I have an early version of the FSQ-106ED and I've tried a couple different focuser options, both of which are serviceable but neither are ideal:
Starizona Microtouch. I already had the old Microtouch controller so I picked up the motor for ~$250 if I recall. With the older controller, the resolution was 10 micron per step, which is quite poor IMO for a f/5 scope. The Microtouch attaches to the fine focus wheel via a gear that you install. The motor housing clamps onto the fine focus assembly, so no brackets required which is nice. While it did work reasonably well, I went away from this approach for a couple of reasons:
- My Tak fine focuser had some slippage when pulling against gravity, which eventually adds up. I doubt it has that much real effect on autofocus accuracy but I prefer my focusing to be more precise and repeatable. After I removed it, I was able to tighten up on the fine focus assembly nut which I think would have reduced but not quite eliminated the slippage.
- The 10 micron per step resolution as mentioned before.
- I was not a big fan of having a separate control box that I have to mount on the scope and run a wire over to the motor. That's one of the nice features of the ZWO EAF and the Sesto Senso for example.
- I measured backlash of ~50 microns which I partly attributed to having the extra gear between the motor and the fine focus shaft.
ZWO EAF. I got this thinking that since the motor would be connected directly to the pinion shaft, I could eliminate the slippage and drift over time and hopefully improve the backlash. I was half right:
- Step size was 3-3.5 microns which I believe is fine for f/5, and probably even if using a reducer. I know some manufacturers tout having 1 micron or sub-micron resolution but I don't think it's really necessary.
- It is true that there is no more slip with the EAF, however it's not quite that simple. The flexible coupling provided by ZWO with the EAF is great for dealing with misalignment between the motor shaft and the pinion shaft, but it has very poor torsional stiffness. This means that the motor has to "wind up" the coupling which acts like a spring before the focuser will actually move. This imparts a kind of backlash and very poor reproducibility and fine movement. I found a cheap coupling on Amazon that has a split-ring clamp design which avoids some of the set screw issues with a truly solid coupling design, yet it still has excellent torsional stiffness and transmits the motor torque with essentially no spring effect. This is a must-upgrade feature IMHO.
- I had no issues with the ZWO bracket for Tak focusers, although it's not quite as clean looking as the fully intregrated clamp housing design of Microtouch and Sesto Senso.
- And then the backlash. Oh boy is there a lot of backlash, in the ballpark of 250 microns if I remember correctly, or something like 75-80 steps. This is even after upgrading the coupling, which means that essentially all of this comes from the internal gearbox used to convert the stepper motor output into something that has decent resolution for the coarse focus knob. The good news is that it is pretty repeatable, and the backlash compensation in the driver works well. Since I set up my autofocus to approach from the same direction each time, it's not a big deal.
In terms of the Tak focuser itself, I'm not a huge fan but I haven't forked over the big bucks to upgrade it to a NightCrawler or FeatherTouch 3515. I did stiffen it up as much as reasonable by tightening the teflon pads on the underside of the drawtube. As a side note even top of the line focusers like FeatherTouch have to be "tuned" differently for visual vs imaging. In order to get a nice "feel" with the Tak focuser it ends up too droopy for imaging. Tighten it up for less droop and now it doesn't feel buttery smooth if focusing by hand. The 3.5" FeatherTouch is able to get much closer to an ideal balance but there is still a trade-off. Such is life or at least my experience. I also minimize how much the tube is racked out to just a few millimeters for the FSQ. And, I added a Guide scope ring that supports my adapters near the camera and bolts to the scope dovetail. Since the autofocuser has to be able to move the drawtube, the guide scope ring screws aren't very tight, but I do believe it offers some additional support when not pointing straight up. If you're using an OAG then that's probably not necessary and the guiding can accommodate for any drawtube sag.
Hope this helps. It would sure be nice to have a NightCrawler which I'm sure will make the focuser just "disappear" and work without having to think about it, but personally I have so far been unable to justify the expense.