How much light is considered pollution ?

[medwatt]

Hello,

I'm still in the process of learning the optimum ways of finding objects in the sky but I'm a little confused about light pollution. I see the term has been thrown around a lot but have never had seen any quantitative measurement attached to it. Obviously I can tell when there is a lot of light !! What I'm not sure of is how of of background light can start creating havoc. For setting max and min, max is say, New York city and min is a village. Where I live is far from New York, but I can see some light bulbs radiating from neighboring houses. How much does that affect my stargazing.

Thanks 

[Ags]

Can you see The milky way? Is it invisible, can you barely make it out, or does it seem bright?

Can you see the andromeda galaxy in Autumn?

How many stars can you see? Dozens, hundreds, or thousands?

For me the three answers are invisible, no, dozens.

If you Google Bortle Scale, you will find a common measure of light pollution.

If you Google Samir kharusi skyfog DSLR, you will find a way to measure your light pollution with a DSLR.

[Ags]

The main problem with the neighboring lights is they probably prevent your eyes from fully dark adapting.

[medwatt]

Are you asking what I'm seeing with my naked eyes or aided ? 

[Beulah]

If you Google Samir kharusi skyfog DSLR, you will find a way to measure your light pollution with a DSLR.

This is great reading and I'll give it a try - thanks!

medwatt, I hope this link helps as well - scroll to page 6 of 19 for your region - authored in 2001 so it's a bit out of date but can be a good start : http://www.lightpollution.it/cinzano/download/0108052.pdf

[Ags]

Sorry, i meant naked eye.

[Langy]

Even in built up areas with street lights I think you may be quite amazed how much you can really see.

The front of my house (on a housing estate) is right on the road with street lights running all the way up and down the road. Now if I shield my eyes from the lights and then look up, on a clear night I can see hundreds and thousands of stars. If I'm further away from my house and look towards Carlisle then you can see an orange haze in the sky. So the real light pollution is really coming from Carlisle which will obscure views low to the horizon in that direction.

I think you may find that even in the city or big town that if you can shelter yourself from direct light you will see lots of stars above your head.

[triton1]

If I can see Polaris and the rest of the stars in Ursa Minor from my garden with the house lights on then I,ll get the telescope out.Always seems to work for me.

Regards Jonn

[ollypenrice]

To quantify your sky quality you could use a Sky Quality Metre. These are providing a sort of universal currency, certainly in astrophotographic circles. http://www.optique-pro.fr/autres-accessoires-d-astro-photographie/geoptik-sky-quality-meter/p,8850?utm_source=froogle&utm_medium=cpc&utm_term=8850&utm_campaign=froogle-1402&gclid=CNGbqvLBs7wCFYYBwwod_lEAyQ

A few local lights are not what LP is about. Protect yourself from them and you have a dark site. But a general skyglow is a different matter. One good test is to look at the darkness at the zenith and at the horizon. At a true dark site there will be little difference. I have one of the best mainland European sites possible but the first 15 to 20 degrees above the horizon are certainly not as dark as the rest. For that you need the Atacama.

Olly

[Ags]

If I can see Polaris and the rest of the stars in Ursa Minor from my garden with the house lights on then I,ll get the telescope out.Always seems to work for me.

Regards Jonn

If I see stars and no clouds, i get the telescope out :-) 

But that hardly ever happens.

[triton1]

Ok if you see looooooads of clouds and one star forget it but if you can see Ursa Minor your in with a shout but if your weathers like ours is I,d polish the scope and wrap it up in a duvet and leave it to sleep.(for ages)

[acey]

If you can see stars that are 6th magnitude or fainter then you have no significant light pollution.

If you can see the Milky Way then light pollution is low.

If you can't see all  the main stars of Ursa Minor (and the sky is completely free of cloud or haze) then there is significant light pollution.

Apart from man-made light pollution there are also natural causes of skyglow: moon, twilight, aurorae, zodiacal light and airglow - so the sky is never completely black. If it were (or if you were in space looking through a visor of high optical quality) you'd see stars to mag 8 or fainter with the naked eye.

I use an SQ meter, and my personal rating (for deep sky viewing) would be:

<19.5 Heavily light polluted. Some targets will be visible (M42, M13, M57, M31, M81/82 etc) but views will be badly compromised.

19.5-20.5 Significantly light polluted, not suitable for galaxy viewing (though some will be visible in a telescope).

20.5-21.0 Milky Way is visible. Not great, but dark enough to get the scope out and have a go: about as good as it gets at my latitude in summer.

21.0-21.5 genuinely dark: Milky Way is bright and clearly visible. Any NGC object should be visible, given sufficient aperture.

21.5+ Very dark - make the most of it.

