Submitting Data to the CBA

The CBA is a world-wide network to study light curves of variable stars. Members’ telescopes are usually in the aperture range 8-32″, generally in backyards. This is how we manage to be effective: by using garden-variety telescopes where people actually live. Most members have CCD cameras. These remarks describe how we carry out and analyze our observations.

Our usual goal is time-series photometry of cataclysmic variable stars, in order to keep watch over the many periodic processes in these binaries — orbital motion, white dwarf rotation, accretion disk precession, and a few others that have not yet acquired a clear physical interpretation. So what you basically need to do is find your way to a program star, and do sequential photometry of say 100-200 1-2 minute integrations. Repeat for ~50 nights.

Still reading? Promising, if true. For such valor, we get to be the Timekeepers of the CVs.

In an ideal world, everyone would use the same comparison stars. And in our charts we designate a couple of neighbor stars as C1 and C2 for this purpose. But in reality everyone must make their own intelligent choice, because of differences in chip size, saturation levels, etc. That’s still OK, because in period-finding analysis we sometimes subtract off the average delta magnitude anyway. In selecting a comparison star, one wants a bright nearby star (the bluer the better) that does not saturate the chip.

Filters. Well, some people got ’em, and V filters can be mighty useful. But most “broad-band” filters transmit only ~10% of the photons available in unfiltered light; that’s a very high price to pay for the improvement in calibration. Usually too high for us, with our normal fascination for faint stars and periodic signals. So we usually go unfiltered.

Data reduction. Big issue. Most cameras come with little software packages allowing photometry of individual frames. A few observers profitably use these for years, but most people quickly conclude that time-series photometry is a critical need. We have no uniform solution, but several CBAers use Munidos (written I believe by Rudolf Novak and/or collaborators, available at the CBA-Brno site), and others use EZPhot (written by Jerry Gunn, CBA-Illinois). These get the job done well, and have nice learning curves that get you to the finished product pretty fast. Consider them.

What you want (actually what we want, but we’re hoping we can convince you to want it too) from data reduction is a time series of differential (Var-Comp) photometry and Julian Date. A file that looks like this:

   347.6890   1.437
   347.6905   1.448
   347.6920   1.420

Sometimes people add a third column, which could be C1-C2, or airmass. Both are somewhat useful.

Time should be truncated (ugly big digits removed) Julian Date, with no heliocentric correction unless you swear that all your dates will forevermore be HJD. Time should refer to the middle of the integration, not the beginning or end.

Our website is, and is a medley of messages, past and future dreams, and resources. You can send data us at our data archive at cba-data .at. Please write or visit the website with further questions!