Local Sidereal Time; Atmospheric Refraction - Paramount Fitness ME II User Manual

Hour angle can be used to describe a telescope's mechanical orientation with respect to a horizon-based
hemisphere. For Paramount mounts with AutoHoming™, when the mount finds home, the home position
is a fixed, mechanical mount orientation. In the northern hemisphere, when homing is successful, the
mount always points to hour angle 2 and declination 0.
Smaller hour angles equate to positions of optimal observing, so your telescope will probably spend a
great deal of the time pointing to and tracking objects near hour angle zero (or, near the meridian).
Note that TheSkyX Professional Edition can be used to configure the Paramount's "flip hour angle" (page
122) in order to maximize the length of time an object can be tracked past the meridian.

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 visible along 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 60 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 these zones is called zone time. Zone time places all locations on Earth
into various time zones. By definition, 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 relies on TheSkyX Professional Edition having the correct time zone for your observing
site. When TheSkyX Professional Edition's time zone is not correct, the position of the telescope cross
hairs will be offset by the time zone hour error when viewing horizon-based Sky Charts.

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).
Paramount User Guide
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