19
Telescopes with long focal lengths often
have a "blind spot" when pointing near the
zenith, because the eyepiece-end of the
optical tube bumps into the mount's legs
(Fig. q1). To adapt for this, the optical tube
can be very carefully slipped up inside the
tube rings (Fig. q2). This can be done
safely because the tube is pointing almost
vertically, and therefore moving it does
not cause a Dec-balance problem. It is
very important to move the tube back to
the Dec-balanced position before
observing other sky areas.
Something which can be a problem is that
the optical tube often rotates so that the
eyepiece, finderscope and the focussing
knobs are in less convenient positions.
The diagonal can be rotated to adjust the
eyepiece. However, to adjust the
positions of the finderscope and focussing
knobs, loosen the tube rings holding the
optical tube and gently rotate it. Do this
when you are going to view an area for
while, but it is inconvenient to do every
time you briefly go to a new area.
Finally, there are a few things to consider
to ensure that you are comfortable during
the viewing session. First is setting the
height of the mount above the ground by
adjusting the tripod legs. You must
consider the height that you want your
eyepiece to be, and if possible plan on
sitting on a comfortable chair or stool.
Very long optical tubes need to be
mounted higher or you will end up
crouching or lying on the ground when
looking at objects near the zenith. On the
other hand, a short optical tube can be
mounted lower so that there is less
movement due to vibration sources, such
as wind. This is something that should be
decided before going through the effort of
polar aligning the mount.
Fig.q
1.
Telescope pointing at the Zenith
2.
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