Supatrak ASCOM Driver

Supatrak ASCOM Driver

The normal guidelines for releasing this sort of thing say that you should get some beta testers to try it out first. That’s a bit of a problem here. No one is likely to build the circuit to link their SupaTrak mount to their PC (see this page for details) without the incentive of actually being able to computer control the mount when they have finished with the soldering iron. So, I’m releasing this driver into the wild on a very ‘alpha’ basis.

The driver should install (Install ASCOM Platform5 first), will pass the ASCOM compliance tests (with the exception of a few minor items regarding blind commands and internal UTC clocks), and works for me with Cartes du Ciel.

The following features are supported (not an exhaustive list, just the highlights)

  • Movement on both axes. The driver supports the same range of rates that seem to be used by the hand controller – that is 32x sidereal down to zero and ~187x sidereal to 800x sidereal
  • Slewing (Synchronous and Asynchronous) to co-ordinates in both AltAz and RADec
  • Sidereal Tracking
  • Accurate Slewing (computer controls slew movements rather than the mount, typically much more accurate). Accurate Slewing also allows you to limit the maximum slew rate (for noise abatement purposes) [disabled by default]
  • 1-star alignment – the first ‘Sync’ command during a connection session will update the offsets applied to the altitude and azimuth axis so that the altitude and azimuth values will be correct rather than offset from an arbitrary position (note that the mount has no way to measure the altitude/azimuth absolutely, it only measures changes from its power on position).
  • N-Star alignment – a second ‘Sync’ command during the same connection session will use the two sync points to calculate the misalignment of the mount azimuth axis from vertical. After that point, all functions will correct for this misalignment. Syncing to more points should give a slightly more accurate result. [disabled by default]
  • Altitude Axis monitoring and limits. The altitude axis is limited to above 0 altitude and can also to a maximum altitude to minimize problems with the scope hitting the mount. Obviously this only works properly once alignment has been completed.

There is currently no attempt in the driver to correct for the mechanical backlash in the altitude and azimuth drives, mostly because I have had no chance to measure how big the backlash is. To limit backlash problems, all slew operations will finish with movements in the +ve directions (both altitude and azimuth). You should try to ensure that you approach the alignment points from the same direction.


  • Basic slewing uses the slew command built into the mount – this can be inaccurate, particularly on long slews where high speed is used. I have seen this miss by more than 0.5 degrees on occasion.
  • Accurate slewing tries to get within a (measured) 15 arc seconds of the target location. Usually it does better than this, on rare occasions it hunts backwards and forwards each side of the target. I have added some safeguards that seem to have cured this (and it will give up after too many tries), but it’s hard to prove that it will never break
  • Because the flow control lines are not connected and there is no flow control handling in the driver, if you use the handset buttons while software is also talking to the mount, you may get occasional error messages from the driver. Generally, using the handset while connected seems okay (tested in Cartes du Ciel)

Using the Driver

Don’t forget to look at the properties page to set the default COM port and tweak various parameters. You can also turn on the experimental features (like n-star alignment) that are disabled by default here. In the properties page you can also bring up the ‘Test Drive’ window to directly control the scope for debugging or tinkering purposes.

After you tell your application to connect to the driver, the first sync will re-calculate the zero points for the altitude and azimuth axes, the second will calculate mount devaition from vertical (if n-star enabled).

I would write more here, but the sky is clearing…


The driver is free software and is released under the terms of the Gnu GPL license. As an additional right, you are allowed to use the driver with any software (free or commercial) which communicates with it via the ASCOM interfaces (without granting this right, using it with commercial software is a bit of a legal gray area).


Version 1.00 of the driver can be downloaded here .

Version 1.01 of the driver is available here. This version adds the following improvements:

  • Fix for various small bugs, including tracking now working across the meridian (previously would only change speed, not direction of movement)
  • Support for entering measured backlash values and applying them to avoid (maybe) having to always approach a point in the same direction
  • The handset can now be used (relatively robustly) to control the mount while the driver is connected. If you use the handset to adjust position while tracking, the tracking location will be updated (i.e. it shouldn’t just drift back to the old RA/Dec)
  • The install may work on Vista with UAC now – I don’t have a Vista machine to test this on, so I’d be interested to hear if this works out.

Note that 1.0.1 is currently pretty much untested. Mind you, if you are reading this, I expect you enjoy living on the bleeding edge…

Support / Discussion

Drop by at the SharpCap Forums to contact me.