PRECISE POLAR ALIGNMENT; SETTING CIRCLES
Precise Polar Alignment
Important note:
For almost all astronomical observing requirements approximate
settings of the telescope's latitude and polar axis are acceptable! Do not allow
undue attention to precise polar alignment of the telescope to interfere with your
basic enjoyment of the instrument.
If desired, more precise polar alignment may be obtained by first accomplishing
basic polar alignment as detailed in
Polar Alignment Procedure
, then returning to
this procedure:
NOTE: This procedure moves the telescope physically to precisely line up with
the celestial pole. Do not use the Electronic Controller arrow keys to move the
telescope electronically or polar alignment will be lost.
1. Orient the entire telescope, including tripod or tripod legs, so that the polar axis is pointing toward Polaris
(Fig. 15).
2. While observing through the SP 26mm eyepiece of the telescope, adjust the length of the adjustable tripod
leg until Polaris is visible in the eyepiece. Use a combination of (a)
lifting and turning the entire telescope
(or
nudging the position of one of the fixed tripod legs) and (b)
adjusting the length of the adjustable tripod leg
to place Polaris in the center of the telescope's field.
3. Repeat step 2 of this procedure in about 15 minutes to see how much drift has taken place and to make the
alignment more precise.
Although the above procedure is somewhat tedious (since the field of view of the telescope with the SP 26mm
eyepiece is only about 1°), it is a worthwhile effort if precise polar alignment is desired (e.g., if photography of the
Moon or a planet is to be performed). With Polaris placed in the center of the telescope's eyepiece, the telescope is
now polar aligned within about one or two degrees - a level of alignment precision more than sufficient for almost
any observing application.
To provide the most stable platform from which to polar align the ETX-90EC it is recommended to purchase
the
#883 Deluxe Field Tripod
. The tripod head tilts easily to the local latitude angle for quick polar alignment, and
locks in a 90° position to facilitate Alt/Az viewing (see
OPTIONAL ACCESSORIES
).
Setting Circles
The ETX-90EC is equipped with R.A. and Dec Setting Circles (14 and 18, Fig. 1) to aid in locating faint celestial
objects
when the telescope has been polar aligned.
Setting circles emulate the celestial coordinates found on star
charts or in sky catalogs. Any charted object is easily located by coordinates in R.A. (in hours, minutes, and
seconds, from 0h 0m 0s to 23h 59m 59s and Dec (in degrees from 0° to ±90°).
With the ETX-90EC polar aligned the Electronic Controller arrow keys (1, Fig. 5) are used to move the telescope in
Right Ascension (left and right keys) and Declination (up and down keys).
NOTE: The Dec setting circle is located on the left arm of the telescope fork mount. The right arm of the mount
contains a graduated circle (mounted behind the knurled knob of the vertical lock), without Declination numbers.
•
Right Ascension Setting Circle:
Since celestial objects move
in Right Ascension the R.A. setting circle (Fig. 36) must be
reset as each object is located during an observing session.
The R.A. pointer is located on the drive base 90°
counterclockwise from the telescope's computer control panel (11, Fig. 1) immediately under the R.A. circle.
NOTE: The R.A. circle has two rows of numbers from 0 to 23, corresponding to the hours of a 24-hour clock. The
upper row of numbers is used by observers in the Earth's northern hemisphere, the lower row by observers in the
Earth's southern hemisphere.
WARNING! Never use the
Meade ETX-90EC Astro
Telescope to look at the Sun!
Looking at or near the Sun
will
cause
instant
and
irreversible
damage to
your eye. Eye damage is often painless,
so there is no warning to the observer
that damage has occurred until it is too
late. Do not point the telescope or its
viewfinder at or near the Sun. Do not
look through the telescope or its
viewfinder as it is moving. Children
should always have adult supervision
while observing.
Fig. 36: Section of Right Ascension setting circle.
