Schmidt-Cassegrain Instruction Manual
|APPENDIX A: EQUATORIAL
Celestial Coordinates: Declination and Right Ascension
NOTICE! Never use a telescope or spotting scope to look at the Sun! Observing
the Sun, even for the shortest fraction of a second, will cause irreversible
damage to your eye as well as physical damage to the telescope or spotting
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,
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
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
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
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.
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
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
b. If the star drifts North
(or up), the telescope's polar axis is pointing too far West (Fig.
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
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.
b. If the star drifts North,
(or up) the telescope's polar axis is pointing too high (Fig.
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