Comment by dkirkman | Hacker News Reader

Comment by dkirkman | Hacker News Reader

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Street lights that emit narrow spectral lines are not a problem at all for spectroscopy, except of course at the line wavelengths. I have a bit of experience here -- a few more than 400 nights observing spectroscopically at Lick Observatory, where we have just a wee bit of light pollution!

All lights, narrow or broad band, present problems for optical broad band imaging.

If we could get all cities to go with low pressure sodium lamps (that only emit at a few wavelengths), astronomers would be very happy. The only problem is that city residents tend to not like the monochromatic look you get from not having a broadband light source. So San Diego, for example, switched to low pressure sodium in the 80's to try to protect the skies for Mt. Palomar. But residents started screaming (it's leading to more crime!) so the city went back to high pressure sodium.

Thanks for posting. I would have thought narrow lines would produce the problem that those specific lines would become unavailable for study, or would be freely mixed with the "real" data in confusing ways. It didn't occur to me that they could simply be subtracted in a deterministic way.

On that basis, I might have argued for high pressure sodium (to smear the lines) instead of low pressure, and I would have been quite wrong.

Given these issues, it's no wonder that a mountaintop in Hawaii, and another one in the Atacama desert, are now the preferred locations for optical astronomy.

> Thanks for posting. I would have thought narrow lines would produce the problem that those specific lines would become unavailable for study, or would be freely mixed with the "real" data in confusing ways. It didn't occur to me that they could simply be subtracted in a deterministic way.

Your original thought is basically correct: we do lose the ability to do any meaningful work at the wavelengths of the lamp emission lines. But the damage is concentrated at a few wavelengths. So we trade having noise over the entire spectrum for a few really, really noisy pieces of spectrum. It's generally a excellent tradeoff.

But even low pressure sodium lamps have some emission spread over a large wavelength range, so it is becoming increasingly difficult to do optical work near major urban areas. In the long run most of those sites will probably transition to mostly IR work.

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