I'm not sure if it is possible to have any reasonable seeing after about 12 am local time from here in the NE suburbs of Adelaide during summer.
I've done a lot of nights of long duration photometry, from twilight to twilight, and the trend is always for the FWHM to improve a bit from about 9pm to 11pm, then deteriorate steadily from there until dawn. I don't know, possibly it is something to do with the difference between the ground temperature and air temperature in a built up area. The one or two occasions where the seeing has been OK in the morning has been when the previous day was unusually cool.
Things seemed a lot more stable in winter, just that there were fewer clear nights (but actually I think this summer has been cloudier than the last).
-Ivan
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