8
That’s 5 hours and 35.4 minutes in right ascension, and -5
degrees and 27 arc-minutes in declination (there are 60 arc-
minutes in 1 degree of declination).
Before you can use the setting circles to locate objects, the
mount must be polar aligned, and the setting circles must be
calibrated. The easiest way to calibrate the setting circles is to
point the telescope at an identifiable bright star, center it in the
eyepiece, then set the setting circles to the star’s published
coordinates, which you can find in a star atlas or astronomical
software program, or perhaps on the internet.
Calibrating the Right Ascension Setting Circle
1. Identify a bright star near the celestial equator (Dec. = 0°)
and look up its coordinates in a star atlas.
2. Loosen the R.A. and Dec. lock knobs on the equatorial
mount, so the telescope optical tube can move freely.
3. Point the telescope at the bright star near the celestial equa-
tor whose coordinates you know. Lock the R.A. and Dec. lock
knobs. Center the star in the telescope’s field of view with the
slow-motion control cables.
4. Loosen the thumb screw located just above the R.A. setting
circle pointer; this will allow the setting circle to rotate freely.
Rotate the setting circle until the pointer indicates the R.A.
coordinate listed in the star atlas for the object. Retighten the
thumb screw.
Calibrating the Setting Circles
Using a star atlas or astronomy planetarium program, identify
a bright star visible in your sky. Some smartphone astronomy
apps allow you to hold your phone up to the sky and the app
will identify the stars and constellations visible in the direction
you’re pointing to. Note the right ascension and declination coor-
dinates of the star.
Let’s take as an example Altair, in the constellation Aquila. Its
coordinates are:
R.A. 19 hr 51 min, Dec 8° 52'
1. Loosen the R.A. and Dec. lock knobs on the equatorial
mount, so the telescope optical tube can move freely.
2. Point the telescope at Altair. Lock the R.A. and Dec. lock
knobs. Center the star in the eyepiece with the slow-motion
control cables.
3. Rotate the R.A. setting circle until the metal pointer indicates
19 hr 51 min.
4. Then rotate the Dec. setting circle until the metal pointer
ind8° 52'
Figure 13. A
) Balance a telescope in right ascension by sliding the counterweight along the shaft until it counterbalances the telescope.
B
) Balance the telescope on the declination axis by sliding the dovetail mounting bar forward or back in the mount’s saddle.
A.
B.
R.A. lock knob
Saddle
clamp
knobs
Counterweight lock
knob
Counterweight
shaft
Dec. lock knob
