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Old 21-06-2021, 02:11 PM
Hans Tucker (Hans)
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Hans Tucker is offline
Join Date: Jul 2008
Location: Melbourne
Posts: 2,205
Originally Posted by Don Pensack View Post
Alas, city lighting has become more broadband. Here in Los Angeles, someone recently measured around 130 different wavelengths of sky brightness from light pollution, covering the entire spectrum.
The use of broadband filters as "light pollution reduction" filters has pretty much come to an end in modern municipalities where LED lighting is common.
Today, we have the headlights on cars, incandescent lighting, fluorescent lighting, LCD lighting, LED lighting, arc lights, neon, argon, krypton, and xenon lighting, sodium vapor, mercury vapor, halogen lighting, and too many other sources to list.

Worse, a filter that removes only some of the light pollution wavelengths and passes the rest will often have internal reflections between layers of coatings that scatter MORE light than the use of no filter at all.

However, for emission nebulae in particular, which emit light basically at 3 closely-grouped wavelengths, the use of a narrowband (not a broadband or LPR) filter will help a lot. The filter will suppress nearly all of the light pollution wavelengths, improving the view of emission nebulae.
Unfortunately, these filters will not help the visibility of stars or galaxies, or reflection nebulae, or dark nebulae, etc.

In today's urban environments, the best way to view most deep sky objects is to travel to darker skies.

Still, for star clusters (open and globular), or double stars or carbon stars, you can simply increase the magnification to make them more visible. As you bump the power up, the background sky in the eyepiece dims, but the stars do not. Here in LA, I can barely tell the bright star cluster M11 is there when I look at it with 50x. At 150x, though, the cluster has become populated with lots of faint stars, and at 200x, the view is similar to what I see in dark skies.
So at least for any object containing stars, increasing the magnification will help.

Galaxies, however, emit light across the spectrum, and because they are extended objects, increasing the magnification also dims them. Other than super-bright galaxies like M104, seeing them really requires darker skies than a city environment.

Ironically, dark skies is the environment where the broadband ("LPR") filters work best--by turning up the contrast a tiny bit.
Thanks Don. Oh to be 10 again when street lights went out at 12am and I could see the LMC and SMC from suburbia. Maybe it is time to utilize the ASV membership and head for darker skies.
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