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. For
example, the LightBridge Mini 130 focal
length is 650mm long.
Primary mirror diameter (reflectors) or
Objective lens diameter (refractors) is how
big the mirror or lens is on your scope.
Telescopes are always described by how
large their primary mirror/lens is. For example,
the objective lens on the LightBridge Mini
130 is 130mm or 5.1 inches. Telescopes
come in many different sizes. They can be
70mm, 8 inches, 16 inches, or even 3 feet
in diameter. The Hubble Space Telescope’s
primary 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 slower
the focal ratio, the longer the exposure time
16
is needed when a camera is hooked up to the
telescope. For example, the Lighbridge Mini
130mm Reflector has fast focal ratio at f/5.
CALCULATING THE MAGNIFICATION OF
YOUR EYEPIECE
The power of a telescope is how much it
magnifies objects. Each telescope has its
own set of focal lengths and, therefore,
different magnifications when used with
various eyepieces. For example, the
Lighbridge Mini 130mm used with the 26mm
eyepiece magnifies an object 25 times. The
9mm eyepiece used with the Lighbridge Mini
130mm will magnify objects 72 times.
You can calculate how much magnification
an eyepiece will have with your specific
telescope. Just divide the telescope focal
length by the focal length of the eyepiece.
Focal Length of the Telescope
÷
Focal Length of the Eyepiece
=
Magnification
Fig. 7
eyepiece
barlow
ASTRONOMY RESOURCES
• The Meade 4M Community
27 Hubble, Irvine, CA 92618
• Astronomical League
Executive Secretary
5675 Real del Norte, Las Cruces, NM 88012
• The Astronomical Society of the Pacific
390 Ashton Ave., San Francisco, CA 94112
• The Planetary Society
65 North Catalina Ave, Pasadena, CA 91106
• International Dark-Sky Association, Inc.
3225 N. First Avenue, Tucson, AZ 85719-2103
(c) nimax GmbH
