I re-focus on every filter change and on every sub. I have 2 imaging refractors, one at 500mm FL and one at 925mm FL native. The longer FL refractor will contract proportionately more than the shorter as temperature drops and will require a re-focus after 5 minute or longer sub, especially in the maritime environment where I live on the south coast of the UK. The shorter Esprit 80ED refractor has obviously less material to contract and the change in focus point throughout an evening is noticeably less than the longer WO FLT132.
I thought about setting a temp parameter for both scopes but decided on balance that the time taken to re-focus is better than the disappointment of a discarded sub when I came to processing.
So the factors I have taken into account in my decision related to the thermal characteristics/experience of my two scopes, the prevailing weather and humidity in the UK, scarcity of imaging time vs time taken to AF for each sub. I have especially noticed the softer focus-drift when taking 20 min + Ha subs.
A factor also is that I’ve recently changed to imaging sequentially through each event rather than completing an event before moving to the next: e.g., LRGB, LRGB, LRGB rather than all the Lums, then all the Reds etc. In this way building a dataset for each channel (and therefore distributing that evening’s conditions evenly throughout each channel rather than concentrated in any one channel) and so there is a need to re-focus on each filter change.
There is no right or wrong approach here, just a trade off of the factors affecting your equipment and imaging conditions.