I picked up the Gemini Automatic Star Focuser Pro for the AT66ED so that I can change scopes between the SV503 102ED and the AT66ED without having to remove and refit the electronic focuser. I have used the focuser for a couple of sessions and it took a bit of tinkering but I have it dialed in. This post covers how I configured the Autofocus options in NINA to consistently achieve focus using NINA’s autofocus routine when using the Gemini Automatic Star Focuser Pro on the Astro-Tech 66mm F/6 scope.


The Gemini Automatic Star Focuser is a budget friendly electronic focuser which is available (search Gemini Automatic Star Focuser on eBay) for less than 1/2 the cost of the ZWO EAF. I also have a ZWO EAF on the SV503 which works great as well. Besides the cost the biggest difference between the two (that I have found) is there seems to be a lot more backlash in the Gemini Automatic Star Focuser. All focusers are going to have some backlash, the Gemini Automatic Star Focuser had about 400 steps of backlash compared to around 35 steps in the ZWO EAF. The backlash in both focusers can be easily remediated using the Backlash compensation settings in NINA.

This curve is showing about 200 steps of backlash, my backlash OUT was set to 150 during this autofocus run. Add the backlash in the curve to my backlash setting and I end up with about 350 steps of backlash. Even with the backlash the autofocus run was able to complete successfully, it just took a bit longer.

Autofocus configuration is likely to be slightly (or maybe more than slightly) different between scopes and auto focusers. This is how I set up the NINA profile for my AT66ED using the Gemini Automatic Star Focuser Pro to get decent focus curves when I run NINA Autofocus. The NINA Autofocus documentation is available here.

I use the Hocus Focus plugin which provides some additional functionality including an aberration inspector, improved star detection, improved star annotation, and quicker autofocus. After installing the Hocus Focus plugin it needs to be configured to be used in NINA -> Options -> Imaging. Under Image options set Star Detector, Star Annotator, and Autofocus to use the Hocus Focus plugin. There are some additional settings that can be adjusted when using the Hocus Focus plugin, but since I am using the plugin’s defaults I am not going to cover those in this post.

I did a lot of testing with different NINA autofocus configuration settings. I made multiple adjustments to the Autofocus step size and Backlash OUT till I was able to get consistently repeatable results from the autofocus routine. The NINA autofocus configuration settings can be found in NINA -> Options -> Autofocus.

• Autofocus initial offset steps: 4. This is the number of focus steps to move initially. Multiply this by the Autofocus step size to get how far the focuser will move for the first focus evaluation point.
• Autofocus Method: Star HFR. The Half Flux Radius (HFR), half of the half flux diameter (HFD), is a measure of the diameter around the peak of the star minus the background.
• Curve fitting strategy: Hyperbolic, Typically an autofocus curve follows a hyperbolic shape, so this is the recommended method for NINA.
• Number of attempts: 1, Use brightest n stars: 0, Inner crop ratio: 1, and Outer crop ratio: 1 are all default settings.
• Binning: 1, even though I typically shoot bin 2 focusing should be done at bin 1.
• R^2 Threshold: 0.7, If the R^2 is below this setting the autofocus run fails. The R^2 value is the calculated focus based on the focus point data and the focus curve. Greater than 0.8 is considered good with greater than 0.9 being ideal focus. I’ll probably set this to 0.8 now that I have things dialed in.
• Autofocus step size: 75. This is the size of one focus step or the number of focuser stepper motor steps between focus steps. The step size setting will be dependent on your scope and focuser. It needs to be configured so there is enough HFR change between focus points to calculate the curve.
• Default autofocus exposure time: 5 seconds. This is the length of exposure at each focus step. This may need to be increased dependent on the target and/or filter.
• Disable guiding during AF is set to OFF. I am not using an off-axis guider so guiding can continue during an autofocus run.
• Focuser settle time: 1 second. This is a pause after the focuser completes a move.
• Number of exposures per point: 2. Two exposures are taken at each point and the HFR is averaged. Increases the time an autofocus run takes, but I found it to be more consistent using 2 exposures per point.
• Backlash compensation method: Overshoot, Compensates for backlash by overshooting the target position.
• Backlash IN/OUT the IN is set to 0 and the OUT is set to 400. Only one of these should be greater than 0. It should be set to a number larger than the amount of backlash you have. In my case the OUT will be applied and the final movement of the focuser will be IN. OUT is the direction the NINA docs recommend for this configuration.

This backlash configuration compensates for backlash by overshooting the target position by 400 steps and then moving back to the target position. For example if the focuser was moving to target position 12000, it would move to 12400, then back to 12000. The overshoot removes any slop/blackslash due to the change in direction of the focuser gears.

With these setting I am getting good focus and a consistent focus curve during the NINA autofocus routine.

The R^2 is 1, which means the focus curve aligns with the focus points getting me to a good final focus point. I ran the autofocus a number of times both manual and during my sequence capture of NGC 6888. This curve was consistent across all the autofocus runs.

In my NINA Sequences I run the Autofocus at the beginning of a target sequence and have a trigger set up to re-run the autofocus after every 10 exposures.

Took a bit of testing but after things were dialed in the Gemini Automatic Star Focuser Pro on the AT66ED using NINA’s autofocus with the Hocus Focus plugin provides consistent and repeatable focus.

Hope this post helps you dial in your autofocus configuration.