Building an electric focuser for an LXD75

Hi,
This is my first post, though I've been reading this forum since last November when I got an LXD75 from Lidl. I've been lucky, I've had no problems with it so far.

I'm now planning to make an electric focuser for the scope using a stepper motor from an old printer. I want to make as few modifications to the scope as possible, so that if I ever decide to sell it (or if this project doesn't work!), I can restore it to its original state. With that in mind, I've a couple of questions I hope someone can help me with.

I'm thinking of supporting the motor on a mounting plate clamped between the focuser and the main tube. Would doing this cause any problems other than needing to re-collimate the scope afterwards?

Also, how can I remove one of the focus knobs, so that I can replace it with a gear wheel?

Thanks,
Noel

Have a google for the JMI motofocus for the lxd55/75. It looks like this:



I'll need to have a look at mine at home but that bolts onto *under?) the focusser and you replace one of the knobs to attach it to the shaft, I think the knobs are just held in with a small allen screw. The red buttons are to move the focus in or out and there is a variable resistor wheel someplace on the box to adjust the speed also.

I'll take some pics of mine when I get home if you like.

Noel,

I have designed built and tested a microcontroller based stepper motor unit, with the intention of using it as an electronic fine focusser. I have also built a serial PC interface to the unit. I have gotten as far as testing it and can microstep the motor in both directions from the PC. What is remaining to do is consider the mechanical aspects and also write an ascom compatible driver so the unit can be used with popular autofocussing software. The project has kind of gone of the boil a bit. But your post reminded me that I need to look at it further. Perhaps a bit of collaboration might benefit both of us?

The other thing about the circuit I built is that with minor modification it can also be used as a dual sensor temperature controlled dew zapper (similar to the kendrick units which cost so much).

Thanks for the quick responses.

Albert,

Somehow I missed the JMI unit when I searched the web before posting here. I've had a look at it now and it seems to be fixed to the plate covering the rack and pinion of the focuser. I'd have thought that would put a lot of strain on that area, but it's a possibility. It would be great if you could post a picture of your focuser. I've looked at mine, but I can't find the allen keys holding the knobs on.

Dave,

We do seem to have been working along the same lines. I've designed a circuit for (but not yet built) a controller using TTL logic. It will have a manual control pad, with 3 buttons, 'in', 'out' and 'fast'. I also plan to be able to connect it to a PC via the parallel port and to modify the Meade telescope driver in Cartes du Ciel to control it.

I'd be glad to co-operate on a design for the mechanism. I don't have access to machine tools other than a power drill, so I'll probably be making it out of aluminium.

In your design, how do you intend to prevent the PC from driving the focuser into the ends of its travel? I'm thinking of using reflective opto switches and a strip of reflective aluminium tape on the focuser barrel.

Dave,

After reading your post, I remembered that I've got a Stamp microcontroller I can use for this project. I can move most of the hardware logic into software and it has a built-in RS232 port, so I don't need to use the parallel port. The circuitry for this project just got a lot simpler. Now to find the Stamp manual.

Thanks,
Noel

Hi Noel,

I'm not familar with the stamp as I've always used standard PIC's however one thing you might need to check is the voltages that the serial port on the stamp accept. A PC outputs between -3 to -15 - and + 3 to +15v. You may well need to use a MAX232 IC to convert this to TTL. Thats what I had to do. It's no problem really, just the IC and a couple of .1uF caps does the trick.

en.wikibooks.org/wiki/Serial_Programming:MAX232_Driver_Receiver

The issue with maximum travel was one which I've been thinking about. Mt thought is;

Making a reference point using a microswitch. i.e when you power on the focusser travels until the microswitch is activated then (now knowing its position returns to the last position (which is stored on the MCU each time the focusser stops a slew. The max in and out position can simple be then set so the unit knows not to go past those points.

Like you I only use hand tools so any mechanical design I make has to be with aluminium too. My main scope is a newtonian however I'd like to be able to use it on other scopes too.

My circuit is fairly simple, its basically a PIC16F628, 4 x TIP 120's (overkill I know), and a hex invertor. The reason I used the tip120's is this started life as dew controller so the power transistors were necessary to provide the current to the dew heater tapes. I also added two dallas thermometer IC's so the ambient and optic temperature could be measured and responded to. All of my projects use an LCD panel for the user interface.

Lot of thought still to go into this project.

I've also published another project to control digital slr cameras. See;

webtreatz.com/content/view/45/1/