Autofocus failures

[mikaelA]

If you use the SGP AF Logviewer you dont need to provide focus packs to analyse the curves, simply load the normal SGP logfile for the session. See this thread for how to use it for other things like determining filter offsets and temperature coefficients:

SGP AF Logviewer v1.2 Release

The newest version 1.2 of the AF Logviewer is now available here: The main changes are the ability to handle and display the quadratic curves of the new AF routine used in the 3.1 betas (the program should still be perfectly backwards compatible with SGP 3.0), the ability to read multiple logfiles at the same time, and an improved Position vs Temperature analysis with filter selection including an filter offset calculation. Big thanks to @jmacon and @bulrichl for proposing the filter offset …

If you post logfiles people can also more easily give feedback on your AF settings.

Mikael

[Paulkirk]

thanks, that’s great, I’ll take a look.
Paul

[andrewbarton]

Hi Paul,

I have a similar setup to you. I have a Celestron Edge HD 8 that I use at F7 with a Starizona feather touch focuser. What I found important was to have the backlash compensation going in the correct direction. With backlash compensation, you want to move the mirror towards the bottom of the optical tube further than your target position and then move up, pushing against gravity. If your backlash compensation doesn’t finish by moving against gravity, you will get mirror flop which destroys the accuracy of your FWHM measurements. For me moving in the wrong direction and resulting mirror flop resulted in the types of focus curves you are seeing.

Ideally, you start your focus run toward the bottom of the optical tube and then all moves through the focus curve measurement push the mirror up against gravity. In this way, you minimize the amount of time you spend compensating for backlash.

For my system, I had to configure autofocus in the following way:

• Control Panel: Focus tab - check reverse focuser direction. For me this is necessary to get the focus run to start towards the bottom of the optical tube. As you are using the same focuser as me I suspect this is true for you.
• Control Panel: Focuser “Other” button - focuser backlash compensation checked, with compensation direction set to “IN”. My step size is 500, but your value may be different given your aperture.

If you don’t reverse the focus direction, you will probably need to set the compensation direction to “OUT” but you will likely be doing backlash compensation for every focus move, but it should work.

For my system, once I got everything moving in the correct direction, it was much easier to tune my step size (avoiding donuts).

Sorry if this is a repeat of what you already know. Perhaps someone else will find this information useful.

Regards,

Andrew

[Paulkirk]

Thanks Andrew, that’s very useful. The part of your description I haven’t got and have never tried is

@andrewbarton

Control Panel: Focus tab - check reverse focuser direction

Once I get my mount repaired I’ll try that out and report back.
best wishes
Paul

[andrewbarton]

Good luck Paul!