After attending the star party in Lostock I learnt that trying to do digital planetary capture using manual focus was a nightmare. Nothing more frustrating that trying to focus the planet by hand, waiting for it to stop bouncing around on the laptop screen and then wondering if it was actually an improvement
So my mission was to build a motorised focuser out of whatever I had lying around at minimal cost.
Things I found at home included an old remote control car controller and servos (no car), some circuit board, 555 timer, voltage reg, resistors, wire, screws, a small plastic enclosures and some sheet steel.
Things I had to acquire included, gears, another small plastic enclosure, power connector and a short length of steel rod. The gears I got out of a scanner I picked up from Aussie junk for $3, the enclosure from Jaycar for about $3, power connector from Jaycar for about $1 and the steel rod from a metal distributor, free scrap.
My plan was to connect the servo motor to my focuser and control it using the lever off my remote control controller.
First I had to modify the servo so that it could continue to rotate in any direction and not return back to the neutral position. This required the removal of the internal end stops and replacing the internal variable resistor (used to provide the neutral position voltage) with a resistor voltage divider so the servo always thought it was in the neutral position and therefore stay where it stopped. At that stage I also connected the small gear to the servo motor.
Next I constructed a circuit that would produce the necessary pulses to control the servo motor, used a simple 555 timer. I also added a 5V regulator so I could use my standard 12V connector to power both the circuit and the servo motor (trying to keep consistent on my scope). To set and vary the pulse width to the servo motor (pulse width controls the direction and speed of the servo motor) I needed to select particular resistor values for the 555 timer and include a variable resistor to provide the control. It just so happens that the controlling levers on the remote control controller is purely a variable resistor, so after removing it from the controller, measuring the resistance and adding additional resistor to make up the correct value for the 555 timer I had my means of control. And to include it into the 555 circuit I just wired it in using about 5m of wire, which allowed me to stand away from the scope when controlling it.
Next I pulled apart the original focuser and replaced the adjusting rod with the one I bought attached the other gear (hot glue is great stuff), then I cut out the steel plate attached the circuit to one side and the servo motor to the other and screwed it to the focuser base, using one of the original screw holes and tapping an additional hole in for better stability.
And there you have it, one motorised focuser controlled remotely with a small lever, which gives both directions and variable speed depending on how far you push the lever. Total cost < $10 and due to the gear ratio I used the minor adjustment is small enough to allow for precise focusing.
The only glitch I seem to have at the moment is the servo occasionally sticks at very slow speed, but moves on with a bit of persuasion. I'm pretty sure a replacement of the internal grease will fix that, just need to determine the best type to use.
My long term plans are to build a circuit that will allow control from the serial port, so I can site at my PC for all control of movement and capture, all I need is time
Thanks for reading my spiel and I hope this may spark some ideas for someone out there and hopefully make life that little bit easier. If youi have any questions please ask.
You know I have an old remote control car remote and reciever sitting somewhere in the shed I couldn't bring myself to throw out. You have given me an excellent idea. Thanks
Hi Kisa. Nice job on the MF unit. Bit along the same lines as mine.
I used a very simple, 2 x 'AA', voltage/current regulator tho & an up/centre off/down, switch & a belt drive to the focuser knob.
Makes it very easy to fine focus now, don't they? & no vibrations at high mags.
Good job...... L.
Thanks for all the feedback guys, it's much appreciated. As I mentioned, I have long term goals of having complete remote control of my scope via a PC (similar to Bird, give or take 6.1 inches of Mirror), so I'll let you know how thinks go with the PC control version of the focuser.
I also like the idea raised by Rajah235 to use a belt drive, it would avoid the accuracies required aligning up the gears and will remove any slack. My only concern may be the stress the belt might put on the servo drive shaft, though it's probably minimal.
Another variant that avoids the use of setting up a 555 circuit and dismantling the remote control controller would be to leave the remote control receiver connected to the servo motor and control it using the remote control controller (wireless). The only thing still needed would be to modify the servo motor to allow continual rotation.
Just a thought, I used friction drive on the RA of my Dob base...ever thought of using a piece of rubber tube attached to the end of the drive shaft against the adjustment knob on the focuser?
That is way cool,,,hows the project coming along, I have a similar thing in mind,,,a wee while ago I built a US1metre "thats a model sail boat" It floats in the same way a brick doesnt. but Ive still got the sail winch servo, and the remote unit,unlike a normal servo this one does about 8 revolutions either way. would love to set that up on my piggy scope. did you get anywhere with the PC serial connection.? and the curcuit for driving the servo, can you be more specific...on the design and function. Im extremely interested..
