DIY electric focuser

[PhilPassmore]

I was getting a little frustrated with my slightly clumsy adjustments to focus. Each attempt at adjustment resulting in several seconds of scope movement and inability to view the outcome of any adjustment, meaning I was likely to miss the focus point and having to repeat several times.

I have a dual speed Crayford style focuser, it's a relatively inexpensive Chinese import fitted to my refractor. I have tuned and modified the focuser itself to smooth it and improve the internal drive and support, but I will leave that for another posting.

So the next step for me seemed to be installing a motorised focus system, so I wouldn't have to touch the optical assembly during focusing. 

A stepper motor linked to a driver board looked a cheap and effective solution, so using my usual source for such bits (AliExpress, no affiliation), I obtained both, with the driver able to run off five volts, so useable from a USB power bank or similar. 

 I tapped a couple of existing holes on the focuser, and used them to mount the assembled brackets which were made from some offcuts of carbon fibre from another historic project. I would under normal circumstances have used aluminium, but the carbon was to hand. A piece of right angle was created from an offcut of box section tube. The bracket is slightly isolated from the focuser with some rubber spacer washers, and the motor is isolated with a commercial cork gasket. 

I removed the fine focus knob from the focuser, and connected it to the motor with an adapter that allows a measure of mis-alignment without causing any problems. 

The driver board has various start/stop modes, and I have set mine so one button drives one way, and the other drives it the other way, with releasing it stops the motor. 

There is a speed adjuster on the board, and it's quite fussy getting it to a speed that drives well and smoothly and reliably. I think the limited current capacity of the board causes some stalling issues. 

Hopefully the pictures show what they need to without further explanation, but any questions, please ask away. Phil. 

Edited December 8, 2021 by PhilPassmore
Missing photo

[mboothe]

Good job! We need more diy projects like this…

[AstroKeith]

Nice job Phil,

Looks like the stepper is a bit high on its bracket. Those flexi couplers are great for angular misalignment  but terrible for lateral displacement - that will really strain the motor. If you use a 'jaw' coupler it will cover both misalignments. Image below.

I'm experimenting with a diy focuser where the output focus drive follows the tactile movement of a control knob. The idea is it would perform exactly like you were turning the actual focuser knob. The control knob is a cheap rotary encoder which produces pulses when turned, I'm just translating that into stepper steps (in the right direction!)

[Len1257]

1 hour ago, AstroKeith said:

Those flexi couplers are great for angular misalignment  but terrible for lateral displacement - that will really strain the motor.

We had a few of these fail at work after poor alignment, ours were whizzing around a bit and fatigue saw them off. Just needed shims and some care. Others work well though. 

[Rusted]

Belt drive? Toothed timing belts run nicely on textured focusing knobs.
You can often move the motor into a less obstructive situation.
The advantage of belt drive is that you can overcome the friction to use manual coarse focusing.
Though you do lose position with stepper motors.

[Merlin66]

I use belt driven (to the SW 10:1 knob) setups on all my scopes. Works very well for me.

[banjaxed]

I also favour the belt drive over direct drive as your mounting position is more flexible.

[Rusted]

There is really no need to use the slow motion knob for belt drive.
The little "Skywatcher" 12V DC gearbox motor can drive the coarse adjustment knob.
Do NOT use the "Skywatcher" control paddle. The cheap contacts on the buttons die rapidly.
Feed the motor with a proper 12V DC supply via a DC controller.

Here is an image of the motor on the Starlight FT3.5" on my 180mm refractor with the massive 2" Lacerta wedge attached.
I use the HitecAstro DC focus controller dozens of times per imaging session.
The coarse knob on the other end of the focuser shaft can easily be used for coarse, manual adjustment.

As already stated it is not a stepper motor but can be adjusted [on screen] to a microscopic crawl.
I like the way the little motor can be wrapped around the focuser to make a highly compact set-up.
Many of the [very expensive] commercial systems put the motors right out on the end of the focuser shaft.
Where it can easily catch on the scenery, cables or clothes.

It is not my intention to hijack the OP's thread. I am merely offering valid options which have worked for me.

[PhilPassmore]

Thanks for all the feedback, it's what I value, whether it's supportive of the way I have done things or if it's offering alternatives. It's how we learn and grow. I am very new to the astronomy scene and have so much to take on board. 

[rjsc2001]

Hi. Great job.

Can you post the links to the material on Ali?