Urban Astronomy

If you have been in the country far from any human-made light source you will know that the night sky is a wondrous, luminous marvel, the sky ablaze with light.  Bright stars and planets are caught in a glowing net of dimmer stars and nebulae.


Unfortunately in our cities we make light to excess like we do most other things, and this pollution blots out all but the nearer planets and the brightest of stars. This means doing some serious stargazing is impossible without a pair of binoculars or a telescope, but even with these essential tools you need know a few tricks of the trade to get the best results.


But for amateur astronomers to win the war against artificial light they need to have some understanding of its nature. There are basically two types of light pollution: sky glow and localized glare, also known as line-of-sight glare. Sky glow is an orange color, and is the combined total visible radiation of many thousands of lights from buildings, cars and streetlights. This results in any dim stars being impossible to see with the naked eye.


Localized glare occurs in any settlement including very small towns.  These can be any badly situated lights such as those shining from inside houses and porch lights. Very close, bright streetlights can produce localized glare. Unlike sky glow it doesn’t obscure all the dim stars, but the glare can shine in the eyes of the observer, preventing him or her from seeing much.


Regardless of the amount of light pollution, the larger the telescope you have the more you can see. However, larger apertures can be more affected by heat distortions. So you may want to consider buying a larger telescope, but apart from being more expensive, they are also heavier to carry around.


The use of computerized telescope mounts can greatly assist you in finding the particular stars you want to observe, but this facility may compromise the resolution and focusing functions of the telescope.


If you already have a telescope of a reasonable quality there is no reason to buy another. You can optimize the one you’ve got. You can put a tube extension in front of the telescope, which can slow down the condensation of dew on to the refractor lens or the corrector plate. This can particularly assist with telescopes using Newtonian reflectors, in which the focuser is close to the tube’s front, resulting in light shining into the focuser.  If the tube is extended by at least one diameter this problem is eliminated, as long as the inside of the extension is painted black.


Choose your telescope on the basis of exit pupil, which is the diameter of the beam of light exiting from the eyepiece, rather than on magnification. You need an eyepiece with the right exit pupil. You can calculate the exit pupil by dividing the diameter of the objective lens or primary mirror by the magnification. The exit pupil will decrease as the magnification increases, so it’s a trade-off. An alternative calculation method is to divide the eyepiece’s focal length by the focal ratio of the telescope. To see the exit pupil aim the telescope at a reflective surface or a light, and step back until you can see the light disk just above the lens. The disk is the diameter of the exit pupil. Never aim any magnifying lens at the Sun.


Obviously you need to keep your telescope clean, in particular the outer eye lens, which can pick up oils from you. You should use specialized lens tissue and dampen it with a lens-cleaning solutions. Then wipe gently clean with the tissue. You can also use lens pens, but only apply light pressure to avoid scratches on the optical coatings.


You may be tempted to use light-pollution reduction filters (LPR), which reflect the broad part of the visual spectrum and let the rest through. However they don’t catch building lights, headlights, and incandescent lights.


If you have access to a roof this can be a good place to use a telescope, as many obstructions can be avoided, as well as low-level lighting. However because roofs heat up easily in the sun, they can release this heat at night, causing distortion of your view.


A park may be a good place to observe from, because of the lack of light, but never do this alone, because of the safety risk. It’s better to stargaze in a group of at least four adults.


To avoid localized glare you can build barriers such as black terrycloth, fixed on a PVC frame. If this isn’t feasible, you can put a dark towel over your head when observing through the telescope. Light pollution may drop after midnight, so stay up and see what happens, as long as you don’t need to get up early for work!

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