Yesterday I drove back across Texas after a fairly frustrating observing run at McDonald Observatory. As I've mentioned in my last few blog posts, the problem was clouds. I've dealt with clouds on many runs in the past, but this run was all the more frustrating because the clouds were quite patchy. So, it would look like it was clearing up, but then the clouds would roll back in again.
There are some types of observing that can be done in partly clear weather. If you are just taking pictures or are taking spectra (splitting the light up into its component colors), you can usually work through clouds. If a thick cloud rolls over, you just close the shutter on the camera, wait for the cloud to pass, and then open the shutter again. It's not ideal, but it works.
My problem was that I was trying to do time-series photometry, or measurements of the brightness of a star as a function of time. The analysis of such information requires long stretches of uninteruppted data, three or four hours at a time. It's okay if some thin cirrus (the high, whispy clouds) are drifting overhead, but when the clouds are thick enough to block all light, you're in trouble. One or two of those thick clouds can be dealt with, but I had thick clouds overhead every 10 minutes or so. And that just doesn't work.
So, out of four nights at the telescope, we got one night of good data. We really needed two or three nights of data for what we were trying to do. And now we will have to wait a year for this target to come around again. Oh well, this happens.