3.3.3  RA Limits 

Whether you are imaging or observing visually, you do need to be concerned about the 

east and west limits of the mount.  The system will never make a complete turn in RA. 

There are default limits set for how far the RA axis can be turned to the east and to the 

west.  During a GoTo operation, if the mount cannot reach its target on the current side of 

the meridian, it will make a meridian flip.   

To change the safety limits, use the Hand Controller to move the scope to where you 

want the new limit to be set, and select the   Setup

Mount Parameter

Set Safety Limit   

menu item.  You have to start up the system accurately in Startup Position and slew to the 

position you want to set as the new limit.  While this is no problem if you are narrowing 

down the allowed range, going beyond the old limits is a bit tricky.  About 10 degrees 

before you reach the current limit, the system will softly ramp down the speed until the 

motor stops near to the real limit. Carefully continue moving with centering speed or 

slewing slowly until the system stops the motor and you hear the buzzer. You are now 

outside the allowed range.  Use the Hand Controller to carefully move beyond the old 

limit to the new limit position, and select the Set Safety Limit command.  The buzzer 

won 

t stop until you go back inside the allowed range or set the new limit with this menu 

item.  

IMPORTANT

: The limits are stored in CMOS SRAM. Whenever the "CMOS reset" 

message is displayed during startup, all values are reset to the factory default.  If you 

need different limits to be set, you will have to set them again!  

While tracking, an audible warning (a short beep about every 20 seconds) sounds some 

minutes before the western safety limit is reached.  This gives you an opportunity to close 

the camera shutter or take whatever other action is necessary.  If you want to continue 

your observation of this object, you' ll need to slew the mount to the east, away from the 

object by several degrees, then return to the object by issuing another GoTo command.   

Gemini will then do a meridian flip and point to the object on the other side of the 

meridian. 

3.4  Polar Alignment 

3.4.1  Definition 

Polar alignment is the process of aligning the mount' s polar (RA) axis with the Earth' s 

rotational axis.  It involves physically adjusting the elevation and azimuth of the mount, 

usually accomplished by turning adjustment knobs.   Although the mount does not need 

to be level to be polar aligned, a level mount can make the alignment process much easier 

since adjusting the elevation knob on a level mount just moves the mount in elevation 

(and the same for the azimuth adjustment).  Moving the elevation knob on a mount that is 

not level introduces some azimuth component as well.  While leveling is an important 

time saver for the Drift Alignment method, it is not necessary at all if you intend to use 

Gemini 

s Polar Axis Correction feature. As mentioned earlier, an accurately polar aligned 

mount can track the motions of the sky by driving just the RA motor. 

Gemini Users Manual 

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