Frequently
Asked Questions
Q: How far can I see with a telescope?
A: Since the distance of a celestial object generally
corresponds to its brightness, the telescope with the largest aperture
(size of the main mirror or lens) will not only allow you to see
the faintest objects but also the most distant objects. For example,
through a 60mm refracting telescope you can see the bright center
of the Andromeda galaxy which is more than 2 million light years
away. With an 8" (200mm) telescope or larger, you can see the
spiral arms of the much fainter Whirlpool galaxy, which is 35 million
light years away! When using a telescope for daytime, terrestrial
observing, the distance you can see is limited by the condition
of the atmosphere through which you are viewing. High amounts of
moisture in the air or heat waves radiating from the ground will
give you a blurry image over great distances.
Q: How do you determine the power of a telescope?
A: The magnifying power of a telescope will change
depending on the focal length of the eyepiece you are using (see
question on determining a telescope's magnification below). A more
appropriate question may be to ask how much light-gathering power
does a telescope have. Light gathering power is a telescope's ability
to see faint, distant objects (see question above) and is solely
determined by the aperture of the telescope's main mirror or lens.
The larger the aperture, the more light-gathering power a telescope
has. Remember it's the entire surface area of the telescope's main
mirror or lens that is collecting the light; therefore every time
a telescope's aperture is doubled, the light-gathering power is
increased by a factor of 4 times!
Q: How do you determine a telescope’s magnification?
A: The magnification of a telescope changes as
the eyepiece is changed. Magnification can be calculated by dividing
the focal length of your telescope by the focal length of the eyepiece.
Always start with your lowest magnification (longest focal length)
eyepiece and work upward from there. A 2x Barlow lens will double
the magnification of whatever eyepiece you use with it. For example:
using a telescope with a 900mm focal length with a 20mm eyepiece
will give you 45x magnification. Using the same telescope and eyepiece
with a 2x Barlow lens will give 90 x magnifications. This would
be the same magnification as a 900mm telescope with a 10mm eyepiece.
Q: How can I clean the corrector plate on my Schmidt-Cassegrain
telescope?
A: The outside of the corrector plate can be cleaned
using a solution of 60% distilled water and 40% isopropyl alcohol
with one drop of clear liquid dish soap per quart of solution. The
solution is applied to a soft tissue or cotton ball and is carefully
rubbed onto the corrector plate using radial strokes from the secondary
mirror housing outward. We do not recommend removing the corrector
plate. The corrector plate is optically matched to the mirrors in
alignment and in rotation.
Q: Everything I see through my refractor is an inverted
image. How can I correct the image?
A: To correct an inverted image, you will need
a 90° star diagonal. When a 90° star diagonal is used, the
mirror flips the image over giving a right side up but reversed
left to right image. You can achieve a fully corrected image using
an erect image prism diagonal.
Q: Why can't I see anything in my telescope when I have
the moon centered in the cross hairs of my finder scope?
A: The most likely cause of the problem is that
your finder scope is not aligned with the main optics. To align
your finder scope, take your scope outside in the daylight and find
an object at least a half mile away as far away as possible (a street
sign, telephone pole, radio tower, mountain top, etc.). Center this
object in your main telescope using your lowest power eyepiece.
Now look through your finder scope. Using the three adjustment screws
that secure the finder scope into its bracket, tighten or loosen
them as needed to bring the cross hairs onto the same target as
is centered in your main telescope. Your scope is now ready to be
aimed at any object in the sky and it will easily be located through
the main optics of your scope.
Q: I have a 60mm refractor with a focal length of 900mm.
Why can't I get a clear image when I try to use my 2x Barlow lens
with my 6mm eyepiece?
A: All telescopes have a maximum magnification
of 60x per inch of aperture (under ideal conditions). Your telescope
would reach this limit at about 140x magnification. The combination
you are using gives 300x magnification (see formula above). Exceeding
the maximum usable limit will cause the image to degrade, becoming
dark and washed out. The Earth's atmosphere also plays an important
part in limiting the maximum magnification you can use. Instabilities
in the atmosphere such as heat radiating from the ground and surrounding
buildings, high altitude winds, and other weather conditions can
cause your image to blur. This also explains why bright stars appear
to twinkle. The best time to use high magnification is on nights
when the stars do not appear to twinkle very much.
Q: Why don’t the images I see through my telescope
look the same as photos I see taken with the same type of telescope?
A: Many pictures you see in magazines and catalogs
are time exposures. The camera shutter is kept open for several
minutes while the telescope tracks the object across the sky. This
allows the film to record fainter detail and colors that can not
be seen with the naked eye. In order to get similar results with
your telescope, you need a camera adapter for a 35mm camera and
a clock motor.
Q: What is the difference between an equatorial mounted
telescope and an Altazimuth mounted telescope?
A: A mount is said to be "equatorial"
if one of its axes can be made parallel to the Earth's axis of rotation
(called the Celestial Pole). This is essential if you want to track
an object over a long period of time as it moves across the sky.
With an equatorial mount the motion of the sky can be cancelled
out by simply turning its axis at the same rate as the Earth's rotation,
but in the opposite direction. Equatorial mounts are necessary for
long exposure astrophotography and for using setting circles to
locate objects. Altazimuth (short for Altitude/Azimuth) mounts are
the simplest type of mount. This type of mount moves in two directions,
altitude (up and down) and azimuth (side-to-side). Due to their
ease of use, Altazimuth mounts are suited both for terrestrial viewing
as well as astronomical observing. Ref 1
Definitions
http://www.celestron.com/tb-trms.htm#APERTURE
http://www.celestron.com/tb-2ref.htm#REFRACTOR
Telescope Reviews
http://www.cloudynights.com/
http://www.scopereviews.com/
http://www.astromart.com/
Reviews and Classifieds
Database for 88 Constellations
http://www.ne.jp/asahi/stellar/scenes/data_e/const88.htm
Other interesting links
http://www.heavens-above.com/
http://www.darksky.org/
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