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Primary mirror diameter is how big the
mirror is on your scope. Telescopes are
always described by how large their primary
mirror is. Your telescope is 114mm or 4.5
inches. Other telescopes are 90mm, 8
inches, 16 inches, or even 3 feet in diameter.
The Hubble Telescope’s objective mirror
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/11. Sometimes, astronomers use
focal reducers to make slow exposure
telescopes have faster focal ratios.
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.
Focal Length of the Telescope
÷
Focal Length of the Eyepiece
=
Magnification
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.
15
HAVE A GOOD TIME,
ASTRONOMY IS FUN!
SPECIFICATIONS
Optical tube focal length. . 900mm
Primary mirror diameter . 114mm (4.5")
Focal ratio. . . . . . . . . . . . . . f/11
Mounting type. . . . . . . . . . . Equatorial
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.
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