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SPECIFICATIONS
Optical tube focal length . . . . . 700mm
Objective lens diameter . . 70mm (2.4")
Focal ratio . . . . . . . . . . . . . . . . . . . f/10
Mounting type . . . . . . . . . . . Altazimuth
Viewfinder Type. . . . . . . . . . . . Red dot
What do the specifications mean?
Optical tube focal length is simply a measurement of the length of the optical tube. In other words, this
is the distance light travels in the telescope before being brought to focus in you eyepiece. Your tube is
700mm long.
Objective lens diameter is how big the lens is on your scope. Telescopes are always described by how
large their objective lens is. Your telescope is 70mm or 2.75 inches. Other telescopes are 90mm, 8
inches, 16 inches, or even 3 feet in diameter. The Hubble Telescope's objective lens has a diameter of
2.4 meters (that's 7.8 feet across!).
The focal ratio helps determine how fast the photographic speed of a telescope is. The lower the focal
ratio number, the faster the exposure. f/5 is faster than f/10. The faster the ratio, the faster exposure
time is needed when a camera is hooked up to the telescope. Your telescope has slower focal ratio at
f/10. Sometimes, astronomers use focal reducers to make slow exposure telescopes have faster focal
ratios.
Altaz mounting simply means your telescope moves up and down (altitude or "alt"), and side to side,
(azimuth or "az"). Other mounting configurations are available for other telescopes, such as equatorial
mounting.
USE THE SPECIFICATIONS TO CALCULATE THE MAGNIFICATION OF YOUR EYEPIECE
The power of a telescope is how much it magnifies objects. Your 25mm eyepiece magnifies an object
28 times. Your 9mm eyepiece magnifies objects 78 times. But if you obtain other eyepieces, you can
calculate how much magnification they have with your telescope. Just divide the focal length of the
telescope by the focal length of the eyepiece.
F
L
T
÷ F
L
E
= M
OCAL
ENGTH OF THE
ELESCOPE
OCAL
ENGTH OF THE
YEPIECE
AGNIFICATION
Look at the specifications. You will see that the focal length of your scope is 700mm
Let's say that you
.
have obtained a 13mm eyepiece. You can tell that what the focal length of your eyepiece is as it is
always printed on the side of an eyepiece. Divide: 700 ÷ 13, which equals 53.8. Round this off to the
nearest whole number and your new eyepiece magnifies objects 54 times.
If you use your Barlow lens with one of your eyepieces, it doubles the magnification of your eyepiece.
Other types of Barlows can triple or further increase the power of an eyepiece. To find out how much
your magnification is when you use a Barlow, multiply your eyepiece's magnification by two.
E
'
2 = M
2X B
YEPIECE
S MAGNIFICATION X
AGNIFICATION WITH A
ARLOW LENS
Your 25mm low-power eyepiece magnifies an object 28 times. Multiply 28 by 2 and you get 56 times
magnification with your Barlow.
It's worth repeating: Keep in mind that a bright, clear, but smaller image is more interesting than a
larger, dimmer, fuzzy one. Using too high a power eyepiece is one of the most common mistakes made
by new astronomers. So don't think that higher magnification is necessarily better—quite often the best
view is with lower magnification value!
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