3 Axis Mounts
Mel Bartels, June, 2003

Why 3 Axis Mounts?

I mean, geez, now you have three drives and three motors and they will have to be under computer control to boot. The attractiveness is that three axis mounts have no field rotation, and, can be aimed so that only a single axis need be moved to track satellites. In addition, three axis platforms free the builder from the constraint of building the latitude into the platform. Really, it's fun to investigate and design three axis mounts since the additional third axis opens up design possibilities, particularly if slewing is restricted to two axes and the third axis is of limited travel and only turned on to freeze field rotation during tracking. Time will tell whether the pluses will outweigh the additional complexity of three motors.

Here's some numbers showing three axis tracking with no field rotation for an alt-alt-az mount located at north 44 degrees latitude with the starting third axis azimith aimed below the pole, locked on object rising in the east. These are distances moved in each axis over one minute intervals.

Test of AltAltAz tracking
please enter Ax3 deg (horizon point that the primary axis is aimed towards) 0
please enter altitude deg 50
please enter azimuth deg 90
please enter the time interval between position readouts in seconds 60
please enter the # of intervals desired 5
initializing to altalt alignment
distances to move for 5 consecutive 60.0 second intervals...

holding field rotation angle= 38.47127922207323
delta deg ax3 (rotation about sky's zenith)= 0.17341020112166144
delta deg az (primary altalt axis aimed at horiz)= 0.2542758006895452
delta deg alt (right angle to primary altalt axis)= 0.16352753004075063

holding field rotation angle= 38.47127922207323
delta deg ax3 (rotation about sky's zenith)= 0.17281749047329675
delta deg az (primary altalt axis aimed at horiz)= 0.2524196497933847
delta deg alt (right angle to primary altalt axis)= 0.1600756042490719

holding field rotation angle= 38.47127922207323
delta deg ax3 (rotation about sky's zenith)= 0.17226899746154095
delta deg az (primary altalt axis aimed at horiz)= 0.2506008462102014
delta deg alt (right angle to primary altalt axis)= 0.15670522105866547

holding field rotation angle= 38.47127922207323
delta deg ax3 (rotation about sky's zenith)= 0.17176430313304056
delta deg az (primary altalt axis aimed at horiz)= 0.2488188373024876
delta deg alt (right angle to primary altalt axis)= 0.1534148103074487

holding field rotation angle= 38.47127922207323
delta deg ax3 (rotation about sky's zenith)= 0.17121915275688593
delta deg az (primary altalt axis aimed at horiz)= 0.2470043090547482
delta deg alt (right angle to primary altalt axis)= 0.15005198680428075

The three motors are all moving at a rate with a magnitude of 1/7 to 1/4 degree per minute, which is analogous to the sidereal tracking rate, a very reasonable and easy rate of motion to achieve.

A 48 inch three axis scope has been located in Cloudcroft New Mexico for years. See Manly's Unusual Telescopes, page 88, and his discussion of three and four axis mounts on pages 84-91.

A computer controlled drive system is not a forgone conclusion. Decades ago before digital computers, some amateurs used a mechanism called an equatorial transformer that took mechanical action generated by a small equatorially aligned machine and forced the 2-3-4 axis mount into following its motion.

Richard Berry has suggested a table mount with three motorized pistons.

Here are some of my back of the envelope (what's the appropriate nickname when using a CAD program?) sketches:

eof