Decided to share some pics of my self-made Solid Focuser as I call it.

Ultra-fast systems lead to big problems during focusing and I think there is still no universal solution to this problem. I was not satisfied with regular focusing solutions for lens like this that use stepper motor and belt. Primary reasons are hysteresis and lack of absolute positioning.

My solution I think is much more robust as it lacks any hysteresis, has high reproducibility and does have absolute positioning. This approach is inspired by focusers from astroholgi. I actually wanted to buy one of those, but did not have any luck with that, I wonder if they are still in business.

The core approach is really simple: attach a lever to the lens and then have a compression spring on one side and a linear actuator on the other side. That's it. The actual realization is much more complicated due to various factors and high precision and reproducibility requirements, so it took me more that a year to manufacture a good working one. I think you can pretty much see how this works from the pictures above.

There are two limiting switches in there. During start-up procedure the lever hits both of them and thus knows its absolute position.

One nice bonus is that I made this focuser wireless. It draws power from QHY camera and has a battery-powered remote (in one of the revisions) that uses a simple knob to adjust position. So no wires run to the mount to run the focuser.

Both remote and the focuser controller itself (red box) can be connected to USB and controlled via ASCOM (I wrote drivers for it) so I can use SGP to autofocus for example. I still focus manually though because this gives much better results.

The whole thing is manufactured by me from scratch: all the aluminum parts (I machined them in our local university machine shop), PCB design, soldering, software (everything is controlled by Arduino Nano boards), ASCOM drivers, etc. There is also a contraption that holds the camera (QHY16200A) fixed and allows for precise tilt control, which is absolutely crucial for a system this fast. There is also a plate for guide scope there. Don't remember everything right now, quite a bit of work!

Quite proud of it right now, it seems to be working flawlessly. The ultimate dream is to get rid of focusing forever: just turn it on, go to pre-defined position for each filter and you are ready to go. Not quite there yet, the focusing position still changes a bit between imaging sessions. This is probably due to the fact that I have to drive it back and forth to our HAS dark site and back. Once it's focused on the spot, I am still good for the whole night and if I do not disassemble the rig and do not travel with it, all the focusing positions stay where they are. I think for a permanently installed rig that would work without any problems.