Light pollution, sky glow versus near field

[BenM]

Basically, I'm wondering whether I can defeat the HPS street lamps near my house.

My house is obviously a first choice observing location because I'm often there, there's hot coffee, the loo... well you get point. Also, I can keep a much larger scope there than I would want to travel with, even just down the road.

Light pollution in my area is very minimal thanks to the fact that I'm on one side of some 10,000 foot mountains. On this side of the mountains there is only a few towns with no more than a few thousand people. My own neighborhood is about 700 houses and we're surrounded by total darkness -- nothing, no street lights, no motorways, no other houses for miles. Other small towns nearby are 20 miles away.

Therefore the sky glow or atmospheric light pollution is quite low.

However, I still have street lights in my neighborhood and all the neighbor's windows with lights on. Because of this I can see maybe magnitude 4 stars, Jupiter and Saturn from my backyard, but if I go just outside the neighborhood I have outstanding viewing, perhaps magnitude 6 objects.

Because of the low atmospheric sky glow, I'm wondering if I can somehow "shoot past" the near field pollution (my neighbors' windows and the two street lights) and reach the dark skies above or if it's hopeless.

Would I be better off with a 10" f4.7 newtonian on my patio or a 6" f5 (short tube) Newtonian a half mile away?

I don't want to go over 10" because the weight and bulk of 12 and 14" make it difficult even to pull out on the patio and cool down also takes longer.

I read some anecdote about shooting through holes in the clouds and getting great results because the cloud layer blocked the light from the upper atmosphere where it was dark. It didn't quite seem intuitive because I would have thought that the reflection would enter the scope as well.

Since I don't have any experience using a scope of my own (I've only used others at ideal locations), I wonder just how much the near field lights will interfere. Obviously they interfere a great deal with naked eye observations but how much can a scope "shoot past" that?

[jwallis]

Hi Ben.

You could buy a light pollution filter which you screw into the end of your eye peice. This should help cut out most of the glow.

First Light optics sell a baader filter which would do just the job. Have a look on their website and see what you think.

[OzDave]

My usual imaging site sounds like it has a similar lighting problem to yours. Basically a village in the middle of nowhere with hardly any light pollution, except for 1 street light and neighbouring houses nearby.

I'm not sure what magnitude objects I can see as I've never tried to test, but I know that on good nights I can see the glow of the milky way faintly.

I find that I can get really decent images in this environment although I haven't tried pushing past about 3 minutes of exposure. I also use an Astronomik CLS clip filter for my Canon 450D. I've not tried imaging without it at this site.

I also image from the city where light pollution is truly awful and only the very brightest stars are visible. Without an LP filter there, the results are awful. With a LP filter, things improve dramatically.

So I think although it may be better to get to as dark a site as you can in general, you will probably find that you can "shoot through" the nearby lights so long as the scope doesn't point too near them. Try to image near Zenith and you should get pretty decent results.

David

[Dave-Ryan]

What country are you in. Obviously not the UK with 10,000 foot mountains.

[BenM]

In the US, state of Nevada. Most people think of Las Vegas but I'm nowhere near there. There's 800 miles of bombing ranges between me and sin city which must be like some kind of poster child for egregious light pollution.

I'm wondering if I can't get a remote for those street lights. You know, secretly wire a little switch inside the base of the pole and when I want to I just hit the clicker. Oh well. I'll check out the filters. I know that low pressure sodium vapor lamps is easier to filter. Those are the really dreadful orange, almost monochromatic lamps. Mine are the high pressure sodium lamps that emit quite a bit more spectrum. They have sort of a blush or peach colored tinge. Hopefully we'll all get some very directional LED's or something in the next 10 or 15 years.

[BenM]

Thanks for the tips, I'm looking at the Baader Moon and Skyglow filter and the Astronomik CLS filters (for 2"). I think something like that should help.

I'm sort of wondering whether the problem is that the neighbor's lights and the street lights reduce my pupils for naked eye viewing or if there is a lot being reflected off particles that will enter the scope as well. When I tried with some binoculars (10x21, I know) it wasn't good.

[pete_l]

My first thought is that your visible limiting magnitude is simply caused by your eyes not getting fully dark adapted, due to direct lighting from the small number of houses and lights in the immediate surroundings.

You say that moving to just outside the neighbourhood improves things a lot: so it doesn't appear as if you've got massive light-domes of pollution you need to counter, just the local direct stuff at ground level.

If that is the case, then the LP will be limited to fairly low angles, near the horizon. That's not going to be too bad, since observing at low angles has all sorts of other disadvantages: more air mass and turbulence. The good news is that the sky higher up will still be dark as there won't be too much artificial light being directed upwards to scatter down. It's that back-scattered light which causes the biggest problem, as it comes from the same direction as the astronomical targets we want to see, and this artificial light reduces their contrast ans can even swamp them completely.

I'd suggest an easy fix first. Get a blanket or towel that you can drape over the eyepiece end of your telescope and keep your head under while observing. That will keep any direct, local LP out of your eyes and allow them to reach max. sensitivity, provided you can keep your eyes dark adapted and not have to duck out of the cover too often. (If so, keep your "observing" eye closed and do whatever needs to be done just with your other eye open, that way you only lose dark adaptation in one eye,