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This note presupposes an equatorial
mounting, properly aligned with the Earth's axis of
rotation. If you are using a portable mounting, the
accuracy with which your setting circles can point your
telescope is directly proportional to the accuracy with
which you have aligned your mounting to the pole. A
few extra minutes spent achieving proper alignment can
pay big dividends if you plan to use setting circles.
Alt-azimuth mountings, such as a Dobsonian
mount, can use altitude and azimuth circles, but a calculation
must be done to convert the target's celestial, equatorial
coordinates (as tabulated) into topocentric, alt-azimuth
coordinates. Consult a reference such as the League's
Math for Amateur Astronomers or one of the
many available computer programs to accomplish the coordinate
transformation.
Computerized, digital setting circles
are now available on many commercial telescopes. Their
use may differ from that described here for analog or
dial-type setting circles. In that case, follow the
directions that come with the computerized mountings.
Declination Circle Only
Many observers find it convenient
to use only the declination circle and to "sweep" in
right ascension to find an object once its declination
has been set. First, point your telescope at a bright
star of known coordinates. Then adjust your declination
circle or declination pointer (depending on your particular
telescope design) to the star's declination. Your declination
circle is now "calibrated" until you move your mounting.
To find another object, turn your
telescope until the target's declination reads on the
declination circle. Lock the declination axis. Point
your telescope in the general direction of the target
and move the telescope back and forth about the polar
axis until the target comes into the field of view.
This back and forth movement about an axis is called
sweeping. Use caution with this method while
trying to find an object in the daytime sky. Do not
sweep near the Sun or its image could enter the field
of view and cause permanent eye damage.
You might be tempted to simply point
your telescope at Polaris and set its declination of
89.1 degrees for your declination calibration. While
this would work in theory, errors in polar alignment
decrease your pointing accuracy as you move farther
away from your calibration star. It is much better to
calibrate your circle with a star near the zenith or
at least near the declination of your zenith. That way
you will never be more than 90 degrees away from your
calibrated declination.
Guide Star Method
This method extends the "declination
only" technique to include the right ascension or hour
circle. In this case, you point your telescope at a
guide star whose coordinates are known and then rotate
your scope to read the target coordinates. However,
this must be done with certain considerations.
After the guide star is centered in
the field, set the declination circle to read the guide's
declination. If the scope is not clock driven, re-center
the object and set the hour circle to read the right
ascension of the guide. If the scope is clock driven,
check to be sure you did not decenter the guide while
setting the declination and then set the hour circle
to the guide's right ascension. Quickly rotate the telescope
in both axes to the coordinates of the target.
Always set the declination circle
first and then set the hour circle. Move to the target
as soon as the hour circle has been set. Remember, the
Earth continues to rotate -- one degree in four minutes
-- while you are making your adjustments. Try to find
a guide star close to your target, within a few degrees
if possible. This will make the movements quicker and
will lessen the effect of inaccuracies in polar alignment.
Note that if your hour circle turns with your clock
drive, it will remain accurate as the Earth rotates.
The Sidereal Method
This method requires that you know
your local sidereal time at the time of observation.
You must either keep a clock set to sidereal time or
you must calculate the sidereal time for the time of
the intended observation.
First, calibrate your declination
circle according to the methods above. Next, determine
the target's hour angle by subtracting its right ascension
from your local sidereal time. A positive hour angle
indicates the target is west of your meridian; a negative
hour angle indicates the target is east of your meridian.
Now, turn your telescope to read the target's declination
and then use the hour circle to point the calculated
hours (and minutes or fractional hours) east or west
of you meridian.
Note that this is the usual method
for finding bright planets in the daytime.
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