- VisuTrak -

A CoSaTrak Implementation - by Willie Koorts

My first attempt at motorising an altazimuth mount for satellite tracking had quite a number of shortcomings. After seeing the excellent results obtained by Greg Roberts' VideoTrak system, I revisited my well stocked junk box - and so did Greg who passed on a few very useful essentials! After a very productive week in June 2000, the end result was this system on the right.

Early results were very encouraging and after sorting out a few teething problems, the mount is performing very satisfactory under CoSaTrak computer control. The main problems were with play in the gears causing the motors to stall when the mesh was too tight but when reducing it too much resulted in vibration when trying to keep up with a high speed satellite. The ex-floppy drive stepper motors are performing surprisingly well against the inertia of having to move a fair mass at quite a speed.

An overall view of my 2nd attempt at a satellite tracking mount
A view with the video camera mountedUnlike Greg's video based system, mine was built with visual satellite tracking in mind although I found an easy way to add a bracket to also have the ability to mount a camera or video.

Since all these pictures were taken with my video camera, the best way so show you what the camera looks like when mounted on the bracket was to let it look at itself in a mirror!

By the way, the black thing round the lens of the spotting scope is a poor man's dew-shield - a piece of packing foam rubber.

I found a number of parts, stripped from an ancient Diablo disk drive unit extremely useful. These units were built very solid and to high precision and parts like the disk drive motor (right) and its magnetic coupling to the removable disk (picture below) could almost be used as-is.

To the rotating top part of the ex-drive motor, a 172 teeth spur gear was attached, surrounded by a nice chrome 360° setting circle (both given to me by Greg) mounted to the stationary part of the motor. The rest of the gear train consists of a 14 teeth gear driven trough a 12.5:1 Muffet gearbox (black rectangular box) via a toothed belt to the stepper motor. This yields an overall ratio of 230:1 and by half stepping the 200 steps/rev. motor, moves the mount by 14 arc sec./½step. A track speed of up to 6°/sec was measured, depending on the PC.

The Azimuth drive
The magnetic connection of the top endThe original precision magnetic disk-platter-to-motor coupling was retained to very easily split the mount in the middle. Initial fears were that this coupling might not be strong enough to carry an unbalanced load or could slip when driven but were found not to be the case. The centre cone connection is very accurate and found to be 100% repeatable with assembly and dis-assembly.
A circular bubble level helps to set up the azimuth movement. An adjustable azimuth pointer (scribed line on black ring) indicates the azimuth position of the mount. Three plastic screws (one visible) control the friction of this rotatable pointer. The Azimuth setting circle
The Alitude ForkA lightweight but rigid cast aluminium fork from my junk box was found ideal to carry the Altitude axis while providing room for its drive gear. This fork snugly slides down a groove, machined into the two aluminium brackets that hold it upright. A bolt and wingnut makes dis-assembling the fork very quick and easy.

The bearing blocks and altitude-axis shaft are made up from the cannibalised disk head assembly from the same Diablo disk drive unit.

The big altitude drive gear was improvised by glueing an inverted toothed belt to a ring of matching diameter. Via another 12.5:1 Muffet gearbox and a short belt drive, the overall altitude gear ratio work out to 275:1 meaning about 12 arc sec/½step of the old floppy drive stepper. A track speed of 6°/sec was also achieved in altitude before.

Another chromed scale from Greg became the altitude position circle which is also adjustable for setting it up accurately. An improved pointer to the temporary Allen key seen in this picture is still to be added.

The Altitude Drive
The Mount being AssembledThis fun animation shows the possible stages of assembly of my new mount - none of these require any tools. Not all these splits are essential but since they mostly came about through the spare parts I used, the ability of taking it apart into small parts was retained for when it might come in handy.

Willie Koorts - 10 July 2000

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