Hi, I'm also planning to buy a 2nd hand SXD, but I do not understand Why there is a declination jump? It's because of incorrect polar alignment? I'll use the mount but without autoguiding for astrophotograhpy, is it possible? I heard from my friends that vixen mount is designed with excellent techniques and accuracy, the jump is a mechanical problem or software/ starbook problem?
It is a problem with how the motors are being controlled by the motor board or the software. If you take an SXW/SXD with a Starbook, and remove the cover to access the gears (be careful here), and then turn the mount on so that tracking is active, you will notice that the Dec gears are actually being engaged. The gears themselves will vibrate as it tracks. Not entirely sure if it is the electronics board, or the software telling the electronics what to do, but the Dec motors are being told to do things while tracking, which is not what you want when trying to photograph things. The  amount of jump I saw in my measurements was on the order of 20 arc seconds if my memory is still correct. It wasn't periodic either. This was measured using guiding with signals to the Starbook turned off just to see the drift. Normal drift will still affect the location of star in Dec, but it shouldn't jump that much on the order of 1-2 seconds when it was smoothly drifting in much smaller increments before and after the jump.
If I had to make any sort of guess, the motors were being driven back and forth rapidly within the backlash of the gears. But occasionally it'd get through the backlash and cause the jump. Â
After installing the NexSXD board, the Dec motor is no longer being driven during tracking (no noise from the Dec motor, no vibration in the gears). The mount itself is also worlds quieter, with no sign of the Dec jump while tracking.
Honestly, while this mount is a bit of a hidden gem, I can only really recommend it with a NexSXD/SXW board, which last I checked was in spotty supply. I wound up with both an SXD and an SXW in my possession for a period of time, and they both had roughly the same amount of PE (happy with the smoothness of the PE though), and they both exhibited the Dec jump with the Starbook driving it. And the PE isn't much better than stuff like the Atlas or Sirius mounts, it is just smoother. So Unguided may do well, but you are still going to be limited on the length of exposures you can make in a very similar way. But in my case, where I wanted it as a refactor mount that could do guided AP that was accurate enough that it "just worked", it is totally worth the money I spent. The competition in this space (Losmandy, AP) are more expensive, and even better built, but tend to also be heavier (CW included). This for me was a nice compromise in price, tracking quality (after the NexSXD board), and portability.
