Attaching the finderscope:
An essential prerequisite for the easy location
of objects on both land and in the sky is the
correct location and alignment of the finder-
scope that attaches to the tube of the tele-
scope. It is, in fact, a smaller version of the
main telescope that is designed to have a low
magnification and a wide field of view so that
the desired target may be easily located. The
eyepiece of the finder scope is equipped with
cross-hairs much like a gun sight marking the
centre of the field of view.
Aligning the finderscope:
(i) Setup the telescope on its mount outdoors
in the day time and ensure that it is balanced
as previously described. You may wish to
extend the tripod legs and securely lock them
again once you have the telescope at a com-
fortable working height. Enlisting the help of
a friend will aid this process. Select the lowest
magnification eyepiece in the set (this is the
one with the largest number engraved on the
cap: usually 20mm or 25mm) and place it in
the focuser drawtube as previously described.
(ii) Select a distant prominent object such as
the tip of an electricity pylon or the tip of a
c h u rch spire, though any distant, well-defined
object will do: the further away the better. In
the case of an equatorial mount loosen the
declination and right ascension clamps, or the
altitude and azimuth locks on the alt-az
mount. Sight along the tube and position it
such that the telescope is pointing towards
the distant landmark.
(iii) This first attempt will call for a little trial
and error since the telescope will not be cor-
rectly focused, so as soon as one sees some-
thing blurred through the eyepiece lock the
declination/right ascension clamps (equatorial
mount )or the altitude and azimuth clamps
(Alt-az mount). Use the slow motion controls
as necessary to centre the object in the field
of view.
(iv) Turn the focuser knobs back or forth until
the image becomes crisp and sharp. Do not
be concerned that the image appears upside
down (reflecting telescope) or reversed in a
left to right plane (refractor with zenith prism)
- this is perfectly natural for an astronomical
telescope since there is no 'up' or 'down' in
the sky and you will soon get used to it. With
the image correctly focused you may wish to
use the slow motion controls to perfect the
alignment on the distant target.
(v) Now we can proceed to the Finder. You
will note that the image it offers is upside
down for the same reasons as that of the
main telescope. Depending on the design of
the telescope that you possess, the Finder will
be attached to the main telescope by a brack-
et that will have either three or six radially
spaced adjusting screws: if yours is of the latter
type, adjust the front three screws to permit
the tube of the Finder to be held centrally in
the bracket. Once this has been achieved,
check that the target is still central to the field
of view of the main telescope then slowly
adjust the rear three screws of the finder
bracket until the same object is positioned in
the centre of the cross-wires in the finder's
eyepiece. Again it may help to have a friend to
help you with this procedure. Once achieved,
ensure that the lock nuts of the Finder adjust-
ing screws are secure. Now you can loosen
the locks on the polar (/azimuth) and declina-
tion (/altitude) axes and practice locating
objects during the day.
Telescope basics:
Calculating the telescope's magnification
The magnifying power of any given telescope
and eyepiece is given by a simple formula that
requires a knowledge of the instrument's
focal length and that of the eyepiece. As we
have seen, the focal length of an eyepiece is
usually engraved on its cap: 10mm or 25mm,
for example.
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