T&J,
The light from the PST etalon/ BF is tuned to Ha there are no other wavelengths getting through so any additional filters would do nothing.
I haven't tried it but maybe a transfer lens ie very low power eyepiece projection where the camera body is positioned as close as possible behind the eyepiece would allow you to focus without excessive magnification. The PST works at f10 and the optimum f ratio is around f30 so a maximum of a x3 Barlow would be the limit.
I use a DMK21 with very good results......
Ken
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