Re: Focus repeatability issue

Dale Ghent

Are you sure it was truly clear sky where you had your telescope pointed? Faint high-level clouds can affect the measured HFD/HFR of the star(s) that MaximDL is using. Clouds move, and that could explain the quickly varying values you are getting.

Also consider the resolution of your setup - a single step in your case might be quite small so the CFZ in your case might /seem/ wide because the high resolution of your setup puts a lot of steps in that zone and landing anywhere in that zone will be ok.

It is tough to find exacting repeatability when creating focus offsets in any system simply due to mechanical and atmospheric effects. The CFZ is always going to be more than 1 single step, so my recommendation is do do a run and then use those derived offsets and look for the repeatability of the HFD/HFR values those produce. If they seem consistent and your stars look good, then you have good offsets to work with.


On Jun 23, 2020, at 9:53 AM, Jim Fakatselis <> wrote:

I am imaging with a QSI683WS8 and a Starlight Instruments focuser with the Starlight stepper motor on an AP130mm EDF refractor Using MaximDL.

I was attempting to measure the filter offsets with my AstroDon series E gen 2 filter set. Taking four sets of measurements repeatedly with 5 of the 7 filters I use most, L, R,G,B,Ha. If I try to immediately refocus, my focus setting continues to drift monotonically. Very erratically. Can’t get repeatability at all.
This past evening four V curve runs on just the Luminance filter in rapid succession, (less than 2-3 min per V curve), over and over produced readings like this:
15020, 15101, 15140, 15213 as an example.

The other filters behaved similarly. I can provide Excel plot of data with linear ramp.

Reported temp from HSM stepper is well within 1 deg, measurements were made about 2-3 min apart in rapid succession. Little to no temp effect should be present.

Any ideas on what I should look for to determine source of this erratic behavior?
The last few weeks there has not been an issue like this, this seemed to develop suddenly. No change to imaging train, it’s been untouched. Seeing?

Thanks in advance,

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