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All that is needed is a thin coating from a Day Glow yellow green marker on a clear filter that one screws into the eyepiece.

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I've tried [and posted results here] and it's not very effective for various reasons. The fluorescent 'filter' has to be precisely located at the eyepiece image plane not just anywhere within or on a lens element.

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Here is another thought,that I wonder if Coronada has looked into. They seem to be using a achromat as the objective. I know that is the case in the Ha PST version. The lens is cemented together with a UV curable adhesive. May times these materials also absorb strongly in the near UV even after curing. This one reason I use quartz optics in my research. Just wondering if the adhesive in the lens and also the coatings on "standard" eyepieces isn't absorbing at 395nm and the dimming the image farther ?

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I think this is highly unlikely! From external inspection I've made deductions about the OG and CaK filter construction . The supplied 20mm Kelner probably has uncemented elements too - all to maximise near UV CaK transmission. There are non invasive ways of testing UV transmission through an optical system.

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Nytecam 51N 0.1W
Meade 30cm LX200+ETX-70+e-finder+C8+Ha+CaK PSTs SBIG SGS+homebuilt spectrographs
Starlight SXVF_M9+Lodestar CCDs/Canon 300D DSLR/Fuji E550
My observatory build-ETX-70 imaging-my videos

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