Bruce's flats at two different focus settings and the resulting "flatted flat" showing essentially no difference between them.
Bruce Gary of exoplanet fame asked me an interesting question the other day:
“One matter that seems to never get discussed in what focus setting to use when taking flats when using a telescope that changes length with temperature changes. This may not matter if you’re doing it in the middle of the night, but twilight flats are taken when temperatures are higher. Arne [Henden, director of the AAVSO] once posted an terse answer that you should set the focus setting to whatever is appropriate for the temperature at the time of the flats, and not the temperature that you expect to be working with at night. I haven’t tested this idea yet and I’m wondering if you have, or know of someone who has. ”
My response:
“I haven’t done any testing, but I doubt that focus is that important since dust donuts are going to be primarily on filter wheel and that distance isn’t going to change much. Ditto with vignetting. And certainly pixel to pixel variation doesn’t depend on temperature of the OTA. So my non-empirical take is that temperature changes to focus are irrelevant for flatting with the electroluminescent panels, anyway.”
Bruce, being the consummate experimenter that he is came back with the following:
“You’re right!”
“I just took a set of 4 images at a cold and warm focus setting, then ratio’d them. Wow! And for years I’ve been diligently setting the focus to whatever corresponds to the temperature when doing flats. I just assumed it mattered so I never checked it; your thought about it made me think that it’s an easy thing to test. ”
There: another myth about flat fields exposed. Thanks, Bruce!