How do I set RA limits for my Advanced GT Mount?
March 30, 2011
RA limits restrict the slewing and tracking movements of your mount to the east and west to prevent it from running into itself, the tube running into the mount or attached equipment running into either the mount or other obstacles.
For firmware versions 4.15 and above, there are three options. They are found under Menu > Scope Setup > RA Limits. RA East Limit sets the limit for the tube on the east side of the mount. RA West Limit does the same thing for the tube on the west side of the mount. The ranges of both settings are from +40 degrees to -20 degrees. Disable Limits turns off any user-entered values and allows the mount to go to -20 degrees on either side.
These angles can be visualized by imagining you are looking straight down the polar axis from the north and above the mount. Each limit’s range of 60 degrees is a segment around horizontal on either side of the mount. Then RA East Limit of -20 is a tube position 20 degrees below the horizontal to the left of the mount (east, a little after 8 o’clock) while RA East Limit +40 puts the tube +40 above the horizontal to the left a little after 10 o’clock . RA West Limit of -20 is a tube position 20 degrees below the horizontal to the right of the mount (west, a little before 4 o’clock) and RA West Limit +40 puts the tube +40 above the horizontal to the right a little before 2 o’clock.
Use the limits as follows. First set the mount to the index marks. Power on the mount. With clutches engaged, move the mount using the hand control’s arrow keys until the limit is where you want it to avoid contact or cable pullout. That could be when the mount’s motor housings contact, when your OTA contacts the tripod or mount or your cables for an attached accessory (such as a camera) are stretched taut.
(NOTE: Even though limits in Dec aren’t set, these contact-stretch limits depend on where the scope is pointing in Dec. Be sure to position the Dec axis according to your situation and or needs.)
Next, go to Menu > Utilities > Get Axis Position and view the azimuth number. Translate the number displayed by noting that OTA east is 0 degrees, that the index position is defined by the mount as +90 degrees, OTA west is 180 degrees. A reading of 195 degrees would mean a setting for the RA West Limit of -15 degrees (180-195 equals -15), a reading of 172 degrees would be a RA West Limit setting of +8 degrees (180-172=8). A reading of 356 degrees would mean a setting for the RA East Limit of -4 degrees (0-356 =- 4), while a reading of 7 degrees would be a RA East Limit of +7 degrees.
The default sets the mount to RA East Limit to - 6 degrees and RA West Limit at -6 degrees too. This allows for 180 degrees of motion and GOTO approaches to take up backlash. These approaches can be as much as 6 degrees beyond horizontal in each direction east or west.
For older versions of the firmware, the default is 0 when the OTA is east of the mount and 180 when it’s to the west of the mount. Azimuth readings would be set directly as the RA Limits. For example, 356 degrees and 195 degrees. To account for GOTO approach and anti-backlash for a 180-degree range of motion, set the mount's RA Limits to 354 and 186.