AstroDude42 I read on this forum that it is supposed to set the backlash to 0 (zero) to let ASIAir find out the backlash by itself.
Olli, you might be confusing the EAF backlash parameter in ASIAIR with the focal length parameter in ASIAIR.
1) you are correct that the backlash value in the Focuser Setting window is not used when ASIAIR runs its autofocus routine. However, unlike the focal length, ASIAIR will not be able to set the value of the backlash itself. If you set it to zero, the EAF will assume no backlash when you do manual focusing.
If you do use ASIAIR's "autofocus," what you will notice is that the ASIAIR always starts the EAF value at some point a little away from the previous focus value, and only moves the focuser monotonically in one direction. It does not go back and forth. Because of this, backlash does not come into play.
After getting enough values to form a curve and obtaining a target minima, the ASIAIR again moves the EAF to a value to that original direction, and again more finely move it towards the minima. Again, it travels in one direction only, and therefore there is no backlash compensation needed. Notice that ASIAIR also does not use the Coarse or Fine parameters in Focuser Setting. When autofocusing, it uses the Step Size that is defined in the Auto Focus Settings of Focuser Setting.
2) if you are manually focusing (using the up and down arrows of the Focuser mini-window), only then will ASIAIR use the Coarse, Fine and Backlash values.
Do not set too fine a value for the Auto Focus step size. The curve will be too shallow to obtain an accurate minima, when you do that. Make that autofocus step size at least a bit larger than the size of the critical focus distance of your optical train. Try for autofocus step size that is two or three times the critical focus distance.
You can measure your draw tube extension, then move the EAF by a couple of thousand steps (assuming it does not bottom out) and measure the draw tube extension again. This gives you how much the focuser moved per EAF step. With two of my Takahashi refractors' rack and pinion focusers, the number is around 4 µm per EAF step; but that number can vary widely for different focusers. My belt driven focuser on my Askar ACL200 is much finer, for example. Many Crayford focusers use the same diameter pinion, and as such they will have the same focuser scale.
Once you measure the drawtube change per EAF step, record it in your notebook; in the future, you can set step sizes without experimenting. You can obtain the critical focus distance by comparing similar triangles of geometric optics with the size of the Airy disk -- just do a web search for "critical focus" and "new critical focus."
Chen
