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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/11.7. 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.
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 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.
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 60mm or 2.4 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!).
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