Celestron AstroMaster LT 76AZ Telescope AstroMaster LT Series Manual English - Page 12

Determining Field Of View, General Observing Hints

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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 = Apparent Field of Eyepiece 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 20mm eyepiece that is supplied standard with the AstroMaster LT 60AZ telescope. The 20mm eyepiece has an apparent field of view of 50°. Divide the 50° by the magnification, which is 30 power. This yields an actual field of 1.7°. 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 of 1.7° by 52.5. This produces a linear field width of 89 feet at a distance of one thousand yards. GENERAL OBSERVING HINTS When working with any optical instrument, there are a few things to remember to ensure you get the best possible image. • Never look through window glass. Glass found in household windows is optically imperfect, and as a result, may vary in thickness from one part of a window to the next. This inconsistency can and will affect the ability to focus your telescope. In most cases you will not be able to achieve a truly sharp image, while in some cases, you may actually see a double image. • Never look across or over objects that are producing heat waves. This includes asphalt parking lots on hot summer days or building rooftops. • Hazy skies, fog, and mist can also make it difficult to focus when viewing terrestrially. The amount of detail seen under these conditions is greatly reduced. • If you wear corrective lenses (specifically glasses), you may want to remove them when observing with an eyepiece attached to the telescope. When using a camera, however, you should always wear corrective lenses to ensure the sharpest possible focus. If you have astigmatism, corrective lenses must be worn at all times. 12

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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 ex-
ample in the previous section, we can determine the field of view using the same 20mm eyepiece that is
supplied standard with the AstroMaster LT 60AZ telescope.
The 20mm eyepiece has an apparent field of
view of 50°.
Divide the 50° by the magnification, which is 30 power.
This yields an actual field of 1.7°.
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 of 1.7° by 52.5.
This produces a linear field
width of 89 feet at a distance of one thousand yards.
GENERAL OBSERVING HINTS
When working with any optical instrument, there are a few things to remember to ensure you get the best
possible image.
•
Never look through window glass.
Glass found in household windows is optically imperfect, and as
a result, may vary in thickness from one part of a window to the next.
This inconsistency can and will
affect the ability to focus your telescope.
In most cases you will not be able to achieve a truly sharp
image, while in some cases, you may actually see a double image.
•
Never look across or over objects that are producing heat waves.
This includes asphalt parking lots on
hot summer days or building rooftops.
•
Hazy skies, fog, and mist can also make it difficult to focus when viewing terrestrially.
The amount of
detail seen under these conditions is greatly reduced.
•
If you wear corrective lenses (specifically glasses), you may want to remove them when observing
with an eyepiece attached to the telescope.
When using a camera, however, you should always wear
corrective lenses to ensure the sharpest possible focus.
If you have astigmatism, corrective lenses
must be worn at all times.
12