View Full Version : why aren't cities skies filled with stars?

2006-Mar-10, 03:11 PM
I know that the polutants spewed from cities as well as the big beaming lights dont help, but i'd like to know the principle behind it, why the change in the atmopshere does this to star light and why our own lights seem to halo our cities the way they do.

2006-Mar-10, 03:19 PM
The sky is brighter, which drowns out the dim stars.

Just like when the sun sets, the stars just magically appear as the sky gets darker.

Dave Mitsky
2006-Mar-10, 04:12 PM
Browse this site - http://www.darksky.org/ - for more on light pollution.

I'm moving this thread to Astronomical Observing...

Dave Mitsky

2006-Mar-15, 04:32 PM
I'm not clear about what factors affect seeing, and I would like to know whether it would be a bad idea to invest in a larger refractor for use in may backyard. There are no astronomy clubs in my area, so I can't just borrow a scope and check it out.

My latitude is 44.7 degrees N, so the planets are often only about 20 or 25 degrees above the southern horizon. Is there a rule of thumb for calculating light pollution and its effects? Are factors other than light likely to affect seeing more when I am trying to look at things relatively low in the southern sky (I live in windy foothills)?

You don't have to read any further, but if you're interested in the exact situation: I enjoy relatively dark skies for the United States. The two sources of light that affect me are the glow from several parking lots 1.5 miles to my south, and a single streetlight that is mostly screened by foliage and lies 200 feet away and not in line with anything I want to look at.

There is a low hill between me and the parking lots. When there are low clouds, the reflection from them is visible, bright enough to cast a shadow, and can cover 30 degrees of southern sky, when the height of the clouds is just right. On clear nights, the light is visible in the pines at the top of that intervening hill, but 5 degrees above the hill there is no visible light at all.

As for particulate pollution, the air is crystal clear on cold nights, but we live 20 miles downwind from some 3,500 foot peaks. My house is in a pocket that is generally calm at ground level, but there is a pretty constant eastward flow of air at most times of any clear night. It is not unusual on a clear night at midnight to see a scrap of cloud come over the ridge on the western horizon to move directly overhead within five minutes, which I calculate to be a rate of 80 mph or better.

2006-Mar-15, 06:33 PM
It sounds like you're in a pretty good location as far as light pollution goes, if the nearest source of lights are 1.5 miles away with the exception of a single streetlight. Other things that affect seeing/transparancy (seeing is a measure of how stable the air is, transparancy a measure of how clear the air is) are how much pollution/dust/humidity is in the air, upper level air currents that differ from lowel level air currents, wind, proximity to buildings/parking lots (they radiate a lot of heat up to a few hours after sundown), etc.

I'm assumin you already have a smaller refractor since you're wondering about getting a larger one. It sounds like you might want to consider keeping your current refractor (providing it is of good quality) and invest in a larger reflector instead for those nights of really good seeing. Might be something to consider. Your proximity to the mtns. and it's resulting air currents is cause for some concern, but overall I'd say you're in a fairly good location.

2006-Mar-15, 08:21 PM
Thanks for your response. I'm really sort of betwixt and between in terms of equipment. I have a 60mm refractor I sort of inherited, and a 114mm short-tube Newtonian, both on EQ mounts.

The reflector probably qualifies as a starter scope. It took me until two weeks ago to collimate it well, but I had the strangest view of Jupiter right afterward - my 20mm EP, at 50x, showed no visible moons, and with the 6mm Huygens at 167x, it was a milky yellow-orange disc (still no moons).

It may be that the optics aren't good on my cheapo short-tube Newtonian. That's a real possibility. It's a generic Chinese-made model I got on eBay for $115, delivered, that very closely resembles Tasco short-tubes you can buy for $200 at discount sites on the web. I just ordered a 32mm GSO Plossl, so if it is an EP issue, I will soon find out.

I asked my question about seeing because I wasn't totally confident that I wasn't witnessing some atmospheric effect - although the likelihood that the air itself would be hiding the moons of Jupiter seemed remote. Thanks again for your reponse.

2006-Mar-15, 10:12 PM
Those Huygens eyepieces are terrible I'm afraid, and are usually what come with cheapo scopes, time to upgrade. Do you have a .965" focuser or a 1.25"? 1.25" are much more prevalent. .965" focusers are usually only found on cheapo dept. store scopes as well. I'd suggest a couple of decent 1.25" plossl eyepeices. Also, 167x can be pushing it for magnification, in my experience you need pretty good seeing to go past 100x, and your 114mm reflector will be good for 200x max even on the very best nights, 50x per inch of aperature is about the max you can go. So the max magnification on your 60mm refractor is about 120x on the very best night.

Dave Mitsky
2006-Mar-16, 07:18 AM
I'm not clear about what factors affect seeing, and I would like to know whether it would be a bad idea to invest in a larger refractor for use in may backyard.


Are factors other than light likely to affect seeing more when I am trying to look at things relatively low in the southern sky (I live in windy foothills)?

For a good treatise on astronomical seeing, browse http://homepage.ntlworld.com/dpeach78/seeing2002.htm

The thickness and optical properties of the atmosphere near the horizon will cause increased extinction (diminution of magnitude) and scintillation (twinkling), along with prismatic dispersion (false colors).





What do you mean by large? A 6-inch refractor is large by amateur refractor standards but is unlikely to be terribly affected by typical seeing conditions. A 10-inch Newtonian will give you a lot more bang for the buck and will provide far better DSO views but will suffer more from bad seeing. It could always be stopped down with an aperture mask if it's deemed necessary.

Dave Mitsky

2006-Mar-16, 04:27 PM
Thanks for both your replies. I very much appreciate the links concerning seeing, and it is interesting to hear that 100x is a reasonable magnification limit on the refractor for an average night. I had such good luck on very cold nights, seeing at 100x with my 60mm refractor, that I just assumed the 114mm reflector would deliver 200x without a fuss.

Dave, your advice about the larger reflector raises a question, and the links about seeing were so useful that I hope I'm not imposing on you to ask if you have another link about the optics of what you recommend. I don't understand why stopping a 10-inch reflector down for planetary views would be desirable. Wouldn't that just reduce the 10-inch to a 3 1/2 inch reflector? From what little I know, it seems to me that the Dawes limit would reduce possible detail from about .5 arc seconds to 1.5 arc seconds. Or does the shadow of the secondary somehow affect the quality of the overall image, the way a partial eclipse of the sun distorts the shadows it throws so that all the edges look crescent-shaped?

Dave Mitsky
2006-Mar-16, 05:14 PM
Seeing seems to be related to aperture, although some dispute this assertion*. Stopping down a large telescope reduces its light grasp and resolution but results in an overall "prettier" image. If that floats your boat, then by all means do it.

The ASH 17" f/15 classical Cassegrain has aperture masks of 14", 12", 10", and 6". The 6" produces an unobstructed, clear aperture of 6" at f/42. For me, the seeing must be really bad to warrant using aperture masks. (It's a different story with apodizing masks, however.**) You will always see more at full aperture but may have to wait patiently for long periods of time for the occasional fleeting moments of good seeing.

Dave Mitsky

* http://skyandtelescope.com/printable/howto/scopes/article_508.asp
** http://www.csastro.org/gallery/article4.htm