Hour Angle; Local Sidereal Time - Software Bisque Paramount MX User Manual

Figure 3: Right ascension and declination relative to the celestial sphere.
Declination
The declination (dec) lines on the celestial sphere are similar to latitude lines on Earth, ranging from –90
degrees to +90 degrees. The "declination" of an object is the angle measured from the celestial equator
(0 degrees declination) along a meridian line through the object. Polaris, the North Star has a
declination of just over 89 degrees so it is very close to the north celestial pole.

Hour Angle

Hour angle (HA) is measured from the meridian westward, along the celestial equator and uses the
same units as right ascension (hours, minutes, and seconds). A celestial object on the meridian is
located at hour angle 0. Six hours of time later, the object is located at hour angle +6. Twelve hours
later, when the object rises, its hour angle is –6h.
Hour angle can be used to describe a telescope's mechanical orientation with respect to a horizon-based
hemisphere. For the Paramount MX, the home position in the northern hemisphere is always located at
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 on objects near, across and past the meridian.
TheSkyX Professional Edition can be used to configure the Paramount MX's "flip hour angle" in order to
maximize the length of time a particular object can be tracked near the meridian for longer exposures.

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.
Paramount MX User Guide
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