This is another image of a self-made autofocus solution. The first can be found here: William Optics RedCat Autofocus.

Somehow, I've always liked the SCTs. I guess it's simply the fact that they're fairly easy to collimate (of those scope which may require it from time to time) and they don't show diffraction spikes. Optical performance is a different story if one has a classic SCT without corrector. The Celestron EdgeHD's are optically excellent in my opinion. Nevertheless, they face the same mechanical issues as all the SCTs in the past which is the moving primary mirror leading to mirror flop which easily kicks out focus position. In addition, as any two mirror system, it's sensitive to temperature changes. While manufacturers have added mirror "clamps" to hold the mirror in place to counter the mirror flop issue, it doesn't help on temperature drift. Therefore, there's no such thing as focus and forget.

There are several autofocus solutions available. IMHO, the most professional (and most expensive) one is to add a focus mechanism to the secondary mirror, being available for 11"+ Celestron SCTs. Common alternatives are to add a classical rack and pinion focuser to the optical back or simply a motor to the focus knob. Until now, I've been attaching a Baader Steeltrack to which I attached a focus motor. Once I had focused the SCT with the main focus knob, I've locked down the mirror with the additional locking mechanism. Subsequent focus runs were then done through the Steeltrack, very similar to any refractor autofocus. Are there drawbacks? I would say so.
Using the optional reducer is virtually impossible. I'm always careful in matching the camera position to the optical design goals of the scope and so back spacing can quickly become an issue. With the reducer, I wouldn't be able to use the Steeltrack. Even in it's shortest configuration, the remaining back focus couldn't accommodate off-axis guider, filter wheel and camera. Without the reducer, it's possible but still a pretty tight business. To me, the major drawback is the weight. It weighs one kilogram and shifts the center of gravity a good piece backwards. As such, I have to attach additional counterweights (another kilogram) to the dovetail bar's front to get the whole thing balanced. All this additional weight requires additional balancing on the RA axis as well. So, if I could find a more lightweight solution, it would be ideal.

As mentioned earlier there are several commercial solutions attaching directly to the focus knob of the scope. Maybe some of you have looked them up as well already. Like everything, there are pros and cons. Those solutions that attach a focus motor through a direct coupling, where the axis of the knob and the drive shaft of the motor match, have the drawback that the motor is close to the optical axis and for the 8" SCT, it might lead to collisions with the equipment attached to the scope. Other solutions create a coupling through a belt which I'd prefer but still the motor is on the wrong side for my taste. Inspired by these ideas, I've created the solution shown here.

The focus motor and scope are on the "same" side, leaving the back end of the scope "open" for any equipment. The mounting plate of the motor is directly attached through the three screws that usually hold the focus mechanism. The coupling of motor and focuser shaft is done by a tooth belt. Elongated holes allow to give an initial tension to the belt. The first iteration of my setup didn't include a belt tensioner and it turned out that belt tension can vary with temperature. Initial tests have proven that autofocus works well when using NINA's overshoot mechanism. However, the required steps for overshoot are immense (due to the mirror flop and the additional gear ratio created through my selection of driving and driven pulley). To fix the issue of changing belt tension, I've added a spring loaded bearing which pulls on the belt to keep a good amount of belt tension. The belt tension is yet to be tested in practice but I am highly confident that it'll work since the previous tests were successful.

Feel free to comment.

Björn