Auto Focus - 3 Things to Make It Easy & Successful

[kaymann]

I have needlessly struggled with SC Auto Focus for a long time and then Borodog hinted, well more than hinted, there are two things (I added one) you can do to make it so much easier for yourself:

• Set up exposure for speed of the AF run, and gain for the number of stars selected in Multi-Star Auto Focus to be a high enough count to start.

• Backlash -
  • Do not try to tirelessly figure out the exact amount of backlash your focuser has.
    Instead get a close approximation or simply estimate worst case scenario.
  • For my ZWO EAF all indicators from other owners are it is going to be around 100 or so.
  • Now go into File-SharpCap Settings-Hardware-Focuser. Set the radio button to Overshoot, approach in the (I choose positive) direction. The reason I choose positive is because of bullet list below.
  • I then set the overshoot to well over my estimated backlash. So in my ZWO EAF case I set my overshoot to 200.
    This is effectively the same as putting in the exact amount and having SC overshoot by that exact amount. The amount of time saved in putting the exact backlash in is minimal as the time difference from going from 100 overshoot to 200 overshoot. The idea is to make sure you go far enough in overshoot to absorb the backlash.

• Be out of focus Negative when starting autofocus and then start the SC autofocus routine with the Positive button.
  • I try to get close to focus then I hit the Negative button so it takes me back at least 200 steps. Even when I back it up 500 steps things work perfectly.
  • Doing this negative starting point by pushing the focuser negatively, and then starting AF with the Positive button ensures that backlash has been consumed before the AF procedure even begins.
  • Then SC will continue in the positive direction where there is no need to worry about backlash.

Since starting this new procedure it has resulted in successful auto focus runs every single time. Before doing these three procedures my auto focus would typically run forever and completely fail into nowhere near focus results.

If someone can make my procedures clearer please comment.

[admin]

Hi,

thanks for sharing and writing it up so clearly - that's pretty much the setup that I use, since overshoot backlash handling with a value that is known to be 'generous' is very reliable in getting good focuser positioning.

One of the common causes of autofocus failures (in SharpCap and other apps) is when the application ends up moving the focuser within the backlash range - the application thinks the focuser is moving, and the position number is changing, but because of backlash the focuser is either stationery or very nearly so. The results you get in the graph are then just random noise - fluctuations in the measurement value due to seeing, etc, so either the autofocus fails or finds a spurious 'best focus' position. (SharpCap tries hard to avoid finding the spurious best focus position by applying various statistical checks to the data to try to be sure that any peak or valley curve is actually significant).

Note that once you have got good autofocus working by the manual method you describe, SharpCap will remember the settings you used (number of steps, step size, measurements at each step, direction of scan, etc). If you then use the 'Refocus' button, SharpCap will start the refocus by moving 1/2 the expected scan width in the opposite direction to the scan (negative in your case) to put the image out of focus before the scan begins.

cheers,

Robin