That's a pretty disappointing full-disc image. Even if you were able to use flats or other tricks, there would be significant bandpass variations of the underlying detail.
A couple of things to consider. When you double stack, the reduced FWHM of the filter system decreases the size of the Jacquinot (aka "sweet") spot. For a front mounted air-spaced etalon, the Jacquinot spot goes from about 1 degree for a single 0.7 A etalon down to about 0.84 degrees when double stacked. See here. Both will easily fit the entire solar disc and then some in a field of relatively uniform spectral response and therefore contrast uniformity.
However, when one places the etalon in a collimator optical system, the magnified field angles reduce the size of Jacquinot spot via the amount of magnification. A collimator lens with half the focal length of the objective will double the magnification, and shrink the spot by a factor of 2 - or 0.5 degree which will barely fit a solar diameter for a 0.7 A etalon. Double stacked this will be 0.42 degrees, and thus a full disc with good contrast uniformity is not possible. Due to optical geometry for the etalon size, the etalon would have to be no less than 50% the diameter of the objective to maintain good full disc contrast performance for a single stack, and would need to be even larger with a longer collimator FL for the narrower FWHM of the double stacked system. To add to the complexity, in your system you are dealing with two sets of collimator and refocusing lenses, which may exacerbate the reduction of the Jacquinot spot even more than a narrower FWHM system would otherwise do by itself. It may also be that in the new Lunt modular telescope designs the collimator(s)/etalon(s) have been configured differently than in the previous dedicated telescopes, as I don't recall the Jacuinot spot being as obvious in the LS100THa images I have seen.
Additionally, from your image the Jacquinot spot and optical axis lies off of the mechanical axis of the system. When you state your using "straight through" it is not clear if you're referring to the blocking filter. If not, most of the off-center issue is likely due to the diagonal blocking filter (and sometimes to a lesser extent the focuser alignment), and a straight through BF is therefore recommended. It might also be the case that the second collimator/etalon system involves some tilt to remove ghost reflections, which would also result in an optical axis offset. Also apparent is that the center of the Jacquinot spot is slightly off-band, and produces a "bulls eye" ("sweet ring") pattern as seen in Christian Viladrich's very nice work up on Jacquinot spot issues found here.
Solutions: Refine your etalon tuning, probably doing one etalon at a time as possible.* As noted I would first check your optical alignment of both the objective and the focuser as best as possible. Be sure your etalon modules are exactly perpendicular to the optical and mechanical axis. Replace the BF diagonal with a straight through if not already accomplished. Ditch the secondary DS module and double stack with a front mounted etalon. Lastly ditch the whole internal etalon system and go with a front DS system.
Use your LS100MT system only for close-up work, and get a good 60 mm front DS system for full-disc viewing/imaging.
* A Hydrogen spectrum tube would be ideal for this purpose. Then refine the tuning at the scope as needed.
Edited by BYoesle, 26 November 2021 - 01:50 PM.