When using an eyepiece, you're looking to get the darkest filaments and the brightest plage in active regions:
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The technique described in this thread is to take the eyepiece out (leaving the blocking filter in place) and observe for the brightest and most uniform appearance of the etalon(s) - not the "sun's disc." This is what I do with my double stacked tilt-tuned Coronado filters - and yes they therefore show the central obstruction that you wouldn't see with the unobstructed Lunt etalons. It's my estimation that you're basically illuminating the etalon(s) with collimated H alpha light, and observing when you are centered on the H alpha line, and how uniform the etalon gap is as well. YMMV.
I was paying close attention today and note that some features on the sun are more visible in certain parts of the eyepiece's fov. Is this indicative of being off band, or just a reality of the way the etalons work?
I'm assuming you have the double stacked internal etalons, and yes it sounds like the "off-band" area is what you're likely seeing in known as the Jacquinot ("sweet") spot, which is fairly common with collimator-based internal etalon telescopes, and made worse with double stacking internal etalons. See here and here.
It also can indicate your etalons might be not thermally stabilized, or have another issue altogether that's where this etalon uniformity test comes in. Narrowband solar filter systems are a world unto themselves, you'll need to get experience in solar observing to know what is good and not so good.
... does it make sense to completely depressurize my 2nd etalon, use your technique to get on band with the first one, then try a second iteration of your technique to get on band with both etalons pressurized? Or is it necessary to take the scope apart, remove the 2nd etalon altogether, then tune the first one? Or, should I just attempt to create the most uniform image with both etalons in place?
With my tilt-tuned front (external) DS I leave both in place and do the primary etalon first, then adjust the second.
With your pressure tuned internal DS, I would as a beginner opt for removing the secondary DS etalon and try the single stack primary etalon first, then add the DS etalon and see how that behaves. After a while leaving both in place and tuning will become easier if not intuitive. At some point you may even want to opt for ditching the internal DS etalon and go with a front mounted 100 mm etalon for double stacking, which will give you better contrast uniformity with the internal primary etalon.
Let us know how it works out.
Addendum: I personally highly value contrast uniformity extended enough to include at least the entire solar disc - at least for a 100 mm and smaller aperture telescope. When moving the Sun's disc off-center you might see some detail change off-band, but there should be a region fairly close to the center where the entire disc of the Sun is on-band. If this is not the case, you might want to give Lunt a call.
Edited by BYoesle, 14 March 2021 - 08:59 PM.