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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, and usually between the diagonal
and the eyepiece in a refractor or a maksutov (Fig.f). With
some telescopes, it can also be inserted between the
focuser and the diagonal, and in this position it gives even
greater magnification. For example, a 2X Barlow when
inserted after the diagonal can become 3X when placed in
front of the diagonal.
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 eyepieces in your collection.
Slowly turn the focus knobs under the focuser, one way or
the other, until the image in the eyepiece is sharp (Fig.g).
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 have not yet reached
outside temperature. Refocusing is almost always
necessary when you change an eyepiece or add or
remove a Barlow lens. On some focusers, there is a
tension adjustment. Over-tighten this may damage the
rack and pinion assembly.
Preparing the Mount
This section describes how to achieve a precise polar
alignment with your HEQ5/EQ6 mount. To achieve a
precise polar alignment it is first necessary to prepare the
mount. In the Northern Hemisphere, this includes orienting
the polar scope reticule and aligning the polar scope
reticule. If you are in the Southern Hemisphere, you only
need to align the polar scope reticule. These steps, which
only need to be done once, are outlined first.
If you have already prepared your mount then you can
skip to the final section entitled "Procedure for Precise
Polar Alignment of the HEQ5/EQ6 Mount". If not, follow
the steps to prepare your mount for precise polar
alignment.
Fig.f
(Refracting Telescopes
(Reflecting Telescopes)
Fig.g
(Refracting Telescopes)
Fig.h
(Diagram applicable to both mounts)
10
Eyepiece
Barlow
Diagonal
and Maksutovs)
Eyepiece
Barlow
(Reflecting Telescopes)
(Maksutov Telescopes)
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