It's interesting: The ERF on the scopes are going to be cutting energy below ~600nm, with the BG glass cutting it above this value, leaving a nice peak remaining in the red for the blocking filter element. BG glass does remain transparent above 1500-2000nm though, so, either this is being cut elsewhere or it is not necessary to cut... ???
The KG glass is a damn effective cut at wavelengths above 700nm up to several thousand nm, however it is a soda glass and this absorbs moisture, fogging in the process; also optically is isn't as 'good' as we might like. The BG glass is a phosphate glass and is the presence of humidity a calcium phosphate crystallization can occur on the surface (what has happened to the guys in the CN link) however this is 'better' optically than the KG, which is why I guess lunt are using it.
I do have a gut feeling Joe that these longer wavelengths manifest themselves as 'noise' in our cameras. I know the camera isn't sensitive at these wavelengths in terms of imaging ability, but I do think when a load of this longer wavelength energy is dumped on the chip it increases noise. Sadly with winter here this is a theory I cannot test till next year now (realistically!) - any clear skies now are brief and i'll be concentrating on just grabbing some shots rather than experimenting!
As a sideline I think the Schott BG36 would make a good candidate for a triple band (CaK, continuum, Ha) ERF with a suitable IR blocker...
http://www.schott.com/advanced_optics/e ... 08_eng.pdf
Unusually revealing for CN, I don't think they realise how much they're telling us there :whistle:
