After a break of more than two months of any astro-related activities, I've decided that I could at least try to contribute by providing some technical ideas to the community. Obviously, it's not going to be of the sort of being something completely new. Often our own ideas are consciously or unconsciously based on previous ideas and concepts we've encountered.
Why do I say this? Of course many technical solutions in our hobby can be purchased but usually for a price which often makes me scratch my head. Without getting further into the topic whether equipment prices are fair, let me simply say: I'm trying to build my own solutions if I've got the knowledge, technology, equipment and most importantly TIME. That being said, I'm going to publish some images here on Astrobin, showing solutions which I've built on my own. Maybe some of you may find them useful for their own projects.
Let's dive into no. 1 ...

While I love the image quality of the WO RedCat 51, the helical focuser gave me some headache. The optics is fairly fast (close to f/5.0) and so temperature drift and mechanical instabilities will lead to a significant drift in focus during the night. Some initial tests, where I had the scope focused and the focuser tightened at the session's beginning, showed that during the course of the night, more than 50% of the data became garbage. As I cannot afford to monitor the scope during the night and do manual focusing, I've decided that I either need to implement an automatic focusing solution or sell the scope. Doing some online research showed that most autofocus solutions were home-made, a few could be purchased also from equipment manufacturers. For the latter cases, the price tag always kept me away from pushing the purchase button. Hence, I've turned on my CAD to design some mechanical parts which I intended to print on my personal 3D printer.

Image revision A (more to come...) shows my first solution, although it's already a second iteration. The first iteration simply mounts an autofocus motor (from the commonly known companies) on top of the scopes mounting ring (it provides 4 M4 threaded holes to attach an autoguider to it). The motor's shaft is then fitted with a drive pulley for commonly available toothed belts. The tooth belt is placed around the rubber part of the helical focuser of the scope and the pulley. To tense the belt, the motor is pulled upwards within the elongated holes and fixed with the motor's mounting screws. While this setup worked in principle, there are a few technical issues requiring another iteration of the design:
The friction between belt and rubber of the grip isn't very high and so slip can occur if the belt tension is too low. Another issue is the flexibility of the rubber which leads to a significant backlash if the focus direction changes. The tension of the belt cannot be increased arbitrarily as well, being limited by the stiffness of the motor mount. Therefore, I've been simply adding a fixation ring to the scope. It's essentially a ring with a diameter very close to the scope's body and a latch clamping the belt between ring and rubber grip. To increase the clamping force, there's a nylon screw on the opposite side of the ring to increase tension. This version worked flawlessly over the course of several imaging sessions (see The M81 Galaxy Group, Barnard 33 and NGC 2024, Horse in Flames, IC 405, The Flaming Star Nebula, Caldwell 49, Rosette Nebula, and NGC 1499 California Nebula).

There are just a few items which I'd like to optimize:
1. The clamping ring doesn't allow for full focuser revolutions. In normal operation this isn't an issue as the amount of rotation for an autofocus run is very limited and the clamping latch wouldn't come close to the tight or slack side of the belt. It's only required to have the focuser close to focus and the latch close to the bottom of the focuser to have enough focus travel.
2. The tension of the belt may change over time due to temperature changes. If that happens the backlash increases (with temperature drops) possibly making autofocus settings invalid. A belt tensioning mechanism should be added (I'll show one on the SCT).

As a general note: to properly focus the RedCat, one should employ autofocus routines with overshoot mechanisms such that the final focus movement is always in the same direction. The helical focuser (at several positions) has a significant backlash (you can even feel it if you focus the RedCat manually) which cannot be compensated through fixed backlash values.

Teaser: parts are currently printed for a different solution, leading to revision B. I will publish it, not matter if it works or not. (we can also learn from ideas which turned out not being successful).

That'd be if for my first publication of that sort. Hope you find this interesting or even helpful. If my write up leaves too many gaps, let me know. Feel free to comment.

Björn