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ENGLISH I 11
YOUR FIRST ASTRONOMICAL OBSERVING SESSION
THE MOON
Now you are ready to take your telescope out at night and do some real observing!
Let’s start with the Moon. The Moon takes about one month to go through a complete phase cycle, from new Moon to full
Moon and back again. Try observing it at different points during this cycle.
While you can observe the Moon any time it is visible in the sky, the best time to view it is from two days after a new Moon up to
a few days before a full Moon. During this period, you will be able to see the most detail in craters and lunar mountain ranges.
Consult a calendar or moon phase app to find out when the next new Moon will be.
1. With a clear view of the Moon, set up your telescope with the 20mm eyepiece.
2. Turn on the finderscope and look through it to find the red dot.
3. Move the telescope until you can see the Moon through the finderscope’s window and the red dot is centered on the Moon.
4. Look through the 20mm eyepiece. Gently turn the focus knobs to adjust the sharpness of the image.
CONGRATULATIONS! YOU HAVE NOW OBSERVED YOUR FIRST CELESTIAL OBJECT!
You can view many other celestial objects, such as planets, star clusters and nebulae using this same basic technique.
CELESTIAL OBSERVING TIPS
This section covers visual observing hints for both solar system and deep-sky objects as well as general observing
conditions which will affect your ability to observe.
MAGNIFICATION
You can change the magnification of your telescope by simply changing eyepieces. Both telescopes and eyepieces
have focal lengths that are measured in millimeters. To calculate the magnification of a given telescope and eyepiece
combination, take the focal length of the telescope and divide it by the focal length of the eyepiece. The resulting number
is the magnification factor.
For example, the AstroMaster 90AZ has a focal length of 1000mm. Using the 20mm eyepiece:
1000mm
=
____________
= 50x
20mm
Focal length of Telescope
Magnification =
_________________________
Focal length of Eyepiece
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 it gives you in the scope you are using.
Using the example in the previous section, we can determine the field of view using the same telescope and 20mm
eyepiece. The 20mm eyepiece has an apparent field of view of 40°.
40°
=
______
= 0.9 degrees of arc
50
Apparent Field of View of Eyepiece
Actual Field Of View =
________________________________
Magnification
The 20mm eyepiece has an apparent field of view of 40°. Divide the 40° by the magnification, which is 50 power. This
yields an actual field of view of 0.9 degrees or 54 minutes of arc.
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