Against my usual practice, I took out my C5/PST based setup without going through my Borg 45ED II/Solarmax 40 setup. Maybe it's due to the fact that I've watched some images from a 100mm Lunt's?
Seeing is 2-4/10, transparency is 3/10. Cheapy-ERF/Celestron C5/PST/BF10, DMK 31AF03, Meade LXD55.
And then followed by shots taken with a Tele Vue 2x barlows:
I spent a lot of time tuning my PST etalon today, I even disassembled one of the afocal lens in the hope trying to see how I can fix the internal reflection issue. I'm a bit surprised to see that those afocal lenses are singlets... Anyway, they work... edge blacken, not bad. I removed that tuning ring so that I can have direct access to the rotator inside, that gives a wider range of adjustment. I found a spot in which the internal reflection is gone, and the amount of detail (bandwidth?) is still acceptable. Uneven illumination is still there, but it's not as serious as before.
The images above are taken before the tuning, and the images below are taken after.
1650 (GMT+8), after tuning the etalon, the field is uniform enough:-