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I did a little experiment today with the GG-420 glass. I passed a beam of sunlight through a narrow band interference filter at 404.7nm, which was tilted slightly to decrease the wavelength a bit further toward the 393nm Ca-K line. The room was as dark as I could make it, which wasn't very dark.

I then held a piece of the GG-420 glass behind the tilted filter, and could easily see the fluorescence. When I passed my finger through the beam in front of the filter, I could easily see the shadow image on the glass when looking at it from off to the side. The glow of the shadow image was yellow-green, and I know it wasn't scatter from surface contamination because the glass was very clean on both sides.

I don't know the intensity of the prime focus image in a Ca-K filtered telescope. There may be enough fluorescence to make a difference with this glass, and there certainly may be other glasses that will fluoresce more efficiently at 393nm. Perhaps GG-495?

I think this glass is yellow because it absorbs the UV and fluoresces in that color. My use of it was initially as a UV absorber, and the fluorescence turned out to be great for use as a transmissive focusing screen for little characters, about 80 microns wide, which I view through a microscope.

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Thanks Bob - that's most interesting and constructive to this discussion. It seems we're the only the two doing practical experiments As you say perhaps other 'filters' may work better [for a brighter image] and of course it remains subjective without complex calibration what 'levels of brightness' we're discussing. Please keep up your excellent work.

Last night tried fluorescent yellow, pink, green, orange felt-tipped pens on photo-emulsion and clear cellotape. Yellow on self-adhesive cellotape best [brightest fluorescence] and sticking up to three layers together even brighter but clarity degraded.

There is a violet emission line in regular fluorescent tubes and holding the 'filter' close to the tube show fluorescent especially brilliant at the edge - a sure sign the devise is working.

These 'filters' supported on a small glass yellow photo-filter on the field-stop of 25mm Kelner eyepiece with 'filter' on scope's UV light side. If reversed the yellow glass completely blocks the UV [CaK] light!

Under test this sunny morn in the PST CaK scope, the best 'filter' combos show a good solar image via fluorescence maybe x10 brighter with three lighter active plage areas seen for first time - later confirmed via digicam posted separately.

Without my fluorescent 'filter' eyepiece the ultra dim native CaK view has improved via a black head cloth but still no disk detail visible even with prolonged observation - Ralph's Robin-egg blue solar disk must be wonderful but prefer the fluorescent route to surgery

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