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Sidereal Tracking Platform



Star Astronomy club members built a sidereal tracking platform used to carry a large Newtonian telescope. It had a unique all-thread drive that used a stepper motor from an old dot matrix printer. Using an inexpensive stepper motor driver it was able to track deep sky objects for 30 minutes without needing to reset the platform.

Initial design objectives for my platform were simple. I wanted my tracking platform to:

  • carry my 16” Truss Dobsonian.
  • use a stepper motor drive
  • provide sidereal tracking for my latitude, 40 degrees north
  • work with the Digital Setting Circles I had just completed.

Before starting found I needed a better understanding of critical dimensions of a tracking platform and how it really worked. Being a push-to Dobsonian guy at the time, I really had no idea how or why tracking mounts worked. So I turned to the internet for some basic information.

I found that a tracking platform works using the same principles as a German Equatorial Mount [GEM.] The GEM moves in only two directions, normally referred to as Right of Ascension [RA] and Declination. .

During GEM setup, the mount is arranged so that the centerline of the RA axle and bearings point directly at Celestial north. For those of us in the northern hemisphere, the North Star marks a point in the night sky very near to Celestial North. The Earth rotates about its own RA axle, an imaginary line between our planets north and south poles.   We also know that Earth rotates at a fixed rate of one revolution in 24 hours. This works out to be 15 degrees per hour or one degree every 4 minutes. Once our GEM is aligned with Celestial North all we need do is rotate the RA axle of the mount at the same rate, 4 degrees per minute to match the Earth rotation. When we do this, objects appear to be stationary in the eyepiece of the telescope.

A sidereal tracking platform does not have an “actual” RA axle like a GEM, but it does have a “virtual” RA axle like the Earth. When the platform’s “virtual” axle is aligned with Celestial North and it is rotated at the same rate as the Earth turns objects will appear to be stationary in the eyepiece of a Dobsonian riding atop the platform too.

The second big piece of this puzzle for me was how a stepper motor works. Again, the internet provided plenty of information about them and how to drive them.   The following circuit was gleaned from internet sources.

PIC Stepper Motor Drive Circuit


It was breaded boarded before platform design even started. 

I decided to use a Programmable Integrated Circuit [PIC] as the heart of my stepper motor driver. The PIC drove the platform forward a sidereal speed for tracking objects. The PIC also reset the platform to its exact starting point at high speed. The PIC counted stepper motor revolutions forward and in reverse. Therefore no limit switches were needed to stop the platform at the end of its maximum travel. In addition, my new DSC system could be used normally as long as the platform returned precisely to its starting point after each tracking campaign.

Bread Boarded Test Bench

 I made a detailed set of plans as seen below.  Making plans is something I like to do.  It saves me time during the build.  

You can download a larger pdf version here.


larger pPlatform Plans

Solid Model Rendering I even did a 3d rendering of the platform to help visualize it.  The blue parts are the bearings.  The yellow show the stepper motor and nylon all-thread drive shaft.
Wood Parts Wood Parts too
There aren’t many parts to make. These were cut in one day.  I used 1/2" plywood.  I would use 3/4" plywood if and when I build another one.

I used an old set of rollerblade bearing and axles for the platforms rollers.

The bottom edges of the plywood arches forming the “virtual” RA axle were armored using 1/2” x 1/8” aluminum bar stock. I gorilla glued and screwed the aluminum to be bottom of each arch.

Bearing Side Platform Base

The bottom edges of the plywood arches forming the “virtual” RA axle were armored using 1/2” x 1/8” aluminum bar stock. I gorilla glued and screwed the aluminum to be bottom of each arch.  All plywood parts were assembled with wood glue and finishing nails.  The triangular bearing anchors were ripped from a 2x4. 

Bar Stock Reinforcement
J-B Weld J-B Drying
Finished J-B Gear

The RA drive is the “cool” part of this platform. I copied this idea from my friend’s platform. The RA drive gear was made using J-B Weld epoxy packed under a section of ¼” nylon all threaded rod that was center and taped to the front RA arc. After hardening the J-B was carefully sanded until the ¼” threaded nylon rod could be removed leaving a perfect imprint of the threads behind. A clean section of ¼” nylon all threaded rod was coupled to the shaft of the stepper motor to drive the RA gear.

A curved aluminum bar with tensioning screw at the end was used to hold ¼” all thread drive shaft in J-B RA gear. Simple….

A curved aluminum bar with tensioning screw at the end was used to hold ¼” all thread drive shaft in J-B RA gear. Simple….

Drive Assembly

Full Load

The platform was load tested first using some bricks, then with the full weight of the 16” Dobsonian. It worked…

Initial Load Test

After testing all wood parts were coated with two coats of ebony Polyshade polyurethane.

Two Coats of Ebony Polyshade
Final Controls

Final Controls provide:

  • On-Off Switch
  • Manual and Auto Modes
  • Auto Mode has
    • Normal tracking forward
    • Return to center/level
    • Full rewind to east
  • PIC stops motor at extreme positions, east and west, without need for limit switches.
  • Return to center/level allows use of Digital Setting Circles without need to realign DSC system each platform reset
A small 6" volt sealed battery was selected.  Two were obtained at a very reasonable price from BatteryBob.  Two fully charged batteries is more than enough for one night's observations, even on cold nights.  Battery