Hi Trevor,
The TDM came out in 2009 specifically to improve tracking for the EQ6 and similar mounts.
There is a discussion on this linked below.
http://www.iceinspace.com.au/forum/s...ad.php?t=39857
This is part of the sales blurb.
Fitted to your mount, the Telescope Drive Master (TDM) seeks to resolve both periodic and random errors in a unique manner. A very high-precision rotary encoder, which measures changes in the speed of rotation, is attached to a right ascension (RA) shaft. An electronic control unit then compares the signals from the encoder against an extremely accurate quartz timer, and if the speed that the RA shaft rotates at deviates from the sidereal rate (the speed the Earth rotates), speed corrections are sent to the mount via its ST4 port. This feedback system ensures that the mount rotates at a very stable speed, dramatically improving tracking accuracy for long exposures in the range of five to 10 minutes.
The TDM relies heavily on a very accurate polar alignment that can only be achieved by a ‘drift alignment’ process. This takes a relatively long time to carry out and although it can be done ‘in the field’; it’s more often used by astronomers with a fixed mount, in an observatory, for instance.
It was expensive to buy, around $2K AU. Because it attached to the ST4 port on the mount, you could not use guiding.
Cheers
Bill
