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little bit beyond sharp focus until the image just starts to blur
again, then reverse the rotation of the knob, just to make sure
you've hit the exact focus point.
If you have trouble focusing, rotate the focuser knob so the
drawtube is in as far as it will go. Now look through the eye-
piece while slowly rotating the focus knob in the opposite direc-
tion. You should soon see the point at which focus is reached.
On the underside of the focuser is a large thumbscrew (see
Figure 13). It can be tightened to lock the drawtube once
you've achieved focus, if desired. This is not normally neces-
sary, though, unless you're using an unusually heavy eyepiece
with perhaps a Barlow lens and their weight causes the draw-
tube to slip downward. The locking thumbscrew could prevent
any such slippage.
Note: The image in your Dobsonian reflector will appear
rotated, or upside down. This is normal for reflector telescopes
and is why reflectors are not recommended for daytime
terrestrial viewing.
Aligning and Using the Red Dot Finder Scope
The red dot finder scope (Figure 15) makes pointing your
telescope almost as easy as pointing your finger! It's a non-
magnifying aiming device that superimposes a tiny LED red
dot on the sky, showing exactly where the telescope is pointed.
It permits easy object targeting prior to observation in the main
telescope.
Before you can use the red dot finder scope, you must remove
the plastic tab sticking out from the battery tray. Doing so will
allow the pre-installed 3V CR-2032 button cell battery to make
contact with the finder scope's electronic circuitry to power the
finder's red LED illuminator. The tab can then be discarded.
To use the red dot finder scope properly, it must be aligned
with the main telescope. This is easiest to do during daylight
hours, before observing at night. Follow this procedure:
Aligning the Red Dot Finder Scope
1.
Center a target that's at least ¼ mile away in the main
telescope's eyepiece. It could be a house chimney or
the top of a distant telephone pole, for example. Now
turn on the red dot finder scope by rotating the power
knob so that it clicks ON (refer to Figure 15). The dot
increases in brightness as you rotate the knob. Typically
a dimmer setting is used under dark skies and a brighter
setting is used under light-polluted skies or in daylight.
Position your eye at a comfortable distance from the rear
of the red dot scope. Look through the rear of the finder
scope with both eyes open to see the illuminated red
dot. The target object should appear in the field of view
somewhere near the red dot.
2.
You'll want to center the target object on the red dot.
To do so, without moving the telescope, use the finder
scope's vertical and horizontal adjustment knobs (shown
in Figure 15) to position the red dot on the object.
3.
When the red dot is centered on the distant object, check
to make sure the object is still centered in the telescope's
eyepiece. If it isn't, re-center it then adjust the finder
scope's alignment again. When the object is centered in
the telescope eyepiece and on the finder scope's red dot,
the finder scope is properly aligned with the telescope.
The red dot finder scope's alignment should be checked
before every observing session.
At the end of your observing session, be sure to turn the power
knob on red dot finder scope OFF (hear the click) to preserve
battery life.
Once aligned, the red dot scope will usually hold its alignment
even after being removed and reattached. Otherwise, only min-
imal realignment will be needed.
Replacing the Battery
Should the red dot finder scope's battery ever die, replacement
3-volt CR2032 lithium batteries are available from many retail
outlets. Push the battery tray out with your fingertip from the
side labeled "PUSH" , then pull it the rest of the way out from
the other side. Remove the old battery and replace it with a
fresh one, positive (+) side facing up. Then slide the battery
tray back in by pushing the side of the tray labeled "BATTERY"
into the slot.
Aiming/Pointing the Telescope
Now that the red dot finder scope is aligned, the telescope
can be quickly and accurately pointed at anything you wish
to observe. It has a much wider field of view than the tele-
scope's eyepiece, and therefore it is much easier to first center
an object in the red dot finder scope. Then the object will also
be centered in the telescope's field of view.
Start by once again moving the telescope until it is pointed
in the general direction of the object you want to see. Some
observers find it convenient to sight along the tube to do this.
Now, look in the red dot scope. If your general aim is accurate,
the object should appear somewhere in the red dot scope's
field of view. Make small adjustments to the telescope's posi-
tion until the object is centered on the red dot. Now, look in the
telescope's eyepiece and enjoy the view!
Determining Magnification
Magnification, or power, is determined by the focal length of
the telescope and the focal length of the eyepiece. By using
eyepieces of different focal lengths, the resultant magnification
can be varied.
Magnification is calculated as follows:
Telescope Focal Length (mm)
Eyepiece Focal Length (mm)
For example, to calculate the magnification of the XT8 Classic
Dobsonian with the supplied 25mm eyepiece, divide the focal
length of 1219mm by 25mm:
1219mm
25mm
Other 1.25" telescope eyepieces of different focal lengths can be
purchased to achieve higher or lower powers. It is quite common
for an observer to own five or more eyepieces to access a wide
range of magnifications.
Once you've centered the object in the eyepiece, you can switch
to higher magnification (using an optional shorter focal length
eyepiece), if you wish. This is especially recommended for small
= Magnification
= 48.8 ~ 49x
9
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