Light Polution

[bard9]

Hi there, I was wondering if there is an established formula for assessing light pollution in any given area ie: distances from cities, towns, and villages.

Jimmy

[JamesM]

Sorry, I'm not sure what you're asking. There are a number of websites that detail light pollution maps in order that people can assess where to go to get dark skies. I'll need to see if I can dig them up for you - is that what you're after?

James

[bard9]

Thanks James,

I have looked at some of the web maps of my area although they are not very detailed (10miles East South East of Lincoln). I suspect local variations may have an effect ie:shading by woodland from a local source of LP.

I was wondering as a guide to the suitability of a particular type of scope.

Thanks

Jimmy.

[Ags]

One standard formula is to go outside on a good night, let your eyes dark-adapt, and then look at the Little Bear. If you can see all the main stars, then you can see down to at least magnitude 5 - quite good. If you can't see any Ursa Minor stars except for Polaris, you probably live in a city. The main stars in Ursa Minor are spaced in brightness by a magnitude, so you can easily guage how birght your sky is by how many of them you can see.

[bard9]

Ags,

Yes on a good night I can see Ursa Minor, I wonder if I sometimes confuse LP with poor seeing conditions.

Thanks.

Jimmy

[umadog]

Astronomical seeing has nothing to do with LP. "Seeing" describes the steadiness of the atmosphere (the degree of twinkling):

Astronomical seeing - Wikipedia, the free encyclopedia

Very local light pollution is a problem, yes. Those LP maps are a good guide but obviously won't predict the consequences of your neighbours "security" lights. Then again, this form of localised LP is easy to get away from or shield. The LP maps are all coarse and describe the size and brightness of light domes around cities or even motorways. This form of light pollution is more pervasive and getting away from it means travelling longer distances.

need-less light pollution

The World Atlas of the Artificial Night Sky Brightness

The explanation of the scale on this map is here:

ClearDarkSky Light Pollution Map

Bottom line is that things start to get really good in green or blue regions, but if you're coming from the city you'll be pretty happy with yellow. In black regions the view is so majestic it can be frightening. From my blue dark site I can see the Veil nebula in 8x42 binos. At home in my red region I struggle to see it with a 12" Dob and an OIII light pollution filter. LP is constantly getting worse, however. Write to your MP to raise your concerns!

You ask about telescopes. Regardless of light pollution, it's still a case of "bigger is better." For instance, light pollution filters will work better in larger telescopes since the filter makes the image dimmer overall. DSOs appear brighter in a larger scope so better tolerate the dimming induced by the filter. I definitely use my LP filters more in my 12" than my 8.75". Of course, portability constraints limit the size of the scope you choose.

[JamesM]

On the scope front, I would add that it depends on what you are looking at. If you have a refractor and want to observe double stars, planets even the moon, then light pollution has a limited effect. It is the deep sky objects, which most of us observe most of the time that are particularly effected by light pollution. Have you drawn a short list of scopes or are you still at the, "getting to grips with it all" level of research?

Clear skies

James

[bard9]

Umadog,

Thanks for that info, its given me plenty to digest.

Jimmy

[bard9]

James,

I fancy imaging at some stage in the future, I have considered something like a second hand ED80. Like to view/image nebula I don't know if I would need a newt to view the dimmer objects.

By the way does your tractor not have a cab?

Jimmy

[umadog]

If you want to view nebulae visually then buy yourself a Dobsonian. You need aperture to see these things and you get the most aperture for your money with a Dob. A 6", or better still an 8", would be a very reasonable starter size.

Imaging is almost a different hobby and requires more expensive equipment. You can take very good shots with even a 4" refractor, although you'll need an expensive mount and potentially an expensive CCD camera. I'm not an imager, but I suspect most who are have a dedicated imaging rig and a separate visual rig. My advice is that you don't go looking for a scope which "does it all" because you'll be disappointed. Buy yourself a competent visual scope, learn the sky and have some fun. Then if you still want to do AP, you'll have a much better idea where to spend the big bucks. This way you'll then have a nice visual scope to use whilst taking those long exposure photographs.

[bard9]

Thanks for your comments Umadog, I'll be looking out for an eight inch dob.

Jimmy.