Mount tracking errors are a sad fact of life.
They’re caused by small imperfections in the manufacturing process, which introduce regular, ‘periodic errors’.
Other random errors are also produced by imperfect bearings and uneven lubrication.
For visual use these errors aren’t really a problem, but once you attach a camera to your scope and start taking long exposures, tracking errors will quickly show up as elongated stars.
Many mounts have a feature called periodic error correction (PEC) that enables you to record a set of corrections required to counteract the tracking errors for a whole turn of the mount’s worm drive.
This recording can then be played back every time the mount is used and the periodic errors will be corrected automatically.
Although PEC can be very effective, unfortunately it has no effect on the random errors.
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.
The TDM has three main parts: the electronic control box, the rotary encoder and an adaptor kit for your specific mount.
Telescope House kindly lent us a Sky-Watcher EQ6 PRO mount for this review, on which the encoder was simple to install.
First, the EQ6 PRO’s polarscope is removed and the beautifully machined encoder shaft is installed in the hollow space in the RA axis.
A fixed ring is then screwed onto the mount over the shaft dust cap and held in place by three screws, while the encoder is attached to the fixed ring with four bolts.
There are clamping rings at either end of the encoder and these are tightened onto the encoder shaft.
The encoder cover is then held on the fixed ring with three bolts.
Finally, the original dust cap is screwed onto the cover, making for a very well-fitting and secure installation.
Connecting the control unit was very simple, with just power, encoder and ST4 cables required.
The EQ6 PRO was star aligned and slewed to the chosen object in the normal way using the mount’s hand-controller, and then the TDM was turned on.
The system carried out a quick self-calibration routine to decide the direction of shaft rotation (for northern or southern hemisphere) and the ST4 autoguider port’s speed of correction, then monitoring and auto-correction kicked in automatically.
We were very impressed with the simple operation and our own independent tests revealed that it improved maximum periodic error from 13.86 arcseconds to just 1.41 arcseconds.
However, it’s worth bearing in mind that corrections are only made in RA and no corrections are made for errors produced by external influences like atmospherics, mirror flop, flexure or polar alignment.
Declination corrections will still have to be made using an autoguider.
So this is not a complete solution for perfect tracking.
The real benefit of using this system, though, is that much longer integration times can be used with your autoguider.
This makes it possible to guide on dimmer stars, which can be a great advantage when using an off-axis guider.
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A version of this article appeared in the April 2010 issue of Sky at Night Magazine