8
Eyepieces are commonly referred to by focal length, which is printed on the
eyepiece barrel. The longer the focal length (i.e., the larger the number) the
lower the power. Conversely, the shorter the focal length (i.e., the smaller the
number) the higher the magnification. Generally, you will use low to moderate
power when viewing. For more information on how to determine power, see the
section on “Calculating Magnification.”
In addition, eyepieces are also referred to by barrel diameter. These Celestron
refractors use eyepieces with a barrel diameter of 1-1/4".
To focus your telescope, simply turn either of the focus knobs located directly
opposite the finderscope. Turn the focus knob until the image is sharp. Once
sharp, turn the knob towards you to focus on an object that is closer than the
one you are currently observing. Turn the knob away from you to focus on a
more distant object than the one you are currently observing.
In addition to understanding how the focusing mechanism works, there are a
few focusing hints to remember when using any optical instrument.
•
Avoid looking through 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
focus. In some cases, you may actually see a double image.
•
Never look across or over objects 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 that can be seen under these conditions
will be greatly reduced. Also, when photographing under these conditions,
the processed film may come out a little grainier than normal.
•
When using your telescope as a telephoto lens, the split screen or micro-
prism focuser of the 35mm camera may “black out.” This is common with
all long focal length lenses. If this happens, use the ground glass portion of
your focusing screen. To achieve a very sharp focus, you may consider
using a focusing magnifier. These are readily available from your local
camera store.
•
If you wear corrective lenses (specifically glasses), you may want to remove
them when observing with an eyepiece attached to the telescope. However,
when using a camera, you should always wear corrective lenses to ensure
the sharpest possible focus. If you have astigmatism, corrective lenses
must be worn at all times.
Accurate alignment of the finder makes it easy to find objects with the tele-
scope, especially celestial objects. To make aligning the finder as easy as
possible, this procedure should be done in the daytime when it is easy to find
and identify objects. To align the finder:
1.
Choose a target that is in excess of one mile away. This will eliminate any
parallax effect between the telescope and finder.
2.
Point the telescope at your target and center it in the main optics of the
telescope. You may have to move the telescope slightly to center your
target.
Focusing
Aligning the Finder
