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32 • Astronomy Basics
Latitude Scales
The easiest way to polar align a telescope is with a latitude scale. Unlike
other methods that require you to find the celestial pole by identifying certain
stars near it, this method works off of a known constant to determine how high
the polar axis should be pointed. The Celestron CM-1100 mount can be
adjusted from 13 to 65 degrees (see figure 4-6).
The constant, mentioned above, is a relationship between your latitude and the
angular distance the celestial pole is above the northern (or southern) horizon;
The angular distance from the northern horizon to the north celestial pole is
always equal to your latitude. To illustrate this, imagine that you are standing
on the north pole, latitude +90°. The north celestial pole, which has a declina-
tion of +90°, would be directly overhead (i.e., 90 above the horizon). Now, let’s
say that you move one degree south — your latitude is now +89° and the
celestial pole is no longer directly overhead. It has moved one degree closer
toward the northern horizon. This means the pole is now 89° above the
northern horizon. If you move one degree further south, the same thing
happens again. You would have to travel 70 miles north or south to change
your latitude by one degree. As you can see from this example, the distance
from the northern horizon to the celestial pole is always equal to your latitude.
If you are observing from Los Angeles, which has a latitude of 34°, then the
celestial pole is 34° above the northern horizon. All a latitude scale does then
is to point the polar axis of the telescope at the right elevation above the
northern (or southern) horizon. To align your telescope:
1. Make sure the polar axis of the mount is pointing due north. Use a
landmark that you know faces north.
2. Level the tripod. There is a bubble level built into the mount for this
purpose.
NOTE: Leveling the tripod is only necessary if using this method of polar
alignment. Perfect polar alignment is still possible using other
methods described later in this manual without leveling the tripod.
3. Adjust the mount in altitude until the latitude indicator points to your
latitude. Moving the mount affects the angle the polar axis is pointing. For
specific information on adjusting the equatorial mount, please see the
section “Adjusting the Mount.”
This method can be done in daylight, thus eliminating the need to fumble
around in the dark. Although this method does NOT put you directly on the
pole, it will limit the number of corrections you will make when tracking an
object. It will also be accurate enough for short exposure prime focus plan-
etary photography (a couple of seconds) and short exposure piggyback
astrophotography (a couple of minutes).
Figure 4-6
The altitude scale allows for
settings between 13 and 65
degrees.
Latitude Scale
Altitude Adjustment Knob