thanks to the Covid boredom, I have removed the filters from the Carl Zeiss Jena FLUO -RF-RED-0 filter wheel and checked them out with the spectroscope:
O is just an AR-coated blanc glass.
The RED is a long pass (630nm) filter.
Unlike the astronomy red filters, this Zeiss is free from any fluorescence due to the UV and blue-violet absorption of light by the color centers in filter glass, an annoying fluorescent contrast loss when viewing the Moon and the bright planets.
So, this clean will be one of my Mars filters for 2020.
The third filter RF is a blue-green-yellow pass with a long wavelength cut at 630nm. It may become handy or not, who knows.
The reason, why I have spent 25 EUR on this filter wheel is the blue interference filter
And now, what is so special about the blue
This remarkable filter cuts away the OIII doublet and any light with longer wavelength, and passes the short wavelengths from H-Beta through the blue Balmer Series down to the UV around 380nm.
And this is pretty exactly what’s needed when the OIII emission in the ionized hydrogen nebulae is to be suppressed, and when the other Balmer emission lines are needed, as the H-Beta might not be enough for visual. Other applications are the blue OB stars associations, and the OB-dominated spiral arms in the galaxies.
The design of the blue Zeiss filter is shown in the pic,
JG What makes a difference to the other, and especially to the astronomy interference filters, is the design of a sandwich with three glass plates.
The front mirrors show at the normal incidence of white light a yellowish color reflection, similarly to the Baader blue(RGB)
The thin ring of the multilayer stack has been left at the edge of the mirror for the QC purpose.
The sophisticated design of this filter is expected to suppress any leakage of light with the unwanted long wavelength well below 0.1%
Even if this filter has been manufactured before 1990, there are no traces of disintegration of the layers visible.
The quality control notes are written with a pencil on the filter rim.
Thank you for reading and looking,