5
well. If it isn’t, repeat the entire process, making sure not to
move the main telescope while adjusting the alignment of
the finder scope.
Finder scopes can come out of alignment during transport
of the telescope, so check the alignment before each
observing session.
Focusing the Finder Scope
If, when looking through the finder scope, you notice that
the images appear somewhat out of focus, you will need to
refocus the finder scope for your eyes. First loosen the lock
ring located behind the objective lens cell on the body of the
finder scope (see figure 2a). Back the lock ring off by a few
turns, for now. Refocus the finder scope on a distant object
by threading the objective lens cell in or out of the finder
scope body. Precise focusing will be achieved by focusing
the finder scope on a bright star. Once the image appears
sharp, retighten the lock ring behind the objective lens cell.
The finder scope’s focus should not need to be adjusted
again.
4. Using Your Telescope
Aiming the Telescope
To view an object in the main telescope, first loosen the
azimuth lock knob. Point the telescope at the object you
wish to observe by “eyeballing” along the length of the tele-
scope tube. Then look through the (aligned) finder scope
and move the telescope tube until the object is generally
centered on the finder’s crosshairs. Accurately center the
object on the finder’s crosshairs using the altitude and
azimuth slow-motion controls. The azimuth slow-motion con-
trol will not engage until the azimuth lock knob is re-tight-
ened. The object should now be visible in the main tele-
scope with the lower-power (25mm) eyepiece. If necessary,
use the slow-motion controls to re-position the object within
the field of view of the main telescope’s eyepiece.
And remember to aim well clear of the Sun, unless the front
of the telescope is fitted with a professionally made solar fil-
ter and the finder scope is covered with foil or some other
completely opaque material.
Note About the Slow-Motion Controls
The slow-motion control cables can only move the telescope
a maximum of 30° in any direction. This is because the worm
gears the control cables attach to will reach the end of their
mechanical “travel.” If, when using the slow-motion controls,
you can no longer rotate one of the control cables in a
desired direction, you have reached the end of worm gear
travel, and the worm gear should be reset. This is done by
first rotating the control cable several turns in the opposite
direction from which it was originally being turned. Then,
manually move the telescope closer to the object you wish to
observe (remember to first loosen the azimuth lock knob, if
repositioning in azimuth). You should now be able to use the
slow-motion control again to fine-adjust your position.
Focusing the Telescope
Practice focusing the telescope in the daytime before using it
for the first time at night. Start by positioning the focuser near
the center of its adjustment range. Insert an eyepiece into
the diagonal and secure with the thumbscrew. Point the tele-
scope at a distant object and get it in the field of view. Now,
slowly rotate one of the focusing knobs until the object
comes into sharp focus. Go a little bit beyond sharp focus
until the image just starts to blur again, then reverse the rota-
tion of the knob, just to make sure you hit the exact focus
point. The telescope can only focus on objects at least 100
feet or so away. It will not focus without the diagonal in place.
Do You Wear Eyeglasses?
If you wear eyeglasses, you may be able to keep them on
while you observe, if your eyepieces have enough “eye
relief” to allow you to see the whole field of view. You can try
this by looking through the eyepiece first with your glasses
on and then with them off, and see if the glasses restrict the
view to only a portion of the full field. If they do, you can eas-
ily observe without your glasses by just refocusing the tele-
scope the needed amount.
Eyepiece Selection
Always start viewing with your lowest-power, widest-field
eyepiece. After you’ve located and looked at an object with a
low-power eyepiece, switch to a higher-power eyepiece and
see if the object looks better or worse. Keep in mind that at
higher power, an image will always be fainter and less sharp
(this is a fundamental law of optics). Many observers use
the lowest-power eyepiece practically all the time! Naturally,
higher magnifications are desirable for viewing some
objects, but stay with low powers when searching for an
object and for extended viewing.
To calculate the power, or magnification, of an eyepiece
when used in a specific telescope, divide the focal length of
the telescope (an inherent specification of the telescope) by
the focal length of the eyepiece (the number, in millimeters,
stamped on the eyepiece):
Telescope F.L ÷ Eyepiece F.L. = Magnification
For example, if you’re using the Explorer 90 telescope,
which has a focal length of 910mm, and the included 25mm
Sirius Plössl eyepiece, the magnification would be:
910 ÷ 25 = 36x
We recommend having a selection of several eyepieces of
different focal lengths, so that you can choose the optimal
magnification, brightness level, and contrast for each object
and for different observing conditions.
Useful Limit of Magnification
Every telescope has a useful limit of power of about 45x-60x
per inch of aperture (diameter of objective lens). Claims of
higher power by some telescope manufacturers are a mis-
leading advertising gimmick and should be dismissed. The
Explorer 90 Altazimuth Refractor, with its 90mm-diameter
(3.5") objective lens, is capable of a maximum useful magnifi-
cation of about 200x. Any higher and the images will virtually
always appear very faint and fuzzy. As a general rule, if the
