Trouble with Auto Focus

Hi there - I haven’t been able to get auto-focus to work consistently. For many months, it would fail, return to original position and I’d just keep going since I was always careful to get good focus with the Bahtinov mask. Still, it was annoying that it could never find focus. My rig is a Vixen 200SS (8" Newtonian), SBIG 8300ST, iOptron CEM40, and a Pegasus FocusCube. I ran through the step calculation (stepvalue@4x HFR at infocus position - stepvalue@in-focus position) x 2 divided by (9-1) and came up with 73 steps, 2x2 binning, 8 sec exposures, 9 data points, star limit 6 pixels. Backlash compensation is admittedly a complete guess at 10 steps (by the way, my focus step range is something like 0-6000; no fine focus knob on the Vixen rig). When I run it, my graph looks like a sawtooth. Last night was even worse. I had dead-on focus before the routine ran. The routine decided I was way out of focus, selected a much lower focus point, and proceeded with the first exposure. When I did a mini-stretch, stars were out of focus. I paused the thing and went on a focus fishing-expedition. I tried different step sizes, but always the sawtooth. I finally disabled auto focus, hit a good point manually, and proceeded filming. Not bad results of the Flaming Star Nebula but so much time wasted. Any of you kind folks have any advice?

Hi there,
welcome to the community.

Since nobody has pitched in yet, let me try to help. Based on my own experience, I would suggest the following:

  1. The SBIG 8300ST is quite a heavy camera, even more so if have a filterwheel and maybe an OAG on it. Make sure that you have no slippage in the setup. Couplers are secure and the focuser is moving the tube in and out reliably.

  2. Make sure that the focuser is actually capable of handling the weight and that the draw tube is not being obstructed or cables are getting in the way.

  3. Before starting the autofocus process, get the stars fairly well focused manually. Then start the autofocus.

  4. I use fairly large step sizes to start off with, like 400 steps, just to make sure that the graph actually shows a difference between the individual focus shots. Once you establish that the HFR is changing, I change the step size slightly, in order to dial it in.

  5. Try increasing the exposure time and make sure you have set the Luminance or no filter for the autofocus.

Anyway, I hope there is something useful in my description.

Best Regards and Clear Skies,

Thank you for the advice. It turns out, one of the mounting screws for my Focuscube had literally fallen out and the bracket was rocking back and forth with only one remaining screw holding it in place. That was the primary issue. After replacing the screw, I ran the step calculation again and still had trouble, so I increased the backlash compensation and step size slightly higher than my calculation (from 68 steps to 80 steps). That seemed to do the trick, but I’ll know for sure the next night we have decent weather in Connecticut.