– 10 –
Looking at or near the
Sun
will cause
irreversable
damage to your eye. Do not point this telescope at or near the Sun. Do not look through the telescope as it is moving.
SPECIFICATIONS
Optical tube focal length . . . . . 900mm
Objective lens diameter . . 90mm (3.5")
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 900mm 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 90mm or 3.5 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
36 times. Your 9mm eyepiece magnifies objects 100 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
OCAL
L
ENGTH OF THE
T
ELESCOPE
÷ F
OCAL
L
ENGTH OF THE
E
YEPIECE
= M
AGNIFICATION
Look at the specifications. You will see that the focal length of your scope is 900mm
.
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: 900 ÷ 13, which equals 69.2. Round this off to the
nearest whole number and your new eyepiece magnifies objects 69 times.
If you use a 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
YEPIECE
’
S MAGNIFICATION X
2 = M
AGNIFICATION WITH A
2X B
ARLOW LENS
Your 25mm low-power eyepiece magnifies an object 36 times. Multiply 36 by 2 and you get 72 times
magnification with a 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!
