Paramount GEM User Guide
18 | P a g e
Smaller hour angles equate to positions of optimal observing, so your telescope will likely spend a great
deal of the time pointing to and tracking objects near hour angle zero (near the meridian).
TheSky Professional can be used to configure the mechanical orientation at which the control system slews
the telescope to the opposite side of the pier, or the “flip hour angle” to maximize the length of time an
object can be tracked past the meridian (see page 111 for more information about configuring the flip
hour angle).
Local Sidereal Time
Local sidereal time (LST) is always equal to the instantaneous right ascension of the local meridian. When
you know the LST, you can look at star maps and determine which objects are near the meridian (those
with a right ascension close to the LST). For example, if the LST is 6:10:00, this means that stars with a
right ascension of about 6 hours are presently situated near the meridian.
If everyone reckoned time based on the Sun crossing the meridian, then each longitude on earth would
have a different time of day. That means noon, or the time the Sun crosses the meridian, would come a
few minutes earlier for someone living sixty miles to the east. Only those people living at the same
longitude would share a common time.
In the late 1800’s time zones were established to minimize the problem of having different time in
populous regions. The time within each zone is called zone time. Zone time places all locations on Earth
into various time zones. Time zone zero is at zero degrees longitude, and the time zone increases by one
hour every 15 degrees longitude moving east or decreases by one hour moving west.
The Paramount orients itself by using TheSky Professional’s location, date, time, and time zone settings.
If TheSky Professional’s time zone is not correct, for example, the right ascension coordinate of the
telescope cross hairs on the Sky Chart will be offset by the number of hours that the time zone is incorrect.
Use the Tools, Verify TheSky Time command to compare TheSky Professional’s time with an Internet-
based time clock and make sure these settings are right.
Atmospheric Refraction
The effects of atmospheric refraction on the position of celestial objects, as well as its effect on the
sidereal tracking rate, are often overlooked or even ignored by many amateur astronomers.
The refraction nuisance (and other system errors like tube flexure) means the “sidereal tracking rate” is
simply not good enough to precisely track objects. Refraction also displaces the position of the celestial
pole and makes precise polar alignment more difficult.
Some other interesting and significant facts about how refraction affects an object’s apparent position
include:
• For a sea-level site, the refraction at 45 degrees zenith distance (ZD) is about 60 arcseconds (one
arcminute).
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