Celestron CGE1400 Manuale di istruzioni - Pagina 6

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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.

Actual image orientation as seen

with the unaided eye

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:

Magnification =

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:

True Field = ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

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).

Reversed from left to right, as

viewed with a Star Diagonal

Figure 1-6

Focal Length of Telescope (mm)

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

Focal Length of Eyepiece (mm)

Apparent Field of Eyepiece

Magnification

the eyepiece directly in telescope

Inverted image, as viewed with