they are actually based on same principles, that the motor drive rotate tangentially your instrument along a equatorial plane. the main difference, is the size and weight of instrument you are using. Astrotrac ( and also Fornax )can be described as an miniaturised equatorial platform so that all have same limitations. they have around 2 hours of useful tracking time, and have to reset to zero position first before continue Actually there was a tangent arm drive project started by Manchester Astronomical Society in 1994 which develops very much use the same concept of "screw rod + stepper motor " budget astronomer Reiner Vogel on equatorial platform