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Tuning a double stack is a two (or three - see later) process.
To achieve maximum transmission at the narrower DS bandwidth (0.5A ) firstly remove the front etalon....
Tune the other to achieve maximum contrast and visible surface detail. Once this is achieved do'nt touch it again...it is on band.
Add the second etalon and adjust it to give the brightest image - at this point it will also be on-band.
If all goes well, there surface features, active areas, filaments and spicule forest should be visible across the FOV.
There's one more thing that can be done to improve the uniformity of the FOV and that is rotating the front etalon (without changing the tuning) relative to the other etalon. Use a small piece of masking tape at the edge to monitor the rotation, a quarter of a turn should improve things, but trials will quickly confirm any improvement.
You can also check that the blocking filter diagonal is not contributing to the problem. It can also be rotated in angular position relative to the OTA to see if there is any change in the FOV.
Hope this helps.
Ken
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