KISA, your idea of using a 'modified', (to give full rotation), servo will not work, as they rely on the 'feedback' from the 'feedback' pot, which is driven off the gearbox assy. If you file/cut the mechanical stops off in the gearbox, the centre wiper arm inside the 'feedback' pot will be destroyed/damaged.
Sorry, but your original idea is best. Maybe try my idea of the drive belt onto the focuser knob. I use 'O' rings, (from a Bearing Supplier), as they are very cheap & reliable. Not much stretch at all.
HTH. L.
Guys,
Found the answer!!!
Get the USB 3 stepper motor Controller and driver from softmark ( www.ar.com.au/~softmark) costs about $80 thru the net and works 100%; use the O ring belt drive suggestion and you've got a great focusser motor ideal for remote control. Can control up to three motors at the same time, with slow/ mediium/ fast speeds. Position control by setting the number of steps. Great!!!!
I'm going to use mine to control the Spectroscope, plus focusser and still have one motor "spare"... maybe also the filter wheel!!??
Strongly recommend it; the software is easy to understand and use.
The motor is 6~9V DC, with gearbox. It is mounted in the eccectric cylinder, so by rotating the cylinder I can adjust the tension of the timing belt (which is driving the knob by friction).
Driving electronics is just two MOSFET switches, cofigured in such a way that the polarity of the driving voltage is controlled by position of the 3 position SPDT switch. Actually, the whole thing can be designed with more complicated mechanical switch (4x2 SPDT), but I did not have one at hand :-)
Also, when switch is in neutral position, DC motor is shorted, which provides for quite efficient dynamic braking (therefore no overshoot, which may be very annoying especially when the gear ratio is small).
The whole electronics and switch is housed into a box, together with 9V battery, as an independent unit.
Clear skies,
Bojan
Last edited by bojan; 15-07-2006 at 07:58 PM.
Reason: More clear photo of focuser
It's been quite a while since I last read this post and sorry to those who had questions later down the track that I missed. Here are some answers in case you're still wondering.
Just in reply to RAJAH235's comment about modifying the servo for continual rotation. You are spot on about the feedback pot, and to get around this I had to do a couple of things. Firstly I disconnected the pot and put a resistor voltage divider in its place. Secondly, the pot is directly connected to the plastic gearing inside the servo by a tapered shaft. To get around this I just drilled out the tapered part of the gears so they spun freely on the shaft from the pot, and hey presto, continual rotation
I've also finally got around to making the focuser pc controlled. Which was made really easy from the site I found below.
It was amazingly easy and very cheap and can control up to 7 servo motors.
The PC controller software they use doesn't quite suit controlling a focuser (not to mention they no longer have the source code) but the info that the PIC needs is very simple so it was pretty straight forward to make a GUI that did the job. With the current set up I can move the actual focuser at a variety of speeds and in increments as little as 4.65 microns for the fine tuning.
I've just recently started modifying the PIC code and in the process of writing a driver so the servo focuser is ASCOM compliant. It's early day but fingers crossed.
If anyone has any suggestions I'd love to hear them and if anyone would like more information please don't hesitate to ask.
Cheers,
Markus
p.s. The focuser is controlled from the serial port, but in case you're wondering, yes it does work fine using a USB-Serial converter.
Thanks gentlemen, you have inspired me to dig through the collection of salvaged parts. Got it stepping, just need to work out how to reverse the motion.
Controller is home brew of 555 timer oscillator and 4017 decade counter, its first 4 outputs go to the motor through some small transistors, the 5th to the reset pin. Good torque on 12v. Little tranny's get a bit warm on continuous operation but since it will be intermittent use, should be OK.
Is the delay without the reset line significant? If not maybe you could use the other 4 outputs of the 4017 in reverse order to your stepper motor through individual trannys (if you have another 4). Then connect a three position switch (for forward, reverse and stop) and wire it up so that turns on/off the individual banks of 4 trannys depending on which direction you want, or turns both backs off when you want it stopped?
![[1ponders]'s Avatar](../vbiis/customavatars/avatar45_9.gif){kind=avatar}
...
L.
but Ive still got the sail winch servo, and the remote unit,unlike a normal servo this one does about 8 revolutions either way. would love to set that up on my piggy scope. did you get anywhere with the PC serial connection.? and the curcuit for driving the servo, can you be more specific...on the design and function. Im extremely interested..
Focusser driver - FOUND the answer!!!!!!
