Telescope Optical terms & Characteristics
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Telescope General Information


"Seeing" is the term astronomers use to describe the sky's atmospheric conditions. The atmosphere is in continual motion with changing temperatures, air currents, weather fronts and dust particles. These factors cause the star images to twinkle. If the stars are twinkling considerably we have "poor" seeing conditions and when the star images are steady we have "good" seeing conditions. Poor seeing is most noticeable when observing planets and the moon, whereas deep sky objects such as nebulae and galaxies are less affected by poor seeing conditions. On deep sky objects, the most important factor is the transparency of the atmosphere (a measure of how dark the sky is on a given night-determined by clouds, dust, haze and light pollution). Seeing conditions and transparency will vary widely from site to site, from season to season and from night to night.

Some manufacturers of small aperture telescopes would like you to believe that they can routinely outperform larger aperture telescopes because of atmospheric turbulence (poor seeing conditions). Occasionally this may be true on planets and the moon (you can stop down the larger aperture simply with cut-out masks to alleviate this problem), but it is never true on deep sky objects (nebulae, galaxies and star clusters) where maximum aperture is needed.

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This is a very important factor in choosing a telescope. If you live in a city polluted with lights you may want to transport your telescope to a dark sky location. If you live in a dark sky location you may have to take the equipment out and set it up. Consider the overall weight and bulk that you will be working with. If you are fortunate enough to have a telescope permanently mounted (or set up), then you should consider the largest aperture telescope you can afford (albeit still considering which type of telescope design fits your needs).

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Look for a telescope that can grow along with you as your experience and interest expand. Make sure the manufacturer has a complete line of accessories so that your telescope and your fun are not limited by lack of equipment. Most manufacturers offer accessories that may be added on at a later time.

If you want maximum versatility, consider that some telescopes are multipurpose for the following- (1) terrestrial viewing, (2) terrestrial photography with the attachment of a 35mm SLR camera, (3) astronomical observing and (4) astronomical photography (astrophotography).

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Most manufacturers are reputable and make good quality products. However, even with the same optical design and same type of mount there are distinct differences between similar units. You need to inspect the units and rely on the advice of telescope dealers, educators, members of astronomy clubs or professional astronomers.

Another very important point is the after-purchase service. Does the manufacturer have a technically competent staff to answer your questions? Can you later purchase an assortment of accessories to fulfill your expanding interest? If you have equipment problems, can you get them repaired promptly?

Also consider the type and length of the product warranty.

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The celestial coordinate system is an imaginary projection of the Earth's geographical coordinate system onto the celestial sphere which seems to turn overhead at night. This celestial grid is complete with equator, latitudes, longitudes and poles.

The Earth is in constant motion as it rotates on its axis. Actually the celestial-coordinate system is being displaced very slowly with respect to the stars. This is called precession and is caused by gravitational influences from the Sun, moon and other celestial bodies.

The celestial equator is a full 360-degree circle bisecting the celestial sphere into the northern celestial hemisphere and the southern celestial hemisphere. Like the Earth's equator, it is the prime parallel of latitude and is designated 0 degree.

The celestial parallels of latitude are called "coordinates of declination (Dec.)," and like the Earth's latitudes they are named for their angular distances from the equator. These distances are measured in degrees, minutes and seconds of arc. There are 60 minutes of arc in each degree, and 60 seconds of arc in each arc minute. Declinations north of the celestial equator are "+" and declinations south are "-". The North Pole is +90 degrees and the South Pole is -- 90 degrees.

The celestial meridians of longitude are called "coordinates of right ascension (R.A.)", and like the Earth's longitude meridians they extend from pole to pole. There are 24 major R.A. coordinates, evenly spaced around the 360 equator, one every 15 degrees. Like the Earth's longitudes, R.A. coordinates are a measure of time as well as angular distance. We speak of the Earth's major longitude meridians as being separated by one hour of time because the Earth rotates once every 24 hours (one hour = 15 degrees). The same principle applies to celestial longitudes since the celestial sphere appears to rotate once every 24 hours. Right ascension hours are also divided into minutes of arc and seconds of arc, with each hour having 60 minutes of arc and each arc minute being divided into 60 arc seconds.

Astronomers prefer the time designation for R.A. coordinates even though the coordinates denote locations on the celestial sphere, because this makes it easier to tell how long it will be before a particular star will cross a particular north-south line in the sky. So, R.A. coordinates are marked off in units of time eastward, from an arbitrary point on the celestial equator in the constellation Pisces. The prime R.A. coordinate which passes through this point is designated "0 hours 0 minutes 0 seconds." We call this reference point the vernal equinox where it crosses the celestial equator. All other coordinates are names for the number of hours, minutes and seconds that they lag behind this coordinate after it passes overhead moving westward.

Given the celestial coordinate system, it now becomes possible to find celestial objects by translating their celestial coordinates using telescope-pointing positions. For this you use setting circles for R.A. and Dec. to find celestial coordinates for stellar objects which are given in star charts and reference books.

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We hope this discussion helps in understanding telescopes and astronomy in general and that you will begin the lifetime enjoyment of our fascinating universe.

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