Because my new PST etalon adapter was modified to accept the PST-BF5 visual back, and the etalon adapter can be used without the PST etalon, I think I will try to use it for deep sky imaging.
H-alpha solar filters usually work like this, first of all, sunlight gets into the system through an Energry Rejection Filter (ERF), it's basically a boardband red color filter, it's used to filter the heat, and also to reduce the incoming light to red color only. Then, it will pass through the etalon, which will give a very narrow band of H-alpha signal, however, harmonics will be passing through the etalon as well, so to isolate the H-alpha line, we will use a dielectric H-alpha filter to cut out the harmonics.
I believe that the PST-BF5 is the dielectric H-alpha filter, thus, maybe it can be used for deep sky imaging. I guess it worths to try as it costs nothing but time.
Of course, the obvious downside is the very limited aperture which will cause serious vignetting.