Celestron C11-SGT User Manual download pdf (Page 10)

8 Intro to Telescopes

TelesCopes and how They work

A telescope’s main task is to collect light to form the brightest possible optical

image of the object it is focused on. This 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 mirror is magnified

by an easily removable component called an eyepiece. By using

different eyepieces, you can easily change the magnification

and the field of view of the image through your telescope.

There are several individual characteristics to help identify

their differences, the most common differences are: Light

Gathering Power, Limiting Magnitude, Resolution and

Magnification. These crucial characteristics provide the

most valuable information to help you easily determine

what you can expect to see through a telescope.

lIght gatherIng power

The most important characteristic of a telescope is

its ability to gather light, which is determined by the

diameter (or aperture); the larger the aperture, the more

light it collects. Objects too faint, such as nebulae and

galaxies, may not be seen by smaller aperture telescopes

no matter how much it is magnified. A telescope’s light

gathering power is

directly related to the

diameter of its lens

(mirror). As the diameter

increases, the light gathering

power increases by the square

of the diameter. If you double

the diameter of the primary lens,

the light gathering ability increases

by four times!

lImItIng magnItude

Astronomers use “magnitudes” to indicate the

brightness of a stellar object which determines what

can be detectable by the instrument. The larger the

magnitude number the fainter the object is and each

magnitude is a difference in brightness by a factor of

2.51 times. For example, a star that is considered 5th

magnitude is 100x fainter than Vega, a 0 magnitude

star (2.51

). With your own unaided eyes, the faintest

star you could see is about 6th 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

its aperture.

resolutIon

The ability of a telescope to render fine detail; higher resolution gives you more detail on

the surface of a planet or separate stars that are close together. Resolution is measured

in terms of degrees, minutes of arc (arcminutes), and seconds of arc (arcseconds). Thus,

something that spans one degree is also 60 arcminutes, or 3600 arcseconds (60x60).

So, something that is one arcsecond is very small – only 1/3600th of a degree.

magnIfICatIon

Frequently referred to as “power” and is a function of the telescope’s focal length and

the eyepiece’s focal length. The focal length is the distance from the primary lens to

the point where the image is formed; the eyepiece magnifies the image. The highest

magnification you can achieve with a telescope is determined by the size and light

gathering ability of the primary lens. The practical limit is about 60x the diameter of

the primary lens (in inches). Since many astronomical objects are relatively large but

faint, medium magnification and a larger diameter primary lens to gather light is the

best combination. When looking at stars, high power is of little use, with the exception

of Binary Stars, since they always look like pinpoints and cannot be resolved as

anything else.

magnIfICatIon formula:

Magnification = Focal Length of Primary in mm / Focal Length of Eyepiece in mm

So, a telescope with a focal length of 2000 mm, using a 25 mm eyepiece:

Magnification = 2000/25 = 80x the power of the unaided eye.

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