Orion Observer 90mm Instruction Manual page 11

Calibrating the Setting Circles
Using a star atlas or astronomy planetarium program, identify
a bright star visible in your sky. Some smartphone apps allow
you to hold your phone up to the sky and the app will iden-
tify the stars and constellations visible in the direction you're
pointing to. Note the right ascension and declination coordi-
nates 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 arrow
indicates 19 hr 51 min.
4. Then rotate the Dec. setting circle until the metal arrow
indicates +8° 52'
Finding Objects with the Setting Circles
Now that both setting circles are calibrated, look up the coor-
dinates of an object you wish to view.
1. Loosen the R.A. lock knob and rotate the telescope until
the R.A. value from the star atlas matches the reading on
the R.A. setting circle. Remember to use the upper set
of numbers on the R.A. setting circle. Retighten the lock
knob.
A.
Figure 22. Collimating the optics. (A) When the mirrors are properly aligned, the view down the focuser drawtube should look like this. (B)
With the collimation cap in place, (with a collimation cap or Cheshire eyepiece in place). Here, only part of the primary mirror is visible in the
secondary mirror, so the secondary mirror needs to be adjusted (tilted). (C) Here the secondary mirror is correctly aligned because the entire
primary mirror is visible in it. But the reflection of the secondary mirror is off-center. So the primary mirror still needs adjustment. (D) Now the
primary mirror is correctly aligned, so the secondary mirror is centered.
2. Loosen the Dec. lock knob and rotate the telescope until
the Dec. value from the star atlas matches the reading on
the Dec. setting circle. Remember that values of the Dec.
setting circle are positive when the telescope is pointing
north of the celestial equator (Dec. = 0°), and negative
when the telescope is pointing south of the celestial
equator. Retighten the lock knob.
Most setting circles are not accurate enough to put an object
dead-center in the telescope's eyepiece, but they should
place the object somewhere within or near the field of view
of the finder scope, assuming the equatorial mount is accu-
rately polar aligned. Use the slow-motion controls to center
the object in the finder scope, and it should appear in the tele-
scope's field of view.
The R.A. setting circle must be re-calibrated every time you
wish to locate a new object. Do so by calibrating the setting
circle for the centered object before moving on to the next one.
Confused About Pointing the Telescope?
Beginners occasionally experience some confusion about
how to point the telescope overhead or in other directions.
One thing you DO NOT do is make any adjustment to the
mount's latitude setting or to its azimuth position (don't touch
the azimuth lock knob). That will throw off the mount's polar
alignment. Once the mount is polar aligned, the telescope
should be moved only about the R.A. and Dec. axes by loos-
ening one or both of the R.A. and Dec. lock knobs and moving
the telescope by hand, or keeping the knobs tightened and
moving the telescope using the slow-motion cables.
B.
C.
D.
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