lin-thaler Thanks for that detailed answers. I know all that about dark frames and have a library for different setups. I just wanna know, if the EAF gets crazy and does not finish when coming to that point, if necessary.
Ah, here is a trick. One night when you don't have anything better to do, and the temperature outside varies quite a bit...
"Seeing" does not even have to be good if you use a Bahtinov Mask.
Make sure to have the EAF temperature probe measuring the OTA temperature somewhere between the Objective and the focuser. Don't measure air temperature, and for heavens [sic] sakes don't use the internal temperature measurement from inside the EAF (it uses the "free" sensor that is built into microprocessor chip); that internal measurement is even more than useless than measuring air temperature.
To get good contact between the EAF probe and the OTA body, you can use a thermal pad that people use with heat sinks (check Amazon as usual, for "thermal pad for heat sink") -- you know, the stuff that leaks oil in the ASI2600 and the ASI1600. That, and some Gaffer tape, should do the trick.
Let everything cool down sufficiently to outdoor temperature, and using a Bahtinov mask, focus really well, recording down the EAF value and the temperature. Do this for each filter that you have in the filter wheel (if they do not have the same glass thicknesses). Wait for temperature to drop a degree or two (yes, some OTAs, especially ones with Petzval configurations, are that sensitive), record another set of readings. Rinse and repeat.
Precompute the focus deltas from temperature to temperature, and from filter to filter.
Keep this record safely in your notebook so you can go back to it in the future.
In the future, you simply have to do one very precise focusing with the Bahtinov mask per night, and only with one filter. For other temperatures, just interpolate from the deltas from this first focus location. For other filters, just look at the deltas between the filter and the reference filter.
If you plot the deltas as curves, you can even see the trend, and be able to extrapolate to temperatures that were not initially measured. On colder nights, you can venture out and add more data to your table of deltas.
If you know the EAF scale (draw tube distance vs EAF step; very easily measured), and the filter thicknesses, you don't even need to Bahtinov the filters, just use 1/3 of the difference between filter thickness and the reference filter thickness.
Chen