Tel70AZAR_5 3/28/07 10:09 AM Page 14
Astronomy and Sky and Telescope magazines.
12HAVE A GOOD TIME, ASTRONOMY IS FUN!
SPECIFICATIONS |
|
Optical tube focal length | 700mm |
Objective lens diameter | 70mm(2.8") |
Focal ratio | f/10 |
Mounting type | Altazimuth |
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 2.8 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.
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.
Looking at or near the Sun will cause irreversible damage to your eye. Do not point this telescope at or near the Sun. Do not look through the telescope as it is moving.
