Personally, the smallest mounts that I would consider for a 12" RC would be an Astro-Physics AP1100, a 10Micron HP2000HPS or Software Bisque Paramount MX+. Thomas is spot on with his comments.
Regarding 10Micron and guiding, I believe that you have a misunderstanding. It's not correct to say that they don't require guiding. I'm not a 10Micron owner, and I'm not familiar with all of their publications, but I believe that even they tell you that beyond a certain image scale and exposure length, they recommend guiding their mounts.
There is much mythology here on Cloudy Nights regarding encoders. The correct way to think about encoders is that they are simply a mechanism to achieve a desired level of accuracy in positioning of the mount's axes - and nothing more. They are also not the only way to achieve accuracy. It can be done with mechanical quality and tolerances as well. Beyond providing for accuracy, they do nothing to help with unguided imaging.
In fact, if you had a mount, with absolutely perfect tracking, you would get poor unguided results with high resolution imaging.
Why is that, you ask? The reason is that the apparent motion of the sky varies with both position in the sky and with environmental conditions. We think of stars moving at sidereal rate in the sky. In theory, if your mount can track perfectly at sidereal rate, that would be all you need. The problem is that all of us with ground based telescopes are imaging through a giant lens called "the atmosphere". On your other thread, we've talked about seeing, which is the effect of scintillation. But that's not the only effect. The atmosphere also refracts light. Because of that, the apparent motion of the sky is only at sidereal rate at the zenith (directly overhead). As you point the mount lower, it's looking through more atmosphere, and that slows down the apparent movement. And at high resolution scales, this effect is not subtle at all. So if you want to accurately track, your mount needs to consider the altitude at which it's pointing. Fortunately, this is just math. Some mounts have a tracking rate called "King Rate". King rate is like sidereal, except that it accounts for altitude.
So for unguided imaging, you just need King rate, right? Wrong!
The problem is that the refractive properties of the atmosphere are affected by temperature, pressure and humidity. So for effective, high resolution imaging without guiding, you need to consider these factors as well. To do that, you need environmental sensors and tracking correction that account for them.
Even when you have that nailed down, you still have other problems to deal with. Telescopes, especially as they get larger, are rubbery things at the microscopic level. The bend and flex as they point in different directions. Optics shift. The refractive properties of lenses change as the temperature of the glass changes. Mirrors shift.
So how do you deal with all of that?
What you need - and this is the "secret" of the 10Micron mounts - is a way to sample points all across the sky (or at least in the parts of the sky where you will image) and compare the actual pointing position, versus the expected position. You can then use that data to build a model that accounts for all of the above factors. The mount can use that model to continually adjust the tracking to accommodate over a dozen factors. 10Micron includes this capability in their hand controller. It's important to note that this is not unique to 10Micron. Both Astro-Physics and Software Bisque (Paramount) also have this capability. The difference is that AP and Bisque use software running on your computer, instead of the hand controller, to do this. Astro-Physics uses software called APCC Pro to do the same thing, and Software Bisque's is called ProTrack. It's also worth nothing that the latest mount from Astro-Physics (the Mach2) includes this capability built into the mount, and will be porting it to their other current mounts as well.
Finally, there are two caveats to keep in mind. Any modeling is only as effective as the behavior of the entire system is repeatable. For example, SCT's focus by moving the primary mirror. Since the primary mirror needs to be free to move, it can shift around somewhat randomly. This random shifting cannot be modeled. For this reason, not even a 10Micron mount can imaging with an SCT unguided. The second caveat is that the real world never completely conforms to our attempts to model it. For this reason, once you reach a certain level of resolution, or try to expose for long enough, the model will diverge from reality. At that point, you need to guide.
I hope that this helps to give a background for understanding the factors involved in answering your questions.
Edited by WadeH237, 20 June 2021 - 09:36 AM.