(cba:news) other comments on faint CBA stars

[Joe Patterson]

Hi Michael et al.,

Yes, time resolution is not so critical since the periods of interest are
~35 min and longer.  But here's the more general point...

For SDSS1238-03, and for faint CVs generally, don't be distracted by the
common rules of thumb for "good photometry".  Some people say "1%
accuracy" as the rule of thumb - in which case, you need >10000 counts,
plus an accurate calibration (standards to <1%).  This is feasible for
practically no CV in the sky... and it is largely useless even where it
is feasible, because CVs show intrinsic erratic variability ("flickering")
which is >>1%.  Someday we will understand that flickering, and whoever
has very extensive and accurate data on it has some chance of making that
breakthrough.  But it's an unsolved problem, and unless you hope to be the
person who finally makes that breakthrough, you're better off considering
the flickering to be THE BASIC BACKGROUND AGAINST WHICH YOU HAVE TO DETECT
PERIODIC SIGNALS.  The latter - the periodic signals - are very different
in that they have (more or less) straightforward physical interpretations,
so that's what I always consider to be the reasonable goal of these
observing campaigns.

The bottom line in all of this is that you can usually consider "5%
statistics" to be a satisfactory statistical accuracy, and you can almost
settle for 3%.  In unfiltered light, my guess is that CBAers can reach
this in about 120 s - for a star as bright as sdss1238-03.  The light
curves won't be pretty enough to send home to Mom, or even to frame in
your observing shack.  But they're likely to be an effective tool for
sniffing out the periodic structure in the star - especially when combined
with the homely light curves of other observers in a global network.

Sorry for being long-winded!  But I'd like to encourage experiemnts on
fainter stars - and to steer people away from very long exposures, since
the latter have only a SLIGHT advantage in signal-to-noise, and they are
much more prone to serious systemtaic error (drive errors, cosmic ray
hits, wind gusts, etc.)

joe