the horizon.  Several methods have been devised to compensate for this effect called 

"differential refraction.   

The King method (published 1931 by Edward Skinner King) helps to avoid star trailing 

in photographs caused by differential refraction. Differential refraction may especially 

affect wide field photographs with long exposures.  

King proposed two measures: 

Offsetting the polar axis slightly towards the zenith, and 

Adjusting the tracking rate. 

The values necessary to accomplish this depend on the geographical coordinates of the 

observing site and the declination of the object being photographed. 

Gemini's Adaptive King rate tracking varies the tracking speed to match the required 

value for the selected object' s declination.  The user must offset the elevation of the polar 

axis himself. 

Closed Loop 

The preceding discussion of tracking rates assumes that the mount is accurately polar 

aligned, allowing tracking to be accomplished using only the RA motor.  However, for 

visual observing sessions, there is no need for extremely accurate polar alignment.  By 

selecting Closed Loop tracking, Gemini can accurately track an object even if polar 

alignment is off several degrees in any direction.  Gemini does this by comparing the 

coordinates of the object being tracked with the actual coordinates of the telescope.  If the 

coordinates do not match, Gemini makes slow corrections in both RA and Dec. axes to 

point the telescope back at the object.  This process of compare and correct is done about 

22 times per second. 

Although Closed Loop tracking will keep an object centered for a long time, it is not a 

substitute for a good polar alignment   especially if you intend to photograph extended 

objects.  A misaligned polar axis will lead to field rotation, an effect where objects in the 

field of view will rotate around the center.  However, after having done a good polar 

alignment, Closed Loop tracking can be used to photograph slow moving objects, such as 

minor planets or comets.  This can work even if the object moves at a varying speed, by 

repeatedly sending actual coordinates to the Gemini using a computer. 

Because Gemini uses modeling parameters to calculate coordinates, Closed Loop 

tracking is only available after a pointing model has been established. 

Comet/User Defined 

The Comet/User Defined tracking rate allows you to program Gemini to set independent 

RA and Dec tracking rates to follow comets and other solar system objects that follow 

unique trajectories. There are several methods you can use to achieve this: 

Gemini Users Manual 

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