Celestron CR-150 HD - Manual (Page 26)

26 • Telescope Basics
You can change the power of your Celestron CR-150 HD telescope just by chang-
ing the eyepiece (ocular). To determine the magnification for your telescope, you
would simply divide the focal length of the telescope (1200mm) by the focal length
of the eyepiece that you are using. In equation format, the formula looks like this:
Focal Length of Telescope (mm)
Magnification = ————————————————
Focal Length of Eyepiece (mm)
Let’s say, for example, that you are using a 20mm eyepiece. To determine the
magnification, simply divide the focal length of your telescope (1200mm) by the focal
length of the eyepiece (20mm). Dividing 1200 by 20 yields a magnification of 60
power.
Although the power is variable, each instrument has a limit to the highest
useful magnification. The general rule is that 60 power can be used for every
inch of aperture. For example, the CR-150 HD is 6" in diameter. Multiplying 6
by 60 gives a maximum useful magnification of 360 power. Although this is the
maximum useful magnification, due to limiting atmospheric conditions most
observing is done in the range of 20 to 35 power for every inch of aperture which is
120 to 210 power for the CR-150 HD.
Determining the field of view is important if you want to get an idea of the size of the
object you are observing. To calculate the actual field of view, divide the apparent
field of the eyepiece (supplied by the eyepiece manufacturer) by the magnification. In
equation format, the formula looks like this:
Apparent Field of Eyepiece
True Field = ———————————————
Magnification
Using the example we started with above, we can determine the field of view
using the same 20mm eyepiece. The 20mm eyepiece has an apparent field of view of
52°. Divide the 52° by the magnification, which is 60 power. This yields an actual
field of .87°, or a little more than three quarters of a degree.
For terrestrial viewing, field size is often referred to as feet at a thousand yards. To
convert this to feet at one thousand yards, multiply the actual field of .87° by 52.5.
This produces a field width of 45.7 feet at one thousand yards.
The apparent field of each eyepiece that Celestron manufacturers is found in
the Celestron Accessory Catalog (#93685).
Determining Field of
View
Calculating Magnification