Skywatching From Towns & Cities
Never have so many people lived under such light-polluted nighttime skies as they do today. Our world may become brighter and brighter at night unless people get mayors and councilors to act on nighttime lighting reduction. With much-improved astronomy equipment now available, today's amateur astronomers have never had it so good to observe and photograph the night sky. Yet, that all goes to waste for urban astronomers as incorrect lighting spreads its tentacles, forcing dark skies to retreat further and further.
The vibrancy of amateur astronomy from towns and cities depends on reduced light pollution, so play a part and make YOUR voice heard by your local government. Councilors must be made aware of the problem.
Understand that the environment in urban areas is public. The night sky is open, but business lighting is private. However, when individual activities become detrimental to the urban environment, that is wrong. Businesses must NOT be allowed to virtually privatize the stars.
Indeed, why should amateur astronomers be forced to find and then journey to dark sky sites or even use light pollution filters just because businesses insist on nighttime lighting or using overly lit advertising? The night sky is everyone's and should be protected.
However, all is NOT AS BAD as it seems, as you will discover in this lesson. Today, equipment for amateur astronomers abounds. Since the start of the Millennium, all kinds of astronomy gear have come onto the market. There are more telescopes and accessories available today than ever before. The thing is, most people with some fondness for the night sky never buy a telescope because they think they cannot see the stars.
Excessive light pollution in urban areas smothers faint, delicate celestial objects like the Milky Way. No one residing in towns and cities has experienced a dark sky without leaving the place. Does that mean astronomy is dead for urban dwellers? Absolutely not. The good news is people residing in and around brightly lit towns and cities CAN get beautiful views of celestial objects through their telescopes.
By knowing what to do.
You already know about light pollution from the previous lesson. However, for those residing in built-up areas, localized or line-of-sight light pollution is a major problem:
Unwanted light illuminating your observing site is often due to poorly located lamps, either from a neighboring residence or, more likely, a streetlight. Line-of-sight light pollution might not wash out the night sky as does sky-glow, but it is really annoying. What follows now are tips to aid urban astronomers with light pollution problems.
Telescopes for Urban Astronomy
Ignore the myth that large instruments are useless in towns and cities because they just gather more light from a light-polluted sky and wash out the view.
Larger apertures will obviously grab more light from celestial objects - no matter where the instruments are located. A telescope that is twice as big as another will always reveal MORE detail.
The only downside to using large aperture instruments in urban areas is that they are significantly affected by tube currents and atmospheric turbulence. Indeed, on nights with lots of jiggling air high overhead, it will be harder to focus on celestial objects than smaller instruments. The thing to understand is that air turbulence has nothing to do with light pollution whatsoever. How to get around the problem of tube currents is explained further along in this lesson.
Best Telescope Size for Urban Astronomy
The best telescope aperture for amateur astronomers - no matter from where they observe - is the one they use most frequently.
Whether it is a basic 3-inch refractor, a handy 4-inch GoTo catadioptric, or a monster 16-inch with all the bells and whistles reflector mounted on a sturdy windproof pillar, as long as it is the most used instrument, that will be the best.
Urban Amateur astronomers should be aware of the problem of storing a telescope, especially a large one. Indeed, how and where to store it should be considered before buying.
The plain fact of the matter is that unless a telescope is quick and easy to set up, it WILL become disused. Even the keenest skywatchers become apathetic when they realize that setting up a large telescope is not easy.
Those residing in apartments should understand from the start that lugging equipment to an observing site usually needs several trips to-and-fro. Ideally, a telescope ought to be one that can be quickly set up in one go.
Homeowners have more choice, especially those with a backyard and perhaps a garage, to store a large telescope.
It is often challenging to locate reference stars under a light-polluted sky in towns and cities because of the usual 'washed-out' sky. However, with today's GoTo instruments, town and city folks have an easy way to find celestial objects - even from locations suffering from severe light pollution – once they know the ropes.
With a 'GoTo' instrument, there will be no need to star-hop when finding celestial objects.
However, do not be fooled into buying a cheap GoTo telescope as it usually comes with inferior optics. You get what you pay for.
