(cba:news) data-taking issues (Joe Patterson) [2007-04-16T15:16:02Z]

Dear CBAers,

Back in NYC now, anxious to look at that great data for BZ UMa and GW 
Lib. Best targets in a couple years, except maybe for the Halloween 
transient.

Both stars are pretty bright, and you'll need to think about saturation.
Some people are fighting this with filters (generally, V filter 
recommended)... some with short exposures... and some with stopped-down 
telescopes.  One of these should work for you.

In general, superhumps and orbital humps have a color similar to the 
star's mean color - hence the amplitude in near-optical passbands (BVRI) 
tends to be similar.  This is one of the reasons we've tended to 
recommend unfiltered photometry: the extra information from filtering 
isn't worth the lowered signal-to-noise.  But this isn't true for all 
stars, and for most stars it has not been studied and hence isn't known 
one way or the other.  I'd find it very interesting if ANY superhumper 
showed a large color effect; superhumps come from heating effects, and 
thermal processes at the relevant temperatures (c. 15000 K) are pretty 
white.  But that's just theory; it is well worth a filtered study if you 
have the filters and can still get good signal-to-noise.  V-I or B-I are 
good choices (we want a good range in central wavelength, but can't 
afford the variable atmospheric transmission typical of U).

BTW these pooh-poohs of filtered photometry don't apply to the DQ Her 
stars (intermediate polars).  There the origin of the pulsed light is
an open question: it could well be a magnetic process (cyclotron 
radiation), which can be a very strange color, and is somewhat commonly 
very red.  PQ Gem is an example of this.  Not much is known about this, 
because extensive multicolor photometry isn't obtained for most DQ Hers. 
An interesting project, perhaps.

Have a wonderful time with these two grand new superhumpers!

joe