[vsnet-chat 882] Re: Magnitudes, sequences, etc

[bas@lowell.Lowell.Edu (Brian Skiff)]

     Greetings Stan.  If you assume the CCD has a linear response, there is
no need for calibration sequences fainter than the usual mag. 10-12.  For
southern observers, the Harvard E-region standards are the thing to use,
since they along with the Landolt equatorial standards define the system in
practice.
     In re attaintable accuracy, you'll find that getting the kind of accuracy
you're used to with photomultipliers is pretty much impossible with CCDs.
There is almost no all-sky CCD photometry that has real external accuracy
below about 0.015 mag. (per observation sigma) published in the literature.
The usual limitation beyond doing the observations and reductions carefully
is flat-fielding.  The best CCD work I've seen is by Alistair Walker 
(formerly of South Africa but now at CTIO) and by Arne Henden (USNO-Flagstaff).
     As discussed last night (western US time!) I wouldn't push too hard on
getting all five UBVRI colors, just V and some color (probably V-R).  You'll
have to measure in two colors to determine V anyway.  Yes, it would be nice
to have all the colors, but the purposes of rapidly getting zero-point stars
around a transient event, you want to minimize the work in order to get
results out as soon as possible.
     Good luck with it.
\Brian

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