"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
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.
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).
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
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
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
Also consider the type and length of the product
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
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
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.
We hope this discussion helps in understanding
telescopes and astronomy in general and that you will begin the lifetime
enjoyment of our fascinating universe.