16
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. 33º and R.A. 18h52m. Unlock the DEC lock
knob and rotate your telescope in DEC until the pointer on the DEC setting circle reads 33º. 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. flexible cables until M57 is
centred in the Red Dot Finder. Now look through the telescope using a low power eyepiece. Centre M57 in the
field 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/finderscope's field of view. The accuracy of your setting circles also
depends on how accurate your telescope is polar aligned.
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).
Fig.l
Plane of local horizon
Nadir
Equatorial Mount
(Northern Hemisphere)
Mount aligned on
North Celestial Pole
Zenith
Right
Ascension
Apparent
movement
of stars
Plane of Celestial
Equator
Meridian
Line
Declination
Object you
are viewing
Polaris
Latitude
W
S
E
N
P
ointing
Y
our
Telescope
To Next Page
Loading...
Need help?
General
