Bart,
You really don't need to worry about the white balance for astrophotography. Some say to stick it on daylight mode, or whatever, so that there's no discrepency between sub-exposures. For example, falling light and the sky getting darker, may induce a slight shift in white balance when left in auto white balance mode.
At the end of the day, we massage our final data in post-processing so much, that white balance is the least of your concern. Much better to do G2V calibration to get colour correct, or, just ignore it altogether, and produce something of an artwork rather than scientifically correct.
But, that's just my ignoramus approach.
H