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The new focuser for the 10" scope has a 10:1 fine control, and
very little torque is required to rotate the fine control knob. It should be
quite a simple matter to construct a stepper motor control to do the focussing,
thus removing vibration from the telescope which is always a problem when you
try to focus by hand. There will also be a
quantitative measurement of the focus position which will be of great value
when doing Bahtinov and Carey mask experiments.
A small but sturdy stepper motor was
purchased, and a Picaxe microcontroller was used as
the basis for the electronics.
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Picaxe board
on the left, L293D motor driver centre.
The Picaxe development board is easy to use. A connection to a
computer USB port allows the programming code to be installed in the Picaxe
chip using a very simple BASIC type language. The
stepper motor had 6 wires of which only 4 are needed, but the people who
frequent the Picaxe Forum
were incredibly helpful in answering my dumb questions, and I eventually got
things wired up correctly and the motor responded by rotating on
command.
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The stepper
motor has 200 steps per revolution. Some experimentation enabled the motor to
be half stepped so that 400 steps per revolution was possible. This rotation
coupled directly to the focus knob will give 3 microns movement per step
which is certainly a much finer control than could ever be obtained by
hand.
The
picture below shows the electronics taking shape on a piece of circuit
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I
had initially considered wiring up 7 segment LEDs to give a readout of focus
position, but the Picaxe Forum suggested using an LCD display. One was obtained via Ebay for less than £5. Hooking the display up to the Picaxe involved quite a bit of 'one
step forward and two back', and a microscopic blob of solder on the circuit
board that was shorting two tracks took several hours to find. But eventually the display started working, only to switch off as
soon as the stepper motor was activated. Again,
several hours elapsed before it became clear that the problem was simply due to
a low battery voltage! If the voltage supply is inadequate these LCD displays
respond by shutting down.
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The
picture above shows the display with a rapidly changing count hence the
disturbed digits to the right. The temporary red LED shows a pulse when the
display is being sent data. The jack plug connected to the PC for programming
can be seen at the rear of the circuit board.
Below is the project squeezed into a
case. The left hand toggle switch rotates the stepper at 'fast' speed, and the
right hand switch is 'slow'. On switch on the
focus number is set to 1000, and then each subsequent step of the motor is
displayed.
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The stepper motor is now in its case, which is simply a length
of 50mm square aluminium tube. Plastic end caps for the tube neatly close the
box.
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A small grommet with the same diameter central hole as the
motor shaft is glued on and a small rubber sucker is glued to the grommet.
Initial tests show that friction between the sucker and the focus knob may be
sufficient to drive the focuser. If not, some adhesive will be
employed.
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The focuser attached to the
telescope. It was a bit nerve wracking drilling holes in the side of the
telescope but luckily all the holes got drilled in the right
places.
The system
works perfectly! When the stepper motor is switched off the focuser can be
turned by hand, which in turn, rotates the stepper motor. When power is applied the motor rotates the focuser under command
of the two toggle switches. Some care in
adjustment had to be made to get the axis of the motor lined up accurately with
the axis of the focus knob. Plastic shims were inserted between the motor case
and the scope tube to enable this to be done.
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