceeded. This appears to be what is going on the left versus right side of your image.
The same thing applies to RichView mechanical pressure tuning. Again ideally the filter CWL is just slightly high, and a small amount of pressure would decrease the etalon gap and bring the filter on-band. Additionally, this pressure would need to be very precisely and evenly applied to both the center and peripheral spacers to maintain a parallel etalon gap. From the two RV etalons I have examined, only the center spacer has pressure applied, and because the etalon is tuned high, a greater amount of pressure is needed to get on-band and significantly deforms the etalon plate. This results in the unevenness of the etalon gap, and therefore differing CWL's across the etalon, and widening of bandpass when the center of the filter is tuned to be on-band:
Left: excess tilt tuning. Right: excess tilt and RichView tuning. Greatly exaggerated for clarity.
The combination of these two issues results in a filter that presents with banding and a broadening of the bandpass, leading to contrast variations across the image and generally having poor overall contrast. See here for additional analysis.
Getting back to "square one:" You should evaluate each etalon individually to see which one requires the least amount of tilt to be on-band. Hopefully the tilt required for this etalon will produce little if any banding. I would hope little if any RichView compression tuning will be needed. This etalon should be designated as the primary etalon. The remaining etalon will be your DS etalon, and should only be tilted enough to remove ghost reflections off the primary image. Thereafter, use the RichView tuning to get the best contrast. I believe in this way you'll have the best compromise with minimal banding and bandpass widening. However, flats will likely be required to achieve the best imaging results as far as contrast variation.
Edited by BYoesle, 20 January 2021 - 07:09 PM.