Analogue Electric Remote Focussing Unit

[Gina]

I have been writing about this in my obsy build thread but as it's a few posts distributed among a large number of other posts and therefore difficult to find, I thought I'd collect the data and re-post about it here - a more appropriate forum.

Part 1

Design Aspects

I wanted something simple and quick to build so rejected the Arduino design many people use. This will not be computer controlled (well not to begin with ). So here are the points on which I based the design.

1. Cheap, simple and quick to build.

2. A cheaply available DC motor/gearbox unit I've used for other things successfully.

3. Simple belt and pulley arrangement to connect gearbox to focuser on scope.

4. Control box in the warm room connected to the motor unit on the scope by cable.

5. Simple analogue voltage control of the motor to control speed and direction.

6. Powered by my main observatory DC supply of 13.8 volts.

Part 2 to follow tomorrow...

[Gina]

Part 2

The Motor/Gearbox and Belt Drive

The Motor/Gearbox used was Maplin part number SN36P :- Multi Ratio Motorgearboxes : Motors & Gearboxes : Maplin Electronics

This is how it is supplied.

The rotation speed of the output shaft is determined by the number of gears attached to the shafts. Each pair of gears gives a 4:1 reduction ratio. The green gears have a clearance hole and rotate freely on the shafts - the orange gears are a tight push fit on the shafts and are used to drive the shafts and to retain the green gears. The gearbox is easily dismantled and reassembled to give a wide variety of reduction ratios. The ring on the long shaft is also a tight push fit and is slid off the shaft to dismantle the gearbox.

For mine I took off a green gear from each shaft and put the gear assembly back together.

Pulley and belt bought from Technobots

Pulley used two of these back to back :- Plastic Pulley 30/25 2.9mm

Belt was :- Pulley Belt 2.5mm x 70mm ID

The shaft on the motor/gearbox is 3.0mm so the 2.9mm hole in the pulley makes a good tight fit. These were the only pulleys shown that had this size hole - nearest otherwise would be loose on the shaft and didn't have a grub screw to tighten it. They only represent half a pulley (like railway wheels) so needed two back to back.

The motor/gearbox was mounted on a plastic box (also from Maplin) which I had sculling around and used in lieu of a bracket to attach the motor/gearbox unit to the scope mounting - more about this in the next part.

[DIYASTRO]

Like its simplicity Gina! Only wish I could keep my stuff simple! I think I must enjoy the headaches!

Boyd

[Gina]

I have had more years than I care to think of keeping things cheap and simple

[angusb1]

Cheers. And with part numbers too

[Gina]

Glad if I can be of help

[Reggie]

That's looking very neat Gina, nothing wrong with cheap and simple, my original 'lectric focuser for my nexstar 6SE was little more than a servo and a 555 timer, total build cost was about £5 (donated servo, pulleys and belts were pulled off random belted and pulleyed items of hardware, like optical drives and printers).

[Gina]

Part 3

Fitting the motor/gearbox to the scope.

Rather than undo the Crayford focuser I decided to use the dovetail bar end stop bolts. These do nothing other than to act as a stop to prevent the scope sliding out of the mount clamp when it is loosened for balancing the scope and attachments. To attach the motor/gearbox unit here requires a right angle bracket of suitable size. Not having a suitable bracket and no materials to make one I used a plastic box I had. That also came from Maplins but not sure which of their dozens of such boxes. This mounting position works out as very convenient.

The belt is just tight enough to turn the slow-motion drive knob on the focuser. This allows manual focussing with the knob slipping under the belt. The belt is also easily taken off the knob and hangs happily on the pulley.

Here are some photos showing how the motor/gearbox unit is attached and the belt drive to the focuser, from various angles.

[iturner2]

Neat

[Gina]

Thanks Ian

[Gina]

Part 4

The Focussing Control Box.

Below is shown the circuit diagram for the Control Box. This uses a potentiometer to control the speed and (ideally) two push buttons to move the draw tube in or out to vary the focus. The push buttons need to be double way with a connection both pressed and not pressed but I was unable to find a source of these so had to resort to an alternative. This is shown in the lower diagram and uses a Double Pole Double Throw (DPDT) centre-off biassed toggle switch. All components were purchased from Maplin Electronics but are also available from several other suppliers.

To make this work in a similar way to two push buttons I made a paddle out of a piece of plastic and hot glued it to the toggle. The diagram below shows how it works. The off position is shown in black with the in and out positions in green and red.

Here are some photos of the circuit board and components and the outside of the unit. The switch is separate and connected to the circuit board by wires. Twin cables connect to a 13.8v bench power supply (Maplin) and to the motor unit, through the cable duct from warm room to pier.