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1. Celestial Coordinates: Declination and Right Ascension
Figure 20: Celestial Sphere
Analogous to the Earth-based coordinate system of latitude and longitude,
celestial objects are mapped according to a coordinate system on the "celestial
sphere," the imaginary sphere on which all stars appear to be placed.
The Poles of the celestial coordinate system are defined as those 2 points
where the Earth's rotational axis, if extended to infinity, North and South,
intersect the celestial sphere. Thus, the North Celestial Pole is that point
in the sky where an extension of the Earth's axis through the North Pole
intersects the celestial sphere. In fact, this point in the sky is located
near the North Star, or Polaris.
On the surface of the Earth, "lines of longitude" are drawn between
the North and South Poles. Similarly, "lines of latitude" are
drawn in an East-West direction, parallel to the Earth's equator. The celestial
equator is simply a projection of the Earth's equator onto the celestial
sphere. Just as on the surface of the Earth, imaginary lines have been drawn
on the celestial sphere to form a coordinate grid. Celestial object positions
on the Earth's surface are specified by their latitude and longitude.
The celestial equivalent to Earth latitude is called "Declination,"
or simply "Dec," and is measured in degrees, minutes or seconds
north ("+") or south ("-") of the celestial equator.
Thus any point on the celestial equator (which passes, for example, through
the constellations Orion, Virgo and Aquarius) is specified as having 0°0'0"
Declination. The Declination of the star Polaris, located very near the
North Celestial Pole, is +89.2°.
The celestial equivalent to Earth longitude is called "Right Ascension,"
or "R.A.," and is measured in hours, minutes and seconds from an
arbitrarily defined "zero" line of R.A. passing through the constellation
Pegasus. Right Ascension coordinates range from 0hr0min0sec up to (but not
including) 24hr0min0sec. Thus there are 24 primary lines of R.A., located
at 15 degree intervals along the celestial equator. Objects located further
and further east of the prime (0h0m0s) Right Ascension grid line carry increasing
R.A. coordinates.
With all celestial objects, therefore, capable of being specified in position
by their celestial coordinates of Right Ascension and Declination, the task
of finding objects (in particular, faint objects) in the telescope is vastly
simplified. The setting circles, R.A (10, Fig.
17) and Dec. (3, Fig. 17) of the LX50
8", 10", and 12" telescopes may be dialed, in effect, to
read the object coordinates and the object found without resorting to visual
location techniques. However, these setting circles may be used to advantage
only if the telescope is first properly aligned with the North Celestial
Pole.
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2. Lining Up With the Celestial Pole
Objects in the sky appear to revolve around the celestial pole. (Actually,
celestial objects are essentially "fixed," and their apparent
motion is caused by the Earth's axial rotation). During any 24 hour period,
stars make one complete revolution about the pole, making concentric circles
with the pole at the center. By lining up the telescope's polar axis with
the North Celestial Pole (or for observers located in Earth's Southern hemisphere,
with the South Celestial Pole) astronomical objects may be followed, or
tracked, simply by moving the telescope about one axis, the polar axis.
In the case of the Meade 7" Maksutov-Cassegrain, 8", and 10"
Schmidt-Cassegrain telescopes, this tracking may be accomplished automatically
with the electric motor drive.
If the telescope is reasonably well aligned with the pole, therefore, very
little use of the telescope's Declination slow motion control is necessary–virtually
all of the required telescope tracking will be in Right Ascension. (If the
telescope were perfectly aligned with the pole, no Declination
tracking of stellar objects would be required.) For the purposes of casual
visual telescopic observations, lining up the telescope's polar axis to
within a degree or two of the pole is more than sufficient: with this level
of pointing accuracy, the telescope's motor drive will track accurately
and keep objects in the telescopic field of view for perhaps 20 to 30 minutes.
Begin polar aligning the telescope as soon as you can see Polaris. Finding
Polaris is simple. Most people recognize the "Big Dipper." The
Big Dipper has two stars that point the way to Polaris (see Fig.
21). Once Polaris is found, it is a straightforward procedure to obtain
a rough polar alignment.
To line up the 7", 8", or 10" LX50 with the Pole, follow
this procedure:
1. Using the bubble level located on the floor of the wedge, adjust the
tripod legs so that the telescope/ wedge/tripod system reads "level."
