T-Square Tracker

(Drawings)

Over the years there have been a large number of star tracker designs put on line market. Most of these are large, quite expensive and not too portable. They vary from hand guide, tripod mounted systems to very sophisticated, digitally controlled motor driven equatorial mounts used by both amateur and professional sky watchers. Most of these are large and rather expensive. The hand-guided systems are great for observing the sky but require constant adjustment and can pose star trailing problems when used for astrophotography. The sophisticated systems are great for both observing and photographing the sky but have the disadvantage of being very expensive and not too portable (portable enough to carry in a backpack over long distances). As a photographer I wanted something that was inexpensive, portable and simple to operate without constant monitoring.

The motor driven tracker design described here is just such a device and is my modification to units previously designed by people like Roger Sinnot and Dave Trott (see Sky and Telescope, FEB 1988). Those units were designed to be either manually controlled or operated with 115-volt synchronous motors. My unit has the advantage of being very inexpensive, less than $50.00, very simple, very portable, and very flexible. The flexibility of this unit comes from its small size, lightweight and the use of a small 12-volt non-synchronous high torque motor. Precise speed control is maintained by a solid-state electronics package. The heart of the electronics is a 12-volt voltage regulator and a 2k-ohm ceramic trimming resistor. The electronics package is not temperature compensated (though it could be) and I will address this later. The 12?volt motor is of very low amperage and therefore allows one to use AA, c, d, or any other 12?volt supply.

The critical elements of the tracker that are required for precise tracking are the linear dimensions from the hinge lines to the drive screw and the elevator board to camera board rub surface and all soldier joints

T-SQUARE TRACKER ASSEMBLY

STEP 1 Cut all wood pieces to the dimensions specified on the sketches. Sand and apply a couple of coats of polyurethane and let dry for several hours.
 
STEP 2 Locate, mark, and drill appropriate wood pieces for the hinges (make sure that all pieces are properly aligned to prevent binding when assembled).
 
STEP 3 Locate and drill 1/2" hole through the elevator board and hinge for the motor shaft clearance. The hinges are case hardened steel and will require patience when drilling.
 
STEP 4 Fold tracker flat, locate and mark the centerline of the motor shaft clearance hole on the baseboard. This is where the 1/4?20 threaded rod drive shaft goes through the base board.
 
STEP 5 Drill hole through the baseboard sufficiently large enough to allow the 1/4?20 cap nut (also drilled through and tatted) to rock fore and aft when installed on the drive shaft.
 
STEP 6 Install the coupler on the motor shaft (I have found that J-B weld works well for this and you may also want to drill and pin the coupler for insurance against slippage).
 
STEP 7 Install the motor on the motor mount board insuring that it lines up with the previously drilled drive shaft hole. It is critical that the centerline of the motor shaft goes through the centerline of the hinge.
 
STEP 8 Assemble the baseboard screw drive mount as shown on the drawing.
STEP 9 Assemble the electronics package as shown on the drawings. Make sure that there are no cold solder joints as these could lead to voltage fluctuations and rpm variations.
 
STEP 10 Drill holes in the project box. Install switch, light, and banana jacks and soldier all leads. Install the pc board in the box.
 
STEP 11 Drill small hole in the project box so you can adjust the trimming resistor.
STEP 12 Assemble the power supply using about 2 feet of the 18?gauge wire for connecting the battery pack to the control box using the banana plugs and 9v connector.
 
STEP 13 Complete assembly and your tracker is ready for calibration.

CALIBRATION

In order to simplify the calibration of the tracker I drilled through the drive shaft and installed a small pointer made from a nail. The reference mark was obtained by installing a small clear piece of Plexiglas to the front of the motor mount. A vertical line was the scribed into the Plexiglas.

The calibration is accomplished by attaching a digital voltmeter to the motor terminals and adjusting the voltage with the trimming resistor until precisely 1 rpm is obtained. It is best to use a stopwatch and let the motor run for 10 minutes and note the number of revolutions. By an iterative process you can get a very precise rpm. When that is achieved, note and record the voltage. That is the calibration for your tracker with that specific motor and electronics package.

Please note that because this specific design is not temperature compensated the voltage, and therefore the rpm, will shift if it is exposed to large changes in temperature. I have determined that for my use it was not worth adding a temperature compensating circuit. Instead I set my tracker outside and let it soak prior to using it, then simply monitor the voltage and readjust it if necessary. I have not found this to be a problem and generally do not make any adjustments after the initial setup.

TED AYERS
20013 COUNTRY CLUB DR
TEHACHAPI CA 93501

T SQUARE TRACKER PARTS LIST

QTY.
Description
1
2'x4' sheet of 1/2" birch or oak plywood
4
2 1/2" narrow hinges
1
3" hinge
1
1/4-20 cap nut
1
1/4-20x1/2" tee nut
1
1/4-20x10" threaded rod
1
2"x4"x1/16" metal plate
1
2K ohms ceramic trimming resistor     #43043
1
12V red lamp assy
Radio Shack
#272-345
 
1
Project box w/PC board #270-283   #18913
1
SPST switch #275-602   #76241
1
Set banana jacks #274-725   #125196
1
Set banana plugs #274-721   #159855
1
Battery holder (8 AA) #270-407   #216240
1
9V Snap connector #270-324   Not needed
1
LM317T 12V voltage regulator #276-1778   #23579
1
220 Ohm 1/2 watt resistor #271-1111   #30470
1
22 Ohm 1/2 watt resistor #271-1003    
1
560 Ohm 1/2 watt resistor #271-1117   #31376
4
Ft 2 conductor 18 gauge wire
1
Edmund Scientific hi-torque 1 rpm motor (catalog #T41-860)    
1
Shaft coupler
1
1/4-20x1" Brass countersunk screw & collar (for mounting ball head)
1
Pint polyurethane

Drawings

Finished Unit

Plywood parts layout and dimensions

Voltage Regulator Schematic Drawing

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