Tuesday, September 18, 2012

Advice for First Time Telescope Buyers...Contd (2)

Number Two: Don't blame the telescope for things it can't control!

Many beginners don't realize that a telescope's performance is often at the mercy of local ambient conditions. Other than the obvious (clouds, fog, etc.), there are several other major factors which limit how much can be seen.

Light pollution is by far the biggest problem facing today's astronomers. Light Pollution has (for all practical purposes) completely ruined the skies in and around of just about every moderate to large city (this means that unless you live out in the country the only things you'll probably be able to see well are the Moon and bright planets). Light pollution is caused by excessive amounts of and/or poorly designed outdoor lights. Light pollution will not degrade the viewing of the Moon and planets, but it can seriously limit the number of non-solar system objects you might otherwise see (objects such as galaxies, star clusters and nebulae). Light pollution has invaded almost all populated areas of the country. Often, the only remedy is to drive to a dark location, generally 50 to 100 miles or so from any major city (and even at this distance evidence of light pollution may remain readily visible). For more information on light pollution, see my article dedicated to this topic: Light Pollution. For a very comprehensive treatment of light pollution, see the International Dark Sky Organization's web site at www.darksky.org.

The next problem that impacts astronomy (but not nearly as much as light pollution) is known as seeing conditions. When you look at an astronomical target, you are seeing it through the Earth's atmosphere, which essentially is an "ocean of air". Very often the atmosphere is highly unsteady, due to thermal variations in the upper atmosphere, air currents near the ground, etc. All of this means that an image passing though it will be distorted to some degree. Have you ever looked at something across a hot asphalt parking lot in the summer? The objects in the distance seem to shimmer. Now, imagine looking throughmiles of turbulent air at high magnification! In short, bad seeing conditions can severely limit the amount of fine detail you can see on the Moon and planets (fortunately bad seeing has much less effect on galaxies, star clusters and nebulae). Seeing conditions are often a function of where you live. Areas like Florida are known for good seeing (New England seeing is often notoriously poor). Also, avoid setting up a telescope in a manner that requires you to look at objects over a house, parking lot, etc. The heat that such items absorb during the day gets radiated at night and can fowl the seeing conditions even more! To see what bad seeing looks like, take a look at this video clip of the Moon (taken through a telescope) during poor seeing conditions. See how it appears to shimmer...

Finally, it is important to give a telescope a chance to cool down to the outside temperature (especially for Newtonian Reflectors and Schmidt Cassegrain type telescopes). Cooling down of the scope is calledthermal stabilization. If a telescope is brought from a warm house out to the cold night, the images seen through it are likely to be very poor at first, perhaps to the point where the telescope won't even seem to focus at all. This is because the optics in the telescope are undergoing a change in size and shape due to the temperature difference. The actual change in the optics is extremely small in human terms; however, at the wavelengths of light, it is very significant. Bottom line: Make sure a telescope has time to cool down to the outside temperature before expecting it to perform at its best. Acceptable performance is usually reached with 15 to 20 minutes, but the very best performance may take one hour or more (depending on the temperature differential and the type of telescope). Storing a telescope in an unheated garage may help to shorten the time it takes to cool; alternatively, set the telescope outside an hour or so before you plan to use it. .

Credit and Source:
Joe Roberts

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