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03
TELESCOPE BASICS
C14 • Dumbbell Nebula • Image courtesy of Ted Inoue
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TELESCOPE CHARACTERISTICS
This section looks at the separate component parts of a telescope. This will help
you in making important decisions when selecting a telescope model. This section
also discusses the uses of a telescope for astronomy or terrestrial (land viewing)
use. It is important to read this section if you aren't sure which Celestron
telescope product is right for you.
What Is A Telescope?
A telescope is, simply, a light collector. Its main task is to form the brightest
possible optical image of the object on which it is focused. This task is
accomplished by the primary optical element, called the “primary” or “objective”
inside the telescope’s optical tube. “Primary” usually refers to the mirror in a
reflecting telescope while “objective” refers to the main lens of a refracting
telescope. The image formed by the primary is then magnified by a removable
component called an eyepiece. By using different eyepieces, you can change the
magnification and the field of view of what you see through the telescope.
When comparing telescopes, there are a number of characteristics that can help
identify their differences. The most common ones are: Light Gathering Power,
Limiting Magnitude, Resolution, and Magnification. No matter what types of optical
designs you compare, these characteristics provide valuable information that will
help you determine what you can expect to see through a telescope.
Light Gathering Power
The most important characteristic of a telescope is its light gathering ability.
The light gathering capability of a telescope is determined by the diameter of its
aperture. The larger the aperture, the more light it collects. When looking at a star,
nebula, or galaxy it’s especially important that as much light be gathered as
possible. Fainter celestial objects may be invisible to smaller aperture telescopes.
Without enough light, dim objects cannot be seen, no matter how much they
might be magnified!
The relationship between a telescope’s light gathering power and the diameter of
its lens or mirror is not directly proportional. As the diameter gets larger, the
amount of light gathered increases by the square of the diameter. So if you double
the diameter of the primary lens, its light gathering ability increases by four times!
To calculate the light collecting area:
Light Collecting Area =π
r
2
(where π= 3.1416 and r = the radius of the primary
For an 8” primary: Light Collecting Area = 3.1416 x 4
2
= 50.24 square inches
For an 4” primary: Light Collecting Area = 3.1416 x 2
2
= 12.56 square inches
Limiting Magnitude
Astronomers use a system of “magnitudes” to indicate the brightness of a stellar
object. An object is said to have a certain numerical magnitude with 0 (the star
Vega ) as the baseline. The larger the magnitude number, the fainter the object.
Each magnitude is a difference in brightness by a factor of 2.51 times. For
example, a star that is considered 5th magnitude is 100 times fainter than Vega,
a zero magnitude star (2.51
5
). The faintest star you can see with your unaided eye
is about sixth magnitude (from dark skies) whereas the brightest stars are
magnitude zero (or even a negative number).
The faintest star you can see with a telescope (under excellent seeing conditions)
is referred to as the “limiting magnitude”. The limiting magnitude of a telescope
is directly related to aperture. Larger apertures allow you to see fainter objects.
A rough formula for calculating the visual limiting magnitude of any telescope is:
7.5 + 5 LOG (aperture in cm). For example, the limiting magnitude of an 8”
(20.32cm) aperture telescope is 14.0.
Limiting Magnitude =7.5 + 5 LOG 20.32 = 7.5 + (5x1.3) = 14.0.
Atmospheric conditions and the visual acuity of the observer will often reduce
limiting magnitude.
Resolution
Resolution is the ability of a telescope to render fine detail. Higher
resolution lets you see more detail on the surface of a planet or
separate stars that are close together. Resolution is measured in terms
of degrees of arc (called degrees), minutes of arc (called arcminutes),
and seconds of arc (called arcseconds). Thus, something that spans
one degree of arc is also 60 arcminutes, or 3600 arcseconds (60 x
60). So, something that is one arcsecond is very small — only
1/3600th of a degree.
Resolution for a given telescope is calculated using the formula:
Resolution = ((1.22 x l ) / D) x 206265 where resolution is in
arcseconds, “l” is the wavelength of light (.00055mm for common
visible wavelengths), “D” is the diameter of the primary in millimeters
and 206265 is the number of arcseconds in one Radian.
Magnification
Magnification is frequently referred to as “power” and is a function
of the focal lengths of both the primary and the eyepiece. The focal
length is the distance from the primary lens or mirror to the point
where an image is formed. The eyepiece magnifies the image formed
by the primary.
The highest magnification you can reasonably achieve with your
telescope is once again determined by the size and light gathering
ability of the primary. The practical limit is about 60 times the diameter
of the primary in inches. So, an 8” telescope should not be expected
to produce reasonable images if the telescope/eyepiece combination
produces a magnification greater than 480x. In practice, the amount
of magnification that can be used will often be reduced by
atmospheric conditions.
Since many astronomical objects are relatively large but faint,
moderate magnification and a larger diameter primary to gather light
is the best combination for viewing most celestial objects. When
looking at stars, high power is of little use since they always look like
pinpoints (stars are so far away they can not be resolved as anything
other than a pinpoint).
To calculate magnification:
(Focal Length of Primary in mm)
Magnification = —————————————
(Focal Length of Eyepiece in mm)
So, for a 8” SCT using a 25mm eyepiece:
Magnification = 2000/25 = 80 times the power of the unaided eye.
The Mount and Tripod
To a large extent, a telescope is only as good as its tripod and mount.
A telescope magnifies everything, including vibration. That’s why many
telescopes with good optics are rendered useless when supplied on an
inexpensively made mount. Since you’ll be using a mount’s controls to
track the slow and steady apparent movement of the stars, a suitable
mount’s adjustments should be smooth, yet precise.
Altazimuth vs. Equatorial
There are two basic types of mounts: Altazimuth (Alt-Azimuth) and
Equatorial. Altazimuth mounts are the simplest type of mount with two
motions: altitude (up and down/vertical) and azimuth (side-to-
side/horizontal). Good Altazimuth mounts will have slow motion cable
controls to make precise adjustments, which aid in keeping tracking
motion smooth.
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