Darkest I've measured is 21.7, darkest possible is 22. Any urban/suburban site is going to be brighter (lower) than 20.5. When I first started it was with an 8" at a site with SQ 18.5-19.0, and I managed to see quite a lot of DSOs, but as soon as I started using a dark site I realized what I'd been missing. Note that the scale is logarithmic: SQ 19.5 is about 500% brighter than SQ 21.5.

The Bortle Scale is popular in US (promoted in particular by Sky And Telescope where it was first published) but I personally have found it not very helpful and in some respects misleading, notably in its assessment of the naked-eye visibility of M33, which I've never managed. Even with SQ 21.7 I can't see stars fainter than mag 6, though people with younger and better eyes should manage.

For lunar/planetary viewing light pollution is irrelevant as these targets are very bright. In that case steadiness of the air ("seeing") is what counts.

[medwatt]

I still learning the sky. I remember the last week I saw orion the hunter with my naked eyes. I first saw the three stars that constitute the belt and immediately I looked for Rigel and Betelgeuse and saw them both though I was unsure at first because Betelgeuse  kept blinking. I checked with Stellarium and confirmed my find. I know Rigel and Betelgeuse are two of the brighter stars does my spotting of orion say anything about the light pollution in my area ?

[Astro Imp]

What other stars in Orion were visible to the naked eye.

[glappkaeft]

Estimate you limiting magnitude and then use this list to count them: http://en.wikipedia.org/wiki/List_of_stars_in_Orion

[Charic]

As already mentioned the Bortle Scale. Do you use Stellarium?  In the Sky and Viewing conditions( you can set) I have set the scale to match what I can see outside. I'm presently looking at between 4 & 5 on that scale. If I Drive out of town by about 4 Miles its gets so much better. My plan is to view from the middle no-where and get the 1 or 2 scale experience. ( I have a site in mind thats 10 miles from any skyglow in all directions?)

[Jon Sales]

Any light is too much IMHO

[medwatt]

Ok. I checked the skies again tonight. Orion the hunter was visible. Very closed to was was the half moon which was very bright. What I saw of orion was just the four stars that mark the vertices of oions body and the three that make his belt. The image below is the exact orientation of the moon and orion w.r.t my view and has these stars of orion that I could see circled in red.

Another thing. I live in a three story building and I think I can see the roofs of most houses in my vicinity meaning that lights are below my rooftop. How much of that is of concern ? 

[Ags]

Sticking with Orion, can you see (naked eye) a haziness where his head should be? That would indicate reasonably dark skies. 

Easier, does the sword look fuzzy. Again that will indicate fair skies, but the sword fuzziness is more priminent and can be seen even under quite polluted skies.

It's probably better not to answer the questions on a night with a prominent moon.

[medwatt]

Thats what I thought. The moon was bright and orion was just by it. I think I was able to tell that Betelgeuse is of different color. How many minutes does it an average person for the eyes to adapt to seeing magnitude 3 or 4 stars ? Are mag 3/4 stars visible just after coming from a brightly lit room ?

[Ags]

You need about half an hour to dark adapt, but it can be ruined in half a second by a neighbour's light.

I would not worry too much about LP. I have terrible LP, I can only see one star in ursa minor, but I still enjoy naked eye observing, spotting the constellations, detecting star colors, trying to remember star names.

With a pair of binoculars, I can try and soot the brighter DOS like the beehive or the double cluster.

Planets are completely unaffected by LP. In fact, you want to avoid dark adaptation for planets as they will ruin it anyway, and you need your day vision to see the colors.

[medwatt]

So consulting your gps device will even ruin your dark adaptation process ! Seems that there are so many adverse factors. Haven't previous stargazers devised ways to stay 'dark adapted' ? 

[Stargazer_00]

They way i see it there are two forms of bad light

The first is classic light pollution caused by large volumes of condensed light sources, such as urban area or industrial areas, whch produce a significant amount of light to have an effect on the colouration of the sky itself.  This reduces your ability to see the faintest objects are the faint light is washed out by the local light pollution in the atmosphere.  It's called pollution because it effects the atmosphere using light, whereas more common pollution is gaseous.

The other form of bad light is local light.  Bright lights at your observing area are as bad or maybe even worse than light polluted skies as your eyes can not dark adapt therefore you simply wont be able to see that faint light because the aperture of your eye is too small!   Even if you are under perfectly dark skies, a halogen light bulk pointed at your face will ruin your observing.

Local direct light issues can be resolved easily by finding the source and having it turned off.  Or just moving a little away from it.

Light pollution is a bigger issue and can only really be avoided by finding somewhere that isn't polluted.

[AndyWB]

Personally, I tend to use Ursa Minor to judge the limiting magnitude for where I am - I carry a chart something like: 

Ursa Minor has the advantage of staying high in the sky, and always being visible, from the UK.