One thing I like to do if possible is to take references before and after the target, particularly when the exposure time is long. For the nova Vul 2024 spectrum I used HD187362 an A1v-A3v star depending on which reference you take and with B-V = 0.07 so matching the expected colour for that spectral class. I used it with the Pickles A2v spectrum. The response changed a bit between the two measurements 2 hours apart, partly due to the decreasing air mass but mainly I suspect due to a drift in focus as I took it early in the evening and everything was still cooling down. I used the average response and the end result should still be within my normal claimed precision of +-10% 4000-7500A
Cheers
Robin
