Using The Setting Circle; Pointing Your Telescope - Dörr DANUBIA DELTA 20 Notice D'utilisation

07.8 USING THE SETTING CIRCLES
The quickest way to fi nd objects is to learn the Constellations and use the Red Dot Finder, but if the object
is too faint you may want to use setting circles on your mount. Setting circles enable you to locate celestial
objects whose celestial co-ordinates have been determined from star charts. Your telescope must be polar
aligned and the R.A. setting circle must be calibrated before using the setting circles. The DEC. setting circle
was set at the factory, and does not require calibrating the same manner as the R.A. setting circle.
Reading the R.A. setting circle
The telescope's R.A. setting circle is scaled in hours, from 1 through 24, with small lines in between represen-
ting 10 minute increments. The upper set of numbers apply to viewing in the Northern Hemisphere, while the
numbers below them apply to viewing in the Southern Hemisphere (Fig.k).
Setting (calibrating) the R.A. setting circle
In order to set your Right Ascension circle you must fi rst fi nd a star in your fi eld of view with known coordi-
nates. A good one would be the 0.0 magnitude star Vega in the Constellation Lyra. From a star chart we know
the R.A. coordinate of Vega is 18h 36m. Loosen the R.A. and DEC. lock knobs on the mount and adjust the
telescope so that Vega is centred in the fi eld of view of the eyepiece. Tighten the R.A. and DEC. lock knobs to
lock the mount in place. Now rotate the R.A. setting circle until it reads 18h36m. You are now ready to use the
setting circles to fi nd objects in the sky.
Finding objects using the setting circles
Example: Finding the faint planetary nebula M57; "The Ring"
From a star chart, we know the coordinates of the Rings are DEC. 33° and R.A. 18h52m. Unlock the DEC. lock
knob and rotate your telescope in DEC. until the pointer on the DEC. setting circle reads 33°. Re-tighten the
DEC. lock knob. Loosen the R.A. lock knob and rotate the telescope in R.A. until the pointer on the R.A. setting
circle reads 18h52m (do not move the R.A. circle). Re-tighten the R.A. lock knob. Now look through the Red
Dot Finder to see if you have found M57. Adjust the telescope with R.A. and DEC. fl exible cables until M57 is
centred in the Red Dot Finder. Now look through the telescope using a low power eyepiece. Centre M57 in the
fi eld 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/fi nderscope's fi eld of view. The accuracy
of your setting circles also depends on how accurate your telescope is polar aligned.
07.9

POINTING YOUR TELESCOPE

A German Equatorial mount has an adjustment, sometimes called a wedge, which tilts the mount's polar axis
so that it points at the appropriate Celestial Pole (NCP or SCP). Once the mount has been polar aligned, it
needs to be rotated around only the polar axis to keep an object centred. Do not reposition the mount base
or change the latitude setting. The mount has already been correctly aligned for your geographical location
(ie.Latitude), and all remaining telescope pointing is done by rotating the optical tube around the polar (R.A.)
and declination axes. A problem for many beginners is recognizing that a polar-aligned, equatorial mount
acts like an alt-azimuth mount which has been aligned to a celestial pole. The wedge tilts the mount to an
angle equal to the observer's Latitude, and therefore it swivels around a plane which parallels the celestial
(and Earth's) equator (Fig.l). This is now its "horizon"; but remember that part of the new horizon is usually
blocked by the Earth. This new "azimuth" motion is called Right Ascension (R.A). In addition, the mount swivels
North(+) and South(-) from the Celestial Equator towards the celestial poles. This plus or minus "altitude" from
the celestial equator is called Declination (DEC.).
Pointing to the NCP
For the following examples, it is assumed that the observing site is in the Northern Hemisphere. In the fi rst
case (Fig.m2), the optical tube is pointing to the NCP. This is its probable position following the polar-align-
ment step. Since the telescope is pointing parallel to the polar axis, it still points to the NCP as it is rotated
around that axis counter-clockwise, (Fig.m1) or clockwise (Fig.m3).
Pointing toward the western or eastern horizon
Now, consider pointing the telescope to the western (Fig.n1) or eastern (Fig.n2) 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.o). 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 fi nderscope, by loosening both the R.A.
and DEC. locks and swivelling the mount around both axes until the object is centred in the eyepiece fi eld. 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 fi eld and
allow tracking by adjusting only in R.A.
Fig.k
R.A. lock knob
R.A. lock knob
Fig.l
Right
Ascension
Meridian
Line
Plane of local horizon
Fig.m
Fig.n
21
R.A. Setting Circle
Pointer
R.A. Setting Circle
Pointer
Equatorial Mount
(Northern Hemisphere)
Zenith
Mount aligned on
North Celectial Pole
Object you
are viewing
Declination
Plane of Celestial
Equator
Celestial Pole
Celestial Pole
Telescope pointing East
Counterweight pointing North
Rotation of the R.A. axis
Rotation of the DEC. axis
Telescope pointing West
Counterweight pointing North
Latitude
Apparent
movement
of stars