Light-Pollution Filters

[SrStewart]

Hey everyone!

Anyone got any insight into LPFs? Currently where I am located we are on a 7-8 rating of light pollution [LINK] .

I really want to push some more out of my scope when I am not going to a friends farm or camping.

I'm looking around currently and hope to get some feedback and some opinions on which ones and if they are really worth it.

Cheers

[Pete Presland]

you might want to read this thread, as there is quite a lot of discussion on the subject.

http://stargazerslounge.com/topic/23551-light-pollution-filter-recommendations/

as for myself I have a Baader Neodymium filter for visual, that does not seem to make a huge amount difference. i also use a 2" for imaging which does help.i believe as the street lights change they will become less effective. Filters won’t make the objects brighter, but in many cases, they will make many of them a lot easier to see.

[Capricorn]

They work because the light pollution concerns the sodium lamps, those yellow ones, also only if they are low pressure sodium.

The light they emit is in simple terms just 2 wavelengths at 589 and 589.6 nm.

All, if all is appropriate, an LP filter need do is to block at around 589.3nm and so these 2 wavelengths are prevented from coming through to your eyes. The other wavelengths are passed and you see these.

They are just a narrow band blocking filter.

Now the problems:

High pressure sodium lights do not emit solely in the 2 wavelengths above, they are more polychromatic - white.

New LED lamps do not emit at the same 2 wavelwngths they also are more "white"

If you were looking at a nebula that have a sodium emission then this sodium light would get blocked also so be less visible.

An LP filter will not work with the newer "white" street lights, they emit at too many wavelengths for filtering to be applicable.

The old sodium lamps were a pain but the narrow wavelengths were "convenient" for filtering out the light.

[DirkSteele]

As Capricorn mentions, the new high pressure street lighting negates the blocking capability of the light pollution filter. However, if you wish to improve the view of emission nebula you could consider a UHC (or possibly OIII filter). Emission nebula radiate a good potion of their light at specific wavelengths (Hydrogen Alpha, Hydrogen Beta and Oxygen III lines). The UHC has a very narrow band pass around these wavelengths and blocks the rest of the visible spectrum. This has the effect of darkening the sky background and enhancing the contrast of the nebula. And at the end of the day, all deepsky observing is a contrast game. Unfortunately these filters will not benefit galaxies or globular clusters as they radiate across the whole visible spectrum.

[Qualia]

I've found the UHC filter useful for given nebulae but as Dirk suggests, they do darken the skies and often sufficiently so to omit the beautiful backdrop of sparklers. This is a small price to pay for observing nebulae themselves, but I wouldn't recommend it as a general use LP filter and certainly not for star-hopping in LP areas. To be honest, for visual use only, I'm very suspicious that such a perfect LP filter really exists and Bunny's and Capricorn's well informed posts above only highlight that doubt. Try to get to a darker site from time to time over the coming colder months; that's what I plan on doing.