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Even though the NexStar can precisely track a celestial object while in the Alt-Az position, it is still necessary to align the
polar axis of the telescope (the fork arm) to the Earth's axis of
rotation in order to do long exposure astrophotography. To do an
accurate polar alignment, the NexStar requires an optional equatorial
wedge between the telescope and the tripod.
telescope's tracking motors to rotate the telescope around the
celestial pole, the same way as the stars. Without the equatorial
wedge, you would notice the stars in the eyepiece would slowly
rotate around the center of the field of view. Although this gradual
rotation would go unnoticed when viewing with an eyepiece, it
would be very noticeable on film.
Polar alignment is the process by which the telescope's axis of
rotation (called the polar axis) is aligned (made parallel) with the
Earth's axis of rotation. Once aligned, a telescope with a clock drive
will track the stars as they move across the sky. The result is that
objects observed through the telescope appear stationary (i.e., they
will not drift out of the field of view). If not using the clock drive,
all objects in the sky (day or night) will slowly drift out of the field.
This motion is caused by the Earth's rotation. There are several
methods of polar alignment, all work on a similar principle, but
performed somewhat differently.
separately, and discussed later in this section.
Calibrating Level
In order for the NexStar to align the optical tube parallel with the
fork arms, it must first use its internal downstop switch as a
reference for leveling the tube. Even though the downstop has been
installed at the factory, the switch still needs to be calibrated for
every individual telescope. To accurately calibrate the downstop, it
is necessary to first do an Alt-Azm alignment (see the Hand Control
section of the manual). Once an alignment has been completed the
NexStar can compare the position of the downstop switch with its known altitude above the horizon. The difference
between the two positions will be stored as an offset when slewing the optical tube to its polar aligned position (90º
declination). To calibrate the downstop switch, do the following:
1.
Align the NexStar in altazimuth using either the GPS
Alignment, AutoAlign or 2-Star Alignment method.
2.
Once a successful alignment has been completed, press
the MENU button and select Calibrate Level from the
Utilities menu.
3.
The telescope will automatically level the tube to its
downstop position and compare its position with its
current altitude above the horizon. This position will be
stored for future use when doing EQ Alignments.
EQ Alignment Procedure
The NexStar has two equatorial alignment modes (one for the
northern hemisphere and one for the southern) that will help you
polar align your telescope by using its internal downstop to position
the tube parallel to the fork arms. Once the telescope has been
positioned towards north (or south) and has been properly polar
aligned, the hand control will ask the user to select the first of two
stars to align the telescope with the sky. The following is an
overview of the polar alignment procedure for the NexStar GPS:
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This is how the telescope is to be set up for polar
alignment. The tube should be parallel to the
fork arm and the mount should point to Polaris.
The Meridian is an imaginary line in the sky that
starts at the North celestial pole and ends at the South
celestial pole and passes through the zenith. If you are
facing South, the meridian starts from your Southern
horizon and passes directly overhead to the North
celestial pole.
30
Figure 6-3
Figure 6-4
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