[Aavso-photometry] Scattered Light

[Greg Crawford]

>If you do astronomy from Nantucket Island in the summer, it looks like you have
>lots of cloudy nights to do long flats

I'm puzzled by this idea of doing flats on cloudy nights. I would have thought
it was necessary to focus immediately before taking flats in order to reproduce
the optical configuration being measured by a flat; and that temperature and
other changes would result in the scope being out of focus on those cloudy
nights. 

Greg C.