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Mellow Yellow Musings


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Presently I don’t normally use coloured filters for lunar observing, although in the past I have done, as well as regularly using a polarising filter of some sort. Recently I had a dedicated lunar session with my 127mm SkyMax MCT. As it was still daylight I decided to use a yellow filter as this tends to turn the background sky black. I was oddly surprised with just how good it was for revealing lunar detail.




A yellow filter absorbs all coloured light except yellow. Yellow is the colour wavelength between 575-585 nm. Green is the colour between cyan and yellow in the spectrum. It has a dominant wavelength of roughly between 495–570 nm.




A yellow filter also absorbs blue light. It provides significantly greater contrast between blue and yellow or white subjects. Which is possibly why yellow filters work well for lunar observing. The Moon is almost monochromatically grey-white when viewed through a telescope. The Baader ‘Yellow Longpass’ filter is 495 nm. A visual longpass filter is a coloured glass filter that attenuates shorter wavelengths and transmits longer wavelengths over the active range of the visible target spectrum.




The Baader colour filters have their origins in an older Zeiss series of colours more specifically aimed at astronomical use than the more commonly used Wratten filters. 




Furthermore, Baader have a specialist range of anti-cyan filters approximating to #8, #12 and #15 Wratten filters. These are: the Fringe Killer, Semi-APO and Contrast Booster. I'm not totally certain if these were originally intended for use with achromatic refractors of varying apertures. The intriguing aspect of the Baader yellow longpass filter is that the 495 nanometre line is virtually green. Albeit the extreme end of the green spectrum that is basically bordering yellow. 




Which makes me wonder if this is the reason why I prefer it so much to the other Wratten yellow filters. I’ve used a Wraten #11 yellow-green for many years specifically for Mars and Saturn. I normally have no difficulty seeing the Cassini Division without filters, but both the Baader 495 nm and the Wratten #11 excel in revealing it much more blatantly. In my experience both filters very effectively aid viewing surface detail on Mars. I also like a Wratten #11 for the Moon in preference to a more conventional shade of yellow. I may now start using yellowish green filters a lot more for lunar observing.



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