9
U
sing the Barlow lens
(for all models)
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 catadioptric
Barlow
Eyepiece
Fig.k
Barlow
Eyepiece
centred in the Red Dot Finder. Now look through the telescope using a low power eyepiece. Centre M57 in the
field of view of the eyepiece.
The setting circles will get you close to the object you wish to observe, but are not accurate enough to put it in
the centre of your Red Dot Finder's/finderscope's field of view. The accuracy of your setting circles also
depends on how accurate your telescope is polar aligned.
(Fig.k). 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 Barlow which is 2X 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, it is greatest value may be that a Barlow can potentially
double the number of eyepiece in your collection.
