6
Actual image orientation as seen
with the unaided eye
Inverted image, as viewed with
the eyepiece directly in telescope
Reversed from left to right, as
viewed with a Star Diagonal
Image Orientation
The image orientation changes depending on how the eyepiece is inserted into the telescope. When using the star diagonal, the
image is right-side-up, but reversed from left-to-right (i.e., mirror image). If inserting the eyepiece directly into the visual back
(i.e., without the star diagonal), the image is upside-down and reversed from left-to-right (i.e., inverted). This is normal for the
Schmidt-Cassegrain design.
Calculating Magnification
You can change the power of your telescope just by changing the eyepiece (ocular). To determine the magnification of your
telescope, simply divide the focal length of the telescope by the focal length of the eyepiece used. In equation format, the
formula looks like this:
Focal
Length
of
Telescope
(mm)
Magnification
=
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
Focal
Length
of
Eyepiece
(mm)
Let’s say, for example, you are using the 40mm Plossl eyepiece. To determine the magnification you simply divide the focal
length of your telescope (the C8 OTA for example has a focal length of 2032mm) by the focal length of the eyepiece, 40mm.
Dividing 2032 by 40 yields a magnification of 51 power.
Although the power is variable, each instrument under average skies 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 C8 is 8 inches in diameter. Multiplying 8 by 60
gives a maximum useful magnification of 480 power. Although this is the maximum useful magnification, most observing is
done in the range of 20 to 35 power for every inch of aperture which is 160 to 280 times for the C8 telescope.
Determining Field of View
Determining the field of view is important if you want to get an idea of the angular 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
As you can see, before determining the field of view, you must calculate the magnification. Using the example in the previous
section, we can determine the field of view using the same 40mm eyepiece. The 40mm Plossl eyepiece has an apparent field of
view of 46°. Divide the 46° by the magnification, which is 51 power. This yields an actual field of .9°, or nearly a full degree.
To convert degrees to feet at 1,000 yards, which is more useful for terrestrial observing, simply multiply by 52.5. Continuing
with our example, multiply the angular field .9° by 52.5. This produces a linear field width of 47 feet at a distance of one
thousand yards. The apparent field of each eyepiece that Celestron manufactures is found in the Celestron Accessory Catalog
(#93685).
Figure 1-6
