NexStar Communication Protocol
This document describes the serial commands supported by the Celestron NexStar hand control. This information
applies to the NexStar GPS, NexStar GPS-SA, NexStar iSeries, NexStar SE Series, NexStar GT, CPC, SLT, Advanced-
GT, and CGE mounts.
Communication to the hand control is 9600 bits/sec, no parity and one stop bit via the RS-232 port on the base of the
hand control.
Note: the GT hand control prior to version 104.0 should be treated as version 1.2 in the tables below. Version 104.0
should be treated as version 4.03.
Get Position Commands
The following commands retrieve the position of the telescope in either RA/DEC or AZM-ALT coordinates.
The position is returned as a hexadecimal value that represents the fraction of a revolution around the axis. Two
examples are given below:
• If the Get RA/DEC command returns 34AB,12CE then the DEC value is 12CE in hexadecimal. As a percentage
of a revolution, this is 4814/65536 = 0.07346. To calculate degrees, simply multiply by 360, giving a value of
26.4441 degrees.
• If the precise GET AZM-ALT command returns 12AB0500,40000500 then the AZM value is 12AB0500 in
hexadecimal. As a percentage of a revolution, this is 313197824/4294967296 = 0.0729 or 26.252 degrees.
The standard commands offer a precision of 1/65536 * 360 * 60 * 60 = about 19.8 arcseconds per unit while the precise
commands offer a precision of 1/16777216 * 360 * 60 * 60 = about 0.08 arcseconds per unit (only the upper 24 bits are
used).
Note: if the telescope has not been aligned, the RA/DEC values will not be meaningful and the AZM-ALT values will
be relative to where the telescope was powered on. After alignment, RA/DEC values will reflect the actual sky,
azimuth will be indexed to North equals 0 and altitude will be indexed with 0 equal to the orientation where the optical
tube is perpendicular to the azimuth axis.
Command Function PC Command Hand Control
Response
Applies to
Versions
Get RA/DEC “E” “34AB,12CE#” 1.2+
Get precise RA/DEC “e” “34AB0500,12CE0500#” 1.6+
Get AZM-ALT “Z” “12AB,4000#” 1.2+
Get precise AZM-ALT “z” “12AB0500,40000500#” 2.2+
GOTO Commands
The following commands direct the telescope to GOTO a specified RA/DEC or AZM-ALT position. As with the Get
Position commands, the values are in hexadecimal and represent the fraction of a rotation around the axis.
Note: GOTO RA/DEC commands will not work unless the telescope is aligned.
Command Function PC Command Hand
Control
Response
Applies to
Versions
GOTO RA/DEC “R34AB,12CE” “#” 1.2+
GOTO precise RA/DEC “r34AB0500,12CE0500” “#” 1.6+
GOTO AZM-ALT “B12AB,4000” “#” 1.2+
