Our opinion at Baader is that, for astronomical objects, a gradual increase and decrease in RGB filter curves is unnecessary and even undesirable. In contrast to terrestrial objects, astronomical objects have discrete emission lines.
The stars themselves obey the black body radiation law, and the colour they shine in is due solely to their temperature - resulting in a continuously smooth, broad spectrum. The nuances of colour seen with terrestrial objects do not occur with deep sky objects. Much more important for RGB imaging is transmission curves with extremely steep slopes for each colour channel - for maximum efficiency and maximum contrast between the individual emission lines.
The transmission of Baader RGB filters is extremely high for each of the three colour channels, while stray light and reflections are reduced to a level not previously seen. For the first time, by using new, more robust manufacturing processes, it is possible to overlap the edges of the transmission curves at the maximum slope of the critical B / G region. Thereby, the crucial Hbeta and OIII emission lines can be perfectly separated but with the OIII line present at maximum intensity in both the 'B' and 'G' colour channels. This leads to much improved colour weighting and, above all, this design enables this most important deep sky emission line energy to be much more intensively utilised than previously.
No significant emission lines are present in the area around 580nm - but this is exactly where the main parts of the emission spectra from mercury and sodium street lamps are to be found. This region is almost completely suppressed by the new Baader RGB filters, making them highly efficient for use under light-polluted skies. The filter blocking has been specifically tailored to the sensitivity limits of conventional CCD chips, thereby minimizing ghosting to an extent well beyond what was previously possible.
- the only parfocal filters with plane-optical polished substrates. Each filter is polished to a precision of 1/4 wavelength
- Baader LRGBC filters are separately hard-coated. The individual filters are not cut out of large plates
- cut out filters have micro-cracks at the edge of the coating layer which tend action to absorb moisture by capillary action
- all Baader filters are individually coated, hence the coating edges are sealed - meaning the penetration of moisture is impossible
- during long term aging tests, Baader filters (including B + W filters/Schneider Kreuznach filters) were boiled in water for an hour and remained completely intact, in marked contrast to cut out filters
- the hard scratch-resistant coating on Baader filters can be cleaned as often as required
- Optical properties:
- no reflections - as is the case with competing RGB filters
- the balanced RGB design allows imaging at the same exposure times - very important for automated serial exposures
- maximum colour contrast for each of the three RGB channels due to extremely steep slopes for all transmission curves
- OIII emission lines in the B and G channel are double weighted, with maximum transmission - for the highest deep sky quantum yields
- R filter delivers maximum transmission and contrast in H-alpha and SII, but blocks IR from 680 to 1200nm
Blocking between the G and R filters range reduces the light pollution arising from mercury and sodium vapour lamps at 580nm, darkens the sky background, improves colour balance and improves colour separation.