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24601
2012-Nov-06, 05:43 PM
i've heard people mention sodium filters for reducing light polution, but when ever i try to google them later, i get water filters.

does anyone here use them?
what is a good source?
what fine print should i be looking for?

TIA

ngc3314
2012-Nov-06, 06:51 PM
Glassbiowers use a dysprosium/neodymium ("dydymium") filter to cut the glare of sodium in their torches. I think that's the same sodium line seen in many streetlights. This would make a big enough difference in light pollution to make up for the overall light loss only under conditions when sodium lamps are a big problem and nothing else is comparably bright. (San Jose's carpet of low-pressure sodium yellow comes to mind). My initial though would be to find a glassblower and try one out, even in a naked-eye view - the broad-band attenuation might be so large that it results in a net loss even with the darker sky.

ETA: Google shows that there are more tailored astronomical filters based on neodymium, so take all the above with a large sodium-inclusive grain.

24601
2012-Nov-06, 11:05 PM
all feedback is welcomed and appreciated.

especially if it gets me something other than water filters.

RickJ
2012-Nov-07, 03:15 AM
Standard LP filters like those from IDS and and others all block the sodium line as well as other light pollution lines.
http://www.sciencecenter.net/hutech/idas/lps.htm
http://www.sciencecenter.net/hutech/idas/filtplt.htm

They'd be far better than filters that just block Na.

From this and other posts of yours I gather you are just starting in astrophotography or you'd know about these filters and some of the other things you've asked about. I suggest you get a good basic book on the subject such as Covington's Astrophotography for the amateur or others of that type.
http://www.covingtoninnovations.com/astro/

A CD book on basic imaging with a DSLR is Lodriguss' beginner's guide
http://www.astropix.com/BGDA/BGDA.HTM

Rick

24601
2012-Nov-08, 01:22 AM
Thanks for the links.

yeah, guilty

I'm just getting back into Astronomy after taking 20 years off to raise kids.
the prices on a lot of hardware have come down to the point where i can start try learning things i couldn't even consider before.
I started at the book store(i'm a bookworm anyway), but most of what they had was worthless.

thanks again for the feedback.