Standard Star Evening Twilight Tests

On the evening of December 29, 2012, the standard star field SDSSJ0200-0000 was observed over and over again in all 5 DES filters starting at 00:25UT (23 min before 12deg Twilight) and ending at 00:49UT (1 min after 12deg Twilight), immediately followed by another SDSS Stripe 82 standard star field (SDSSJ0320-0000), which finished at 00:56UT (6 min after 12deg Twilight). (12 deg Twilight was at 00:48UT.) The Night Summary for this night can be found here.

Following are the residual plots from 21 min before 12deg Twilight to 8min after 12deg Twilight. Even for observations at about 20min before 12deg Twilight, the number of standard stars detected in the SDSS fields (standard star limiting mag is roughly r=18) does not appear to suffer dramatic loss. There does appear to be a slight trend in the residuals (see linear fit in each of the plots), but at < 1% level.

Note that the main field in question (SDSSJ0200-0000) was separated by the Sun by about 108 deg. (SDSSJ0320-0000 was separated from the Sun by even a larger angular distance.)*

Tentative Conclusion: We can start observing standard stars 20 min before the 12deg Evening Twilight. Although yet undetermined, if observing standard stars this early in Evening Twilight, it is probably better to observe those which are not in the Western part of the sky (closer to the Sun and larger sky background gradients). As usual, until other fields are calibrated, SDSS Stripe 82 fields are the highest priority fields, and at least one (and preferably more) SDSS Stripe 82 fields should be observed each night (supplemented, when necessary, by the other standard star fields).