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Gemini Users Manual 27

3.3.1.1 Predefined Rates

Two predefined tracking rates drive the RA axis at a constant speed to compensate for the

Earth’s rotation.

Sidereal

Sidereal rate is simply the rate that compensates for the Earth’s motion relative to the

celestial sphere. It keeps the mount pointed at a constant RA and Dec. coordinate in the

sky and will probably be the rate that you use the vast majority of the time. With sidereal

tracking selected, the telescope makes one revolution per sidereal day (86164.0905

seconds). Note: The sidereal rate assumes that your mount is accurately polar aligned.

None/Terrestrial

This rate turns tracking off completely and allows you to observe stationary objects such

as points on land, or geosynchronous satellites. It is also useful for keeping the telescope

from exceeding its safety limits when the Gemini must be left on for extended periods of

time between observations.

3.3.1.2 Computed Tracking Rates

In addition to the Earth’s rotation, the computed tracking rates take many additional

factors into account such as the Earth’s orbit around the Sun, the Moon’s orbit around the

Earth, and an object’s position relative to the horizon.

Lunar and Solar

Neither the lunar nor solar tracking rates is constant. Since the orbits of Earth and Moon

are slightly elliptical, the apparent speed of Moon and Sun will change. When you select

the lunar or solar tracking rates, the tracking speed will be calculated based on the actual

position and the position one hour later.

Because the Moon orbits the Earth, its RA and Dec. change during the course of the

night. The lunar tracking rate compensates for the RA component of that change; there is

no compensation for the changes in Dec. While sidereal rate can certainly be used for

visual lunar observing, the lunar rate will do a better job keeping the Moon centered in

the telescope’s field of view. Since the Moon is relatively near the Earth, even the

location on Earth where you are observing must be taken into account. The lunar

tracking rate therefore takes the parallax into account by calculating topocentric instead

of geocentric positions.

Gemini computes the tracking rates by calculating the current position of the Sun or

Moon and the position one hour later. While the tracking rate for the (distant) Sun will

not change much during an observing session, the tracking rate for the Moon might vary

significantly as its parallax changes moving from the horizon to higher elevations and

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