I have been using SGP for about 1 tear and have only been able to achieve good focus by adjusting the focus point after using autofocus. A friend suggested that focus backlash might be the problem.
I ran my own tests to determine the amount of backlash required for my focuser. I have an OPTEC focuser mounted to my Celestron CPC1100.
First I did a daylight test to see how many steps it takes to rotate the focuser one revolution. That turned out to be about 1800 steps.
First it takes about 4000 steps to go from the lowest HFR to 3-4 times the lowest HFR.
I then ran SGP and ran the autoguider with different amount of backlash. I found that with 1000 step IN of backlash the focus was not crisp. Changing to 1500 steps IN of backlash improved the focus. Changing to 1750 steps IN backlash improved even more. Moving out to 2000 steps IN did not change the focus noticeably but focus looked great. All of the focus tests generated very good V curves, and all resulted in achieving the same focus point however using 1750 to 2000 steps of backlash resulted in the best focus.
I also did this same test with my Stellarvue 90 mm refractor. Once again I found that about 1500 steps IN of backlash generated the best focus.
Hope this helps…