2. Set the Equatorial Wedge to your observing latitude as described in Appendix
A.
3. Loosen the Dec. Lock, and rotate the telescope tube in Declination so
that the telescope's Declination reads 90°. Tighten the Dec. Lock.
Loosen the R.A. Lock, and rotate the Fork Arms to the 00 H.A. position.
4. Using the Azimuth and Latitude controls on the Wedge, center Polaris
in the field of view. Do not use the telescope's Declination or Right Ascension
controls during this process.
At this point, your polar alignment is good enough for casual observations.
There are times, however, when you will need to have precise polar alignment,
such as when making fine astrophotographs or when using the setting circles
to find new objects.
Once the latitude angle of the wedge has been fixed and locked-in according
to the above procedure, it is not necessary to repeat this operation each
time the telescope is used, unless you move a considerable distance North
or South from your original observing position. (Approximately 70 miles
movement in North-South observing position is equivalent to 1° in latitude
change.) The wedge may be detached from the field tripod and, as long as
the latitude angle setting is not altered and the field tripod is leveled,
it will retain the correct latitude setting when replaced on the tripod.
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3. Precise Polar Alignment
It should be emphasized that precise alignment of the telescope's polar
axis to the celestial pole for casual visual observations is not necessary.
Don't allow a time-consuming effort at lining up with the pole to interfere
with your basic enjoyment of the telescope. For long-exposure photography,
however, the ground rules are quite different, and precise polar alignment
is not only advisable, but almost essential.
Notwithstanding the precision and sophistication of the drive system supplied
with the Meade LX50 telescopes, the fewer tracking corrections required
during the course of a long-exposure photograph, the better. (For our purposes,
"long-exposure" means any photograph of about 10 minutes duration
or longer.) In particular, the number of Declination corrections required
is a direct function of the precision of polar alignment.
Precise polar alignment requires the use of a crosshair eyepiece. The Meade
Illuminated Reticle Eyepiece is well-suited in this application, but you
will want to increase the effective magnification through the use of a 2x
or 3x Barlow lens. Follow this procedure, sometimes better known as the
"Drift" method (particularly if the pole star is not visible):
1. Obtain a rough polar alignment as described earlier. Place the illuminated
reticle eyepiece (or eyepiece/Barlow combination) into the eyepiece holder
of the telescope.
2. Point the telescope, with the motor drive running, at a moderately bright
star near where the meridian (the North-South line passing through your
local zenith) and the celestial equator intersect. For best results, the
star should be located within ±30 minutes in R.A. of the meridian and
within ±5° of the celestial equator. (Pointing the telescope at
a star that is straight up, with the Declination set to 0°, will point
the telescope in the right direction.)
3. Note the extent of the star's drift in Declination (disregard drift in
Right Ascension):
a. If the star drifts South (or down), the telescope's polar axis
is pointing too far East (Fig. 22).
b. If the star drifts North (or up), the telescope's polar axis is
pointing too far West (Fig. 23).
4. Move the wedge in azimuth (horizontally) to effect the appropriate change
in polar alignment. Reposition the telescope's East-West polar axis orientation
until there is no further North-South drift by the star. Track the star
for a period of time to be certain that its Declination drift has ceased.
(Please note that Figs. 22, 23, 24 and 25 show the telescope pointed in
the 90 degree position, and not the 0 degree position that is required for
"Drift" method alignment. This is done to illustrate the position
of the pole star relative to the polar axis of the telescope.)
5. Next, point the telescope at another moderately bright star near the
Eastern horizon, but still near the celestial equator. For best results,
the star should be about 20° or 30° above the Eastern horizon
and within ± 5° of the celestial equator.
6. Again note the extent of the star's drift in Declination:
a. If the star drifts South, (or down) the telescope's polar axis
is pointing too low (Fig. 24).
b. If the star drifts North, (or up) the telescope's polar axis is
pointing too high (Fig. 25).
7. Use the latitude angle fine-adjust control on the wedge to effect the
appropriate change in latitude angle, based on your observations above.
Again, track the star for a period of time to be certain that Declination
drift has ceased.
The above procedure results in very accurate polar alignment, and minimizes
the need for tracking corrections during astrophotography.
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