Equinox Systems GEMINI Uživatelský manuál stáhnout pdf (Strana 30)

Gemini Users Manual 25

Gemini displays RA in hours, minutes, and decimal minutes so that the RA value above

would be represented as 16h 42m9. Declination is displayed as degrees, minutes, and

seconds, so the above Dec. value would be represented as +38° 55' 12".

3.1.2 Precession

While most stars, galaxies and other deep sky objects do not move relative to each other

in any kind of time frame we can notice, their RA and Dec. coordinates do change slowly

over the years. This is because the coordinate system is based on the poles and equator of

the Earth. In addition to rotating on its axis once a day and orbiting the Sun once a year,

the Earth’s axis is precessing like a top once every 26,000 years taking the RA and Dec.

coordinate system with it.

This is why RA and Dec. coordinates always have a date associated with them, called the

Epoch or Equinox of the coordinates. Most sources now use Epoch 2000 coordinates

(abbreviated J2000.0), meaning that the RA and Dec. values were correct on Jan 1, 2000.

In order to accurately point a telescope at an object whose J2000.0 coordinates are given,

a precession calculation must be applied to determine the RA and Dec. values for the

current date.

By default, Gemini assumes that any coordinates you enter are for the epoch of the

current date. You can change Gemini’s default epoch to J2000.0 using the

“Setup→Communication→Coordinate Epoch→Epoch J2000.0” menu item, allowing

you to enter (or transfer from a connected PC) J2000.0 coordinates and letting Gemini

perform the needed precession calculation.

3.1.3 Azimuthal Coordinate System

Although the equatorial coordinate system (RA/Dec.) is best suited for most astronomical

objects, some applications require pointing to a given azimuth and altitude (Alt/Az). One

example is pointing to satellites in geostationary orbits. Gemini supports the azimuthal

coordinate system by allowing you to display the current altitude and azimuth as well as

allowing you to enter Alt/Az coordinates from both the hand controller or over the serial

line.

While the altitude of an object can be easily defined as the distance from the horizon,

azimuth needs an arbitrary zero-point and an orientation. There are two common systems:

one starting in the north, the other in the south. Gemini's zero point in azimuth is in the

north, increasing to the east (90°), south (180°) and west (270°).

In order to fit the display, the precision of Alt/Az coordinates is limited to minutes in

Visual mode. In Photo mode, azimuth and altitude are alternately displayed with an

accuracy of (arc)seconds.

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Equinox Systems GEMINI Uživatelský manuál Strana 30