I finally got around to shooting my new flats last night. Afterwards I slewed over to Vega to sync my mount and check my focus. As soon as I take the first image, I could tell the focus was off. All of the little stars were donuts! I don't recall ever seeing this big of a shift, one night to the next. So I throw the Bahtinov mask on and focus. Then I slew over to my nemesis, the Crescent Nebula, to begin imaging for the remainder of the night. (Or until APT couldn't recenter after the Meridian flip, as it happens!)
Anyway, as I'm about to start the sequence, I notice that the blue filter is selected. Ah! That's from doing the LRG and B flats! So when I was checking the focus earlier, I was doing so using the blue filter. Normally I use L. That got me to wondering if the big focus shift I detected was actually from the filter change and not some other gremlin.
So, if you can use only one filter (LRGBHSO) for focusing, which one is "best", ie the stars will be the sharpest when using the other filters? I guess it coudl be which two filters: one from LRGB and one from LHSO? I think I read somewhere on CN that blue is best for LRGB. Does that mean O3 would be best for narrow band?