As nobody seems to be willing to share her / his experience with the SGP’s new best focus evaluation method, Quadratic Fit, I will show my first data taken with SGP v188.8.131.525.
Takahashi FSQ 106 (no ED; focal length = 530 mm, aperture = 106 mm) with original Rack & Pinion focuser (no gear reduction), Robofocus stepping motor, Seletek Armadillo2 focuser controller,
ZWO ASI071MC Pro
The appended charts show the following:
chart_1.JPG: SGP’s AF window when the AF run is completed
chart_2.SVG: the very first focus curve of 12th Ocotber 2019 (HFR vs. fp) with 9 data points plus data of the validation frame
chart_3.SVG: number of stars vs. deviation from best focus [steps]
chart_4.SVG: temperature trend (focuser and ambient temperature vs. time)
chart_5.SVG: linear regression of best focus vs. temperature
The data were recorded in the night of 12th/13th October 2019, just one night before full moon. Moon rise was at 19:25. Target was M 33 with an angular distance of about 34.5° away from the moon. Sunset was at 19:46. At about 01:15 the until then clear sky began to overcast.
No pre-focusing with a Bahtinov mask or similar aid was performed. Instead the initial focus position was calculated from the linear function derived from prior capturing sessions. chart_2 shows the result of the very first AF run.
In the AF runs the step size was varied: 15 steps (AF runs at 21:23, 21:30 and 22:56), 10 steps (AF runs at 23:47) and 8 steps (AF runs at 01:19). With SGP’s old best focus evaluation method I generally used a step size of 25 steps, resulting in a large HFR ratio of about 8. However, sometimes this setting resulted in too low HFR value at the left and right edges os the AF run. With Quadratic Fit in the inspected range of the step size I don’t recognize an impact of step size on best focus, a step size of 15 seems to be appropriate for my setup.
Chart_3 shows the number of stars found for the AF runs vs. the deviation from best focus in steps. The shape of the curve is always found with my scope: the maximum number of stars is detected about 20 steps beside best focus. I don’t know whether this is typical for SGP’s star finding algorithm or whether this is specific for my telescope though.
Two different temperature measurement devices were used: one for measuring ambient temperature (a device that actually is intended for in-house usage) and an external temperature sensor, connected to the focuser controller, located at the scope tube near the objective cell and well shielded from wind.
In that night I captured 39 light frames (exposure time 5 min), extracted the focuser temperature from the FITS header and added the data to the temperature chart (chart_4; blue +: focuser/AF runs, green x: focuser/light frames, red diamonds: ambient). Temperature dropped only very slightly which is typical of my site (La Palma, pure maritime climate). The deviations between the to curves might be due to wind. I sat in the winter garden, so I just don’t know because I didn’t notice.
In chart_5 best focus values are plotted vs. temperature. During performing AF runs I triggered some manual reruns in order to check the reproducibility. The chart shows 12 AF runs, two data points are hidden because identical results were obtained (for both temperature and best focus) on the rerun. The reproducibility was very good.
Chart_5 also shows the linear regreeion of best focus vs. temperature. For my FSQ 106 a temperature coefficient of -10.2 steps/K is evaluated, this corresponds to -68 µm/K. Note that a negative temperature coefficient means that the focuser has to be moved INWARDS when temperature drops. The coefficient of determination R^2 is 0.9964 indicating an excellent correlation of measured best focus and temperature. Note that the range of deviations of best focus from the regression line is -0.5 to 0.7 steps!
I am very happy with this further development of SGP’s Auto Focus capability and want to encourage you to try this enhancement. I am curious about your experience.