WARNING: if you are new to astronomy, leave GoTo instruments alone until you have gained experience with a standard telescope. If you do not know what you are doing with a GoTo instrument, it will likely put you off for life.
Use a Tube Extension
One way to improve the view in light-polluted skies is to add a tube extension in front of the telescope to block stray light - especially when using a Newtonian reflector. Newtonian telescope focusers are close to the front of the tube, so any incoming side lighting has a chance of getting through. A tube extension painted black on the inside eliminates the problem and boosts image contrast.
Know the Exit Pupil
Using the right eyepiece is essential because successful observing is not always about magnification. However, the exit pupil is the diameter of the light beam exiting the eyepiece. You can get an idea of the exit pupil's size by aiming the telescope at the daytime sky and backing away. Look at the disk of light appearing to float inside the eyepiece. That is the exit pupil. The exit pupil size depends on the magnification used.
The formula used to work the exit pupil out is as follows: -
Exit pupil = eyepiece focal length / telescope focal ratio (f/ number)
d = fe/fnumber
For example, a 20mm focal length eyepiece on an f/8 telescope gives an exit pupil of 20/8 = 2.5mm.
Knowing the exit pupil diameter is crucial because if it is too big or too small, the view will be compromised, especially under light-polluted skies. Light pollution always reduces contrast, and if the exit pupil is too large, the contrast will nosedive. So, watch out.
If the exit pupil is too small, the view will be much too dim.
So now you know.
Attach a FULL-style eyecup to the eyepiece when observing in urban areas to shield the corner of the eye from stray light coming from nearby properties. Full-style or Wing-style eyecups are available. However, for maximum effectiveness, use a Full eyecup.
Light Pollution Filters
Light pollution filters we introduced in lesson one. They are not effective in urban areas because of the various wavelengths of artificial light used by local businesses and residents.
The filters suppress a large portion of the visible spectrum that includes sodium and mercury vapor light wavelengths while allowing others through. They always dim the view, but boost contrast, making it easier to identify particular celestial objects. They DO NOT, however, make them any brighter.
So now you know.
Binoculars are superb for observing the night sky from light-polluted sites because they reveal celestial objects washed out by urban sky-glow. Whereas it might be impossible to see stars of third magnitude and below from towns and cities, binoculars reveal objects down to seventh magnitude. (Stellar magnitudes are covered in Lesson 4.)
Observing From High-Rise Apartments
The roofs of a high-rise apartments are excellent observing sites. If you can gain access to one at night, you will be above trees and most buildings. The best thing is that you will be well above bothersome streetlights.
However, the roofs of tower blocks radiate heat, which causes air turbulence, so the seeing conditions up there might not be that good. Seeing refers to the steadiness of the atmosphere and has little to do with cloud cover. Good seeing is the absence of air turbulence.
Observing from Public Spaces
Grassy areas like public parks are away from streetlights, and the air there is usually calmer and cooler. Find the best location in the park, away from trees. Many towns and cities have astronomy clubs, with access to first-rate observing sites. Keep your fingers crossed that the sky will be clear when they have a meet and pick experienced observers' brains for hints and tips.
When to Observe from Urban Locations
The best time to observe celestial objects from towns and cities is when they are at the highest point in the night sky, where the atmosphere is less dense. Light pollution in urban areas always lessens after midnight, when businesses and residences switch off night lighting. So, the best time to do astronomy in built-up areas is during the early morning hours.
The pre-dawn night sky appears more transparent than the sky does after sunset. That is because the morning air then has lost the detrimental effects of vehicle exhausts and the previous day's heat released by concrete structures.
The air is always colder in the morning, by which time telescopes will have reached the ambient temperature. So, being early to rise is best for urban astronomers.
Observing from Indoors is Okay
Modern window glass is flat enough not to distort the view outside, so there is NO reason why binoculars cannot be used from a window in a darkened room. A telescope can be used if the window is of reasonable size and there is enough space by the window.
When behind a window with a telescope, you will not freeze like you do when outside in the cold. There is also no need to worry about tube currents caused by the telescope suddenly being exposed to cold air. Modern window glass does not really distort starlight.
From behind glass, the night sky - even though light-polluted - can be made give up its secrets.
Another benefit of observing behind glass is that sky maps and planispheres can also be comfortably consulted under red light without being blown about by the wind!
