Light Pollution filters

[Bone_Idle]

I have a lot of light pollution where I live, would a filter help. I don't think I could go for something too strong though as I only have a 90mm refractor.

Any thoughts or recommendations?

[umadog]

With a 90 mm refractor, I'd not bother with a filter. Any good filter would get rid of so much light that you'd not see much. You'd not be able to push the magnification past about 15 x or 30x. You're better off taking some trips to darker skies.

[Canyouguesswhoitisyet]

I have just got a Skywatcher LP filter £20. On trying it last night I could not really see any difference. I am sure there must be but it was not obvious. I have only had one night using it though.

[West End Wendy]

I have a Baader neodymium for my 4" refractor. Makes very little difference for me - good on Jupiter though.

It will depend to some extent on your local street lighting.

Tim.

[Bone_Idle]

Sounds like they're not too effective.

Is there any truth that they improve the views of nebulas? I can see nebulosity in Orion, but it's very feint. Wondering if a filter would help?

Or would a UHC filter b a better option?

[umadog]

LPR filters are effective if you use the right filter on the right object. The more broad-band filters provide limited contrast boost. The UHC filter and its ilk are narrow band filters which work a lot better. They boost contrast on emission nebulae such as planateries and M42. Some objects emit primarily in OIII or HII, in which case they will benefit most from filters which pass only those lines. The Veil Nebula can go from invisible to visible in urban skies using an OIII. Even from darker skies, the OIII filter will bring out filamentary detail in the Veil which isn't visible otherwise. Galaxies are not helped by the narrowband filters. The main issue with these filters is that they work best at 5 to 10x per inch of aperture. In other words, you're limited to very low powers with small scopes.

[Bone_Idle]

Thanks for the explanation Umadog. Sounds like with my scope I'd be restricted to my 25mm EP at best (about 36x).

I did read that the UHC filters were more designed for smaller scopes (as opposed to the OII ones which recommend 200mm or larger), so I might try one of those.

[umadog]

It's true that the OIII filter cuts out more light, but the UHC cuts out an awful lot as well. I think you might be disappointed with a UHC in a 3" scope, but I have to admit that I haven't actually tried that combination. Perhaps someone who has will chime in.

If you do go for it, make sure you can get well dark adapted. It definitely won't work well if you have direct lights shining at you.

[Bone_Idle]

Thanks again. It's beginning to sound like it's probably not worth the £40 or so it will cost. I don't have a really dark spot in my garden, so it would mean moving to a dark site to observe, and that probably wouldn't be an option most of the time.

I think I'll save myself £40 and maybe buy another TMB Planetary or put it towards a RA motor.