8
Fig.e
U
sing the optional Barlow lens
A Barlow is a negative lens which increases the magnifying
power of an eyepiece, while reducing the field of view. It
expands the cone of the focussed light before it reaches the
focal point, so that the telescope's focal length appears
longer to the eyepiece.
The Barlow is inserted between the focuser and the eyepiece
in a reflector (Fig.e). In addition to increasing magnification,
the benefits of using a Barlow lens include improved eye
relief, and reduced spherical aberration in the eyepiece. For
this reason, a Barlow plus a lens often outperform a single
lens producing the same magnification. However, its greatest
value may be that a Barlow can potentially double the
number of eyepiece in your collection.
F
ocusing
Fig.f
Slowly turn the focus knobs under the focuser, one way or the
other, until the image in the eyepiece is sharp (Fig.f). The image
usually has to be finely refocused over time, due to small
variations caused by temperature changes, flexures, etc. This
often happens with short focal ratio telescopes, particularly when
they haven't yet reached outside temperature. Refocusing is
almost always necessary when you change an eyepiece or add
or remove a Barlow lens.
Barlow
Eyepiece
In order for your telescope to track objects in the sky you have to align your mount. This means tilting the head
over so that it points to the North (or South) celestial pole. For people in the Northern Hemisphere this is rather
easy as the bright star Polaris is very near the North Celestial Pole. For casual observing, rough polar alignment is
adequate. Make sure your equatorial mount is level and the red dot finder is aligned with the telescope before
P
olar alignment
Fig.g
0 10 20
30
40
50
60
70
80
90
Setting the latitude
Remove the telescope tube and the counterweights from the mount.
Find the latitude and time zone of your current location. A road atlas
or GPS unit is useful for your local geographic coordinates. Now look
at the side of your mount head, there you will see a scale running
from 0-90 degrees (Fig.g). At the base of the head, just above the
legs, are two screws (latitude adjustment t-bots) opposite each other
under the hinge. All you have to do is loosen one side and tighten the
other until your latitude is shown by the indicator pointer (Fig.i).
Loosening the front T-bolt first will make the adjustments easier. Small
adjustments may be needed after re-attaching the telescope tube and
the counterweights.
Polaris, the "Pole Star" is less than one degree from the North
Celestial Pole (NCP). Because it is not exactly at the NCP, Polaris
appears to trace a small circle around it as the Earth rotates. Polaris
is offset from the NCP, toward Cassiopeia and away from the end of
the handle of the Big Dipper (Fig.g-1).
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