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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 20mm eyepiece that came with your telescope. To determine the magnification you divide the focal length of your telescope
(the PowerSeeker 114AZ for this example has a focal length of 900mm) by the focal length of the eyepiece, 20mm. Dividing 900 by 20 yields a magnification of 45x.
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 PowerSeeker 114AZ is 4.5” inches in diameter. Multiplying 4.5 by 60 gives a maximum useful magnification of 270 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 90 to 158 times for the
PowerSeeker 114AZ telescope. You can determine the magnification for your telescope the same way.
Note on Using High Powers –
Higher powers are used mainly for lunar and sometimes planetary observing where you can greatly enlarge the image, but remember
that the contrast and brightness will be very low due to the high magnification. Using the 4mm eyepiece together with the 3x Barlow lens gives extremely high power
and can be used on rare occasions – you will achieve the power but the image will be dark with low contrast because you have magnified it to the maximum possible.
For the brightest images with the highest contrast levels, use lower powers.
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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 Angular 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 20mm eyepiece that is supplied standard with the PowerSeeker 114AZ telescope. The 20mm eyepiece has an apparent field of view of 22°.
Divide the 22° by the magnification, which is 45 power. This yields an actual (true) field of 0.5°.
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 0.5° by 52.5. This produces a linear field width of 26 feet at a distance of one thousand yards.
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When using any optical instrument, there are a few things to remember to ensure you get the best possible image.
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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.
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Never look across or over objects that are producing heat waves. This includes asphalt parking lots on hot summer days or building rooftops.
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Hazy skies, fog, and mist can also make it difficult to focus. The amount of detail seen under these conditions is greatly reduced.
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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.
ASTRONOMY BASICS
Up to this point, this manual covered the assembly and basic operation of your telescope. However, to understand your telescope more thoroughly, you need to know a
little about the night sky. This section deals with observational astronomy in general and includes information on the night sky.
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To help find objects in the sky, astronomers use a celestial coordinate system that is similar to our geographical co-ordinate system here on Earth. The celestial
coordinate system has poles, lines of longitude and latitude, and an equator. For the most part, these remain fixed against the background stars.
The celestial equator runs 360 degrees around the Earth and separates the northern celestial hemisphere from the southern. Like the Earth's equator, it bears a reading of
zero degrees. On Earth this would be latitude. However, in the sky this is referred to as declination, or DEC for short. Lines of declination are named for their angular
distance above and below the celestial equator. The lines are broken down into degrees, minutes of arc, and seconds of arc. Declination readings south of the equator
carry a minus sign (-) in front of the coordinate and those north of the celestial equator are either blank (i.e., no designation) or preceded by a plus sign (+).
The celestial equivalent of longitude is called Right Ascension, or R.A. for short. Like the Earth's lines of longitude, they run from pole to pole and are evenly spaced
15 degrees apart. Although the longitude lines are separated by an angular distance, they are also a measure of time. Each line of longitude is one hour apart from the
next. Since the Earth rotates once every 24 hours, there are 24 lines total. As a result, the R.A. coordinates are marked off in units of time. It begins with an arbitrary
point in the constellation of Pisces designated as 0 hours, 0 minutes, 0 seconds. All other points are designated by how far (i.e., how long) they lag behind this
coordinate after it passes overhead moving toward the west.
