The American Association
of Amateur Astronomers
Frequently Asked Questions
FAQ 15: Tips for Observing in Light Polluted Areas
Ken Boquist, Bill Geertsen, David Hasenauer,
Lee Maisler, Chris Randall, Roberto Torres,
John Wagoner, Fred Lusen
Sometimes it can be difficult observing in heavily light polluted skies.
But by following a few procedures, your observing session can be more enjoyable
and more rewarding. The following are tips that our crack team of observers
offered to help increase your satisfaction when observing in light polluted
When to Observe
Observe during new moon. Just like observing in dark skies, the moon adds
light to the night sky and reduces contrast.
Observe after 10:00PM. This gives the dust and water in the air a chance
to settle. Dust and water reflect light which can turn a good night into
a bad one.
Observe after 11:00PM. Many stores have closed by this time, and because
they turn off their lights, a city's light glow is reduced considerably.
Observe after 1:00AM. After the stores have closed, most shoppers and workers
have gone home which means that there is a lot less traffic on the streets
and freeways, and light pollution is reduced.
Ask your neighbors over for an observing session. After seeing the effect
of light pollution on observing, they will be more co-operative in turning
off their lights for you.
Try to catch your target objects straight overhead. This is always the
darkest part of the sky.
Select the right objects to observe. Magnitude is not everything. A bright
galaxy may be invisible, whereas a dim planetary may be easily seen. Small,
high surface brightness and stellar objects are easier to observe than
large, diffuse objects.
If you have an alt-azimuth mount (Dobsonian), try to observe near the meridian.
Up-down, left-right motions translate into north-south, east-west motions
and makes following a path on a star chart easier.
Watch Sky Conditions
Pay close attention to the weather. Cool, dry nights are best at any location,
but are more pronounced in the city.
Learn to read the quality of the sky by the observing of stars with the
naked eye. A clear night might seem perfect for observing, but may in fact
be bad for viewing if the seeing is not good.
Observe after a rain storm. The skies appear darker as light is no longer
reflected off of dust particles in the air.
Observe after a cold front has come through. The air is more stable and
the air pollution has been blown out.
Use a dark cloth to cover your head and eyepiece to shield them from stray
Use a dew shield on your telescope to shade it from stray light.
Clean and collimate all optics. Dirty optics scatter light.
Light pollution and O-III filters are good for planetary and emission nebulae.
Use a pirate's eyepatch to keep out stray light.
A right-angle finder with amici prism under a dark cloth is helpful for
Setting circles are a great aid for finding difficult objects, especially
when those objects are quite some distance from a naked eye star.
A good star atlas, a pair of binoculars, and a one power finder (e.g. Telrad)
with a template for that finder, are inportant for finding objects in bright,
non-contrasty skies. Telrad-hopping can sometimes be easier and just as
useful as star-hopping with a finder. Viewing the sky through your Telrad
with binoculars is also a nice trick.
Pick the darkest section of your site and make an extra effort to block
out stray light. Using a light baffle made of a tarp and tent pegs help,
as well as a three-sided wall made out of cardboard. Try to make the immediate
area around your site as darkened and non-reflective as possible. Use existing
structures and foliage to block the direct view of lights.
Use earphones or a radio to mask neighborhood noise. Noise can be very
Finally, attitude is very important. Any observing is better than no observing
Finding Deep Sky Objects in the City
You can see deep sky objects from the city. Some of the brighter Messier
objects are available. You just have to know where to look.
Start by having a good star chart. Overlay a sheet of clear cellophane
over it and plot the brightest stars you can see naked eye. Draw a line
with a colored pencil between the plotted stars and look for objects along the
line that you want to observe. Use the charts grid scale to get the
angular distance from your known star and then use the telescopes field of view
angle to figure how far you have to move the telescope to see what you want to
look at. In the absence of a goto scope this requires a little work and
diligence but it does pay off.
The idea is to practice, practice, practice. From your back yard and in
the house. Set your scope up in the house in a dark room, turn out the
lights, and in the dark (or using your red flashlight) become intimately
familiar with your scope and all its controls. That is one thing you do
not want to be doing at a dark site