Taking pictures of rainbows is - at least for me, usually - a fool's errand.
If I get lucky enough to see one, by the time I get my camera it has either gone away or diminished enough that it isn't worth taking a picture of.
Or, if I have my camera by some fortuitous act of fate and a rainbow appears, I get so excited that I start shooting with some aberrant setting that I had just been using to catch a shot of a hummingbird.
It's always something.
So, the fact that I got this optimum light panorama has to be called an act of god.