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Fig.a
Fig.b
F
ocusing
A
ligning the Finderscope
6
These fixed magnification scopes mounted on the optical tube are
very useful accessories. When they are correctly aligned with the
telescope, objects can be quickly located and brought to the centre
of the field. Alignment is best done outdoors in day light when it's
easier to locate objects. If it is necessary to refocus your
finderscope, sight on an object that is at least 500 yards (metres)
away. Loosen the locking ring by unscrewing it back towards the
bracket. The front lens holder can now be turned in and out to
focus. When focus is reached, lock it in position with the locking ring
(Fig.a).
Choose a distant object that is at least 500 yards away and point
the main telescope at it. Adjust the telescope so that the object
is in the centre of the view in your eyepiece.
Check the finderscope to see if the object centred in the main
telescope view is centred on the crosshairs.
Use the two small alignment screws to centre the finderscope
crosshairs on the object. The screws work in opposition to a
spring-loaded knob (Fig.b).
1.
2.
3.
Slowly turn the focus knobs (Fig.c), one way or the other, until the
image in the eyepiece is sharp. 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.
Fig.c
Fig.d
U
sing the Barlow lens (optional)
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 should be inserted between the focuser and the eyepiece
in your Dobsonian (Fig.d).
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.
Barlow
Eyepiece
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