Dorr DANUBIA ATLAS 2000 Bedienungsanleitung Seite 24

Pointing toward the western or eastern horizon
Now, consider pointing the telescope to the western (Fig.o1) or eastern (Fig.o2) horizon. If the counterweight
is pointing North, the telescope can be swivelled from one horizon to the other around the Dec axis in an arc
that passes through the NCP (any Dec arc will pass through the NCP if the mount is polar-aligned). It can be
seen then that if the optical tube needs to be pointed at an object north or south of this arc, it has to be also
rotated around the R.A axis.
Pointing to directions other than due North
Pointing in any direction other than due North requires a combination of R.A. and Dec positions (Fig.p). This
can be visualized as a series of Dec arcs, each resulting from the position of rotation of the R.A. axis. In
practice however, the telescope is usually pointed, with the aid of a finderscope, by loosening both the R.A.
and Dec locks and swivelling the mount around both axes until the object is centred in the eyepiece field. The
swivelling is best done by placing one hand on the optical tube and the other on the counter-weight bar, so
that the movement around both axes is smooth, and no extra lateral force is applied to the axisbearings. When
the object is centred, make sure the R.A and Dec locks are both retightened to hold the object in the field and
allow tracking by adjusting only in R.A.
Pointing at an object
Pointing at an object, for example to the South (Fig.q), can often be achieved with the optical tube positioned
on either side of the mount. When there is a choice of sides, particularly when there could be a long observing
period, the East side (Fig.q2) should be chosen in the Northern Hemisphere because tracking in R.A. will move
it away from the mount's legs. This is particularly important when using an R.A motor, because if the optical
tube jambs against the mount's legs, it can result in damage to the motor and/or the gears.
Telescopes with long focal lengths often have a "blind spot" when pointing near the zenith, because the eye-
piece-end of the optical tube bumps into the mount's legs (Fig.r1). To adapt for this, the optical tube can be
very carefully slipped up inside the tube rings (Fig.r2). 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 prob-
lem 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.
10 | CHOOSING THE APPROPRIATE EYEPIECE
10.1 CALCULATING THE MAGNIFICATION (POWER)
The magnification produced by a telescope is determined by the focal length of the eyepiece that is used with
it. To determine a magnification for your telescope, divide its focal length by the focal length of the eyepieces
you are going to use. For example, a 10mm focal length eyepiece will give 80X magnification with an 800mm
focal length telescope.
Focal length of the telescope
Magnification =
Focal length of the eyepiece
When you are looking at astronomical objects, you are looking through a column of air that reaches to the
edge of space and that column seldom stays still. Similarly, when viewing over land you are often looking
through heat waves radiating from the ground, house, buildings, etc. Your telescope may be able to give very
high magnification but what you end up magnifying is all the turbulence between the telescope and the sub-
ject. A good rule of thumb is that the usable magnification of a telescope is about 2x per mm of aperture under
good conditions.
800 mm
e.g. = 80x
10 mm
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Celestial Pole
Fig.o
Telescope pointing East
Counterweight pointing North
Telescope pointing West
Counterweight pointing North
Fig.p
Examples of the telescope moved in R.A. and DEC.
Fig.q
Telescope pointing South
Fig.r
Telescope pointing at the zenith
Rotation of the R.A. axis
Rotation of the DEC. axis