TRACKER Mk2 DESIGN

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Some of the design objectives for the new tracker were:

• Hand Controller - to allow manual guiding operations by hand via Fast and Slow buttons.
• Permanent Periodic Error Correction (PEC) - Non-volatile storage of correction data for worm gear errors.
• Autoguiding - Connection port to allow computer controlled real time corrections of tracking errors in RA.

 

Drive Resolution

The stepper motor has 48 steps/rev. This is coupled to 250:1 gearbox, which is connected via a flexible coupling to a worm gear with 180 teeth. The resolution of the drivetrain is calculated as follows:

Click image to enlarge

The motor step rates are calculated based on the period of the earths rotation, taken as 23 hr 56 min 4 sec. (The Moon's period is taken as 27.32166 days). For the tracker to rotate at earth's rate of 15.041364 arc second/second, the step frequency for the motor is:

BCD Switches For Frequency Setting

There are four BCD (Binary Coded Decimal) switches inside the case which allows the motor drive frequency to be set. They set the divide factor for the master clock frequency.

The master clock is taken from an 8640AN Programmable Oscillator, the output of which can be programmed by six control pins. This provides a very convenient way of introducing faster foward and reverse settings. For guiding and slewing with the hand contoller it provides Slow (x0.5) and Fast (x1.5).

8640 CTL	1	2	3	4	5	6	O/P Freq (Hz)
Forward (x1)	1	1	0	0	0	0	100kHz
Slow (x0.5)	1	1	1	0	0	0	50kHz
Fast (x1.5)	1	0	0	0	0	0	150kHz
6x Reverse 6x Forward	0	0	0	0	0	0	600kHz

A set of four BCD switches are used to set a further subdivision using a CMOS 4059 Programmable Divide-by-N Counter. This IC is setup in a mode preset for divide by 10. The BCD values for sidereal rate are calculated as follows:

Following a similar analysis for tracking the Moon gives the BCD switches settings of 414 REM 0.

The implementation of the 398 REM 9 subdivision is as follows.

10^2	10^1	10^0	Remainder	Mode Control
1   2   4   8 J13 J14 J15 J16	1   2   4   8 J9 J10 J11 J12	1   2   4   8 J5  J6  J7  J8	1   2   4   8 J1  J2  J3  J4	Ka  Kb Kc
3	9	8	9
1   1   0   0	1   0   0   1	0   0   0   1	1   0   0   1	1    1    0

 

The accuracy of the crystal clock has been measured using a time interval analyser. For the sidereal rate, the theoretical frequency for 398 REM 9 is;

The measured values are;

Mean Frequency (Hz)	25.068161601
Max Freq (Hz)	25.0681623
Min Freq(Hz)	25.0681608
Error	0.00078 Hz
Error in parts per million	-31

The main cause of the error is the 100 kHz frequency output from the 8640N. This was measured at 99.99700 kHz. This equates to an error of 30 ppm. Using this figure gives a predicted output frequency after the subdivsion of 25.068188 Hz.

The drive circuit for the stepper motor is a Philips SAA1027.

The motor is a four phase unipolar stepper motor, with 48 steps/rev. (Farnell Part No 147-879). This is connected to an ovoid 250:1 gearbox (Farnell Part No. 147-885).

The electronics are housed with the original casing covering the worm gear. The controls are as follows:

Main Power Switch
Switches the 12V DC from the battery, protection via a 2A fuse.

Drive Switch
This is a four position switch:
• DRIVE OFF
• DRIVE ON - Normal (x1)operation using the frequency set by the side mounted BCD switches.
• 6X FOR - Forward drive at 6 times normal rate
• 6X REV - Reverse drive at 6 times normal rate

PEC Switch
This is a four position switch:
• PEC OFF
• PEC ON - Drives tracker using 16-bit frequency subdivision values stored in non-volatile memory (EEPROM). There are 128 stored data values for one rev of the worm.
• REFINE - Drives the tracker using the stored PEC data but allows the data values to be adjusted during each of the 128 steps per rev. The modified values overwrite the original subdivion value at the end of each the 128 steps.
• RESET & H/C - This initiates a new PEC programming cycle when the PEC RESET button on the hand contoller is also pressed. This 'two stage' reset process is to guard against accidentaly overwriting previously stored PEC data.
Following a Reset, the normal subvision value is loaded as a starting point. The hand controller is used to increase or decrease the subdivision value to make the tracker slow down or speed up. The subdivision value is stored in the EEPROMs at the end of each of the 128 steps per worm.

Hand Controller
A hand controller (H/C) from a Meade telescope has been modified to provide the Fast and Slow control buttons. During normal drive operation the Fast is x1.5 and Slow is x0.5,of the normal drive rate.
When PEC is switched on, the same buttons are used to decrease and increase the 4-digit subdivision data word. The 4-digit number is displayed in the left hand LED display.
The H/C has X6 momentary switch to provide 6 time forwards and 6X reverse operation.
The H/C also is used to provide a secondary momentary switch for the PEC Reset command.

Autoguiding Port
An autoguiding port has been added to the base of the tracker case. The RJ12 connector interfaces to the 8640AN programmable frequency oscillator. The autoguiding connectors follow the pinouts for the generic ST-4 autoguider. Four opto-isolators are used to provide electrical isolation for the computers electronics.

 

 

 

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