Equatorial Mounting

Principle

The stars, like the Sun, rise to the east and lie down in the west, in a great apparent movement of rotation of the whole of the vault of heaven. However, to make an observation, it is necessary that the direction pointed by the instrument remains fixed compared to stars. One thus compensates for the rotation of the Ground by an opposite rotation of the instrument: as the Earth turns of a turn by 12 midnight sidereal, around the North-South axis, the instrument must turn around a parallel axis, in the contrary direction and at the same speed. It is one of the axes of the equatorial mounting which fulfills this role, the second axis making it possible to change the direction concerned.

Mounting supports an instrument of astronomical observation, such as the Télescope or the glasses.

Description

Equatorial mounting is made up mainly of an axis, known as axis polar or time, which carries the instrument of observation (often by means of a fork). This axis is graduated in hours (0-24). A second axis, perpendicular to the polar axis, is called axis of the variations. It is graduated in degrees (+90 with -90°).

To be operational, equatorial mounting requires an adjustment called put in station . It is indeed possible to modify the slope and the orientation of the polar axis. The slope compared to the horizontal one must correspond to the Latitude of the site. The orientation must be done towards North or the South, according to the hemisphere.

When mounting is put in station, the polar axis must point towards the celestial Pôle, which corresponds to 1° close with α Ursae Minoris, said the pole star in the northern hemisphere, which means that the polar axis of mounting is parallel to the axis of rotation of the Earth. Thus, the rotation of the instrument around this axis makes it possible to follow the Right ascension of the celestial sphere, and rotation around the other axis changes the variation. Certain star follow-ups require an operation known as reversal of this axis in a sysmetric position, sometimes delicate.

A electrical motor involves the polar axis in rotation, at the speed of 1 turn by 12 midnight. The regularity and the precision are the essential criteria to make astronomical photography with length Exposure time.

In mounting the most worked out, all the axes are motorized. A computer, coupled to a receiver GPS, order the engines for an erection fast, and allows an automatic pointing: it is enough to enter the celestial coordinates of the object, or to select it in a list.

See too

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