Equinox Systems GEMINI Manuel d'utilisateur télécharger pdf (Page 31)

Gemini Users Manual 26

3.2 How the Sky Moves

If you have spent any length of time outdoors looking at the stars, you’ve probably

noticed that everything in the sky shifts position over time during the night. Of course

this is due to the fact that the Earth is rotating on its polar axis. Recall that the celestial

poles in the sky lie directly above the Earth’s poles, so just as the Earth rotates around its

poles, the entire sky appears to rotate around the celestial poles, completing one

revolution in about 24 hours. Objects that lie close to the poles make small circles

around the pole, while objects further from the pole make larger circles, taking them

below the horizon for periods of time during their revolution. So, although an object’s

RA and Dec. don’t change significantly during the course of a night, a fixed telescope

aimed directly at a point on the meridian (the imaginary line passing directly overhead

from north to south) would see stars drift through it from east to west as increasing RA

values cross the meridian. It is important to understand that a fixed telescope points to a

constant Dec. value, but a changing RA value. This also explains why RA is measured in

hours.

There is another coordinate system – one that does not move with the rotation of the

Earth. Because it does not move relative to the Earth, it can be used to describe a

telescope’s position relative to the Earth. It uses the same Dec. value as does the rotating

system to measure angles north and south of the poles. But instead of RA, the so-called

"Hour Angle" is used to measure angles east and west. It too is measured in hours,

minutes and seconds (or sometimes just minutes), but uses the meridian as its zero point.

An object in the east has a negative hour angle. This value is interpreted as the number

of hours it will take for the object to arrive at the meridian. If it is due east, its Hour

Angle (abbreviated HA) will be -6h, meaning it will need 6 hours to arrive at the

meridian. Objects in the west have a positive hour angle, indicating the number of hours

the object is past the meridian. Gemini displays the hour angle of an object (in minutes,

together with the current elevation) at the time you select it from one of the databases.

3.3 Tracking

One of the jobs of a telescope mount and control system is to move the telescope to

compensate for this constant change in RA of the sky. In order to stay pointing at a

constant RA value, the mount must move at the same rate that objects in the sky appear to

move. This is called tracking. If your mount is polar aligned, then its polar (or RA) axis

is parallel with the Earth’s axis, and tracking should involve simply driving the mount in

RA at the same rate the Earth is turning. While this is a very close approximation for

stars, there are a few complications that need to be addressed.

3.3.1 Rates

Gemini can drive the telescope at several different tracking rates depending on the object

you are observing and how close your mount is to being polar aligned. The currently

active tracking mode is displayed briefly whenever you press the MENU button to enter

Telescope Control Mode.

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Equinox Systems GEMINI Manuel d'utilisateur Page 31