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Richard Drumm The Astronomy Bum

Didymium filter fol-de-rol!

20 Oct 2009, 18:56 UTC
Didymium filter fol-de-rol!
(200 words excerpt, click title or image to see full post)

I've been thinking about taking a "portrait" of McCormick Observatory, with star trails behind the dome, but good ol' Charlottesville has been growing over the years and now fills the sky with a diffuse orange glow. So rather than slip over to Bear Mountain and get a REAL dark-sky starfield, I decided to try to knock down the skyglow in-camera as it were.Perusing the internet, I came upon the use of didymium glass filters to do this trick. Glassblowers have been using the glass in safety glasses for some time so that they can see the melting glass they're working on despite the sodium light of the flame they're working with. Photographers (as contrasted with astrophotographers) use didymium filters to boost the color saturation of the red end of the spectrum (as compared to the more yellowish middle) in landscape photography.I found several photographers who claimed the Hoya filter to be the best, so I ordered one for my Nikon:The filter is supposed to knock down the yellow of sodium outdoor lights in particular. After more internet skulduggery I found out more about skyglow & filter specifications Here is a compilation of a spectrum from sodium lights and the didymium ...

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