Also, recording observations in a log-book is easy. In fact, quite a bit of useful astronomy can be done, especially if the window faces south with the ecliptic in view, along which pass the Moon and planets.
Perhaps the most significant benefit of observing from indoors with a telescope is the rock-steady view due to the absence of wind. Even a cheap tripod will remain steady. You can also be observing the night sky wearing a tee-shirt while the outside air temperature plunges below zero.
There are definite advantages for indoor urban observing that most observers fail to notice. Your 'room with a view' will be your observatory, even though the night sky is not pitch dark. Rather than taking a portable instrument to a dark sky site and having to set it up in freezing air, a light-polluted sky from 'behind a window' is no detriment when it is easy to consult astronomical charts, books, and catalogs comfortably. The best bit is that you can be observing in minutes instead of hours.
Double stars can be split. Variable stars can be measured, and planetary details can be studied – all from an urban location behind glass.
A dark sky is not necessary to do amateur astronomy from towns and cities, but really the best time to observe from indoors is just before dawn. That was covered in Lesson 1. The pre-dawn sky usually offers better seeing conditions due to much-reduced traffic and human activities.
However, be aware that faint, extended celestial objects such as nebulae and galaxies are severely affected by urban light pollution because of their low surface brightness, so there is no point trying to focus on them from behind glass.
Or is there?
With determination and patience, it IS possible to make faint objects stand out against the light-polluted background when specific filters and the technique of averted vision are used. Find out about these methods later on in this course.
A Finderscope is Not Essential
A finderscope is not essential when observing under town and city night skies because of the light-polluted bright background. What is essential is the ability to locate the position of a celestial object precisely. Learn how to read sky charts and understand that the less dense atmosphere close to the zenith is pretty good for observing from urban locations.
From the zenith radially out to 25° or so, there is a window in light-polluted skies where faint celestial objects can be seen. The higher in the night sky you point your telescope, the clearer the view will be.
Watch the Weather Forecast
Do not be concerned if it is raining during the day because once the rain has gone past with its associated high winds, good observing conditions often follow. That is because air pollutants over urban areas are blown away and replaced by clearer air.
It is incredible how improved the seeing conditions and sky transparency often become after a low-pressure weather system has gone through. If it is currently raining, wait until it has stopped and the clouds have gone away. After the rain has gone, nebulae that are difficult to see through 8- or 10-inch reflectors may suddenly pop into view. The improved observing conditions also help small telescopes give better results.
The night skies of areas with frequent wet weather, such as northwest Europe and the USA's west coast, where gales are common, benefit from windy weather. So, all is not lost if your usual weather conditions are mostly wet.
Use a Nebula Filter
Urban-based amateur astronomers should always use a nebula filter to capture light emitted by emission nebulae, such as the impressive Orion nebula, Messier 42. The filters finely cut light into separate emission wavelengths:
Nebular filters give a darker sky with fainter stars while revealing the wonderful effects of electricity on space plasma.
CCD cameras (charge-coupled device) are many times more sensitive to light than the human eye or photographic film. So, once an image is captured via a CCD camera - or a DSLR camera adapted for astrophotography, it can be enhanced with image processing software.
The amazing thing is that the light-polluted sky background of urban skies can then be removed with these technological wonders. Together with a CCD or adapted digital camera, ANY telescope in a light-polluted town or city can reveal stars down to magnitude sixteen, even when the Moon is out. That is several times fainter than the same instrument can discern at dark sky sites.
Think about that.
Unfortunately, CCD cameras have narrow fields of view, so focusing them on faint celestial objects is not that easy. CCD astrophotography is difficult because of the difficulty of aiming the telescope at a precise location in the sky without an accurate 'GoTo' system. It is not easy polar-aligning indoor telescopes.
There is also the problem of focusing, especially during long exposures. Nevertheless, with patience, it is possible to obtain outstanding results with digital technology from urban locations.
NOW TAKE THE NAS TEST
HOW WELL DO YOU UNDERSTAND LIGHT POLLUTION?
DISTINCTION: 90-100% GRADE 2: 75-89% GRADE 3: 60-74% GRADE 4: 40-59%