Re: Detecting variables

[Stupendous Man <richmond@stupendous.cis.rit.edu>]

> If I were to take an image of the same part of the sky to 
> look for new variable
> stars what would be the "best" interval between pictures?

  The question you are asking is closely related to the question
"what is the distribution of periods among all variable stars?"
If you know that most variables have periods of, say, 1 to 3 hours,
then you would design an observing scheme which sampled each star
several times an hour.  
  
  The distribution of periods is (to my knowledge) not very 
well known.  There are plenty of studies which have tabulated
the periods of known variables, but they all suffer from 
some sort of selection bias.  Early in the century, most
photometric data came from wide-angle, long-exposure photographs,
which provided (I believe) rather infrequent sampling of any
particular star: several times a week at best, I would guess,
more typically several times a month.  The majority of 
variable stars in the catalogs at that time, therefore, tended
to have long periods.

  Anyway, even the best modern-day catalogs of variable stars
will suffer from the same sort of selection effects, so if you
use one to guide your observations, you may be missing out on
some type(s) of variable stars.  

  Still, it probably makes sense to look at such catalogs.
Let me suggest a few:

        - the General Catalog of Variable Stars (GCVS).
          You can find a copy at the TASS catalogs page:

              http://stupendous.rit.edu/tass/catalogs/catalogs.html

          You can download a copy of the third edition from that
          page directly, or visit the official GCVS site for
          more recent versions or additions.  The catalog contains
          information on periods, magnitudes and amplitudes, 
          so you can figure out what sort of variable star is
          more common within your observing range.

        - The All Sky Automated Survey (ASAS) has just released
          its fourth catalog of variable stars in the southern
          celestial hemisphere.  The latest paper is on astro-ph
 
              http://arxiv.org/abs/astro-ph/0412645

          It includes links to the ASAS web site, from which you
          can download all their catalogs, and again build up
          a table of periods.

        - You might also look at the Faint Sky Variability Survey
          (FSVS), at

               http://staff.science.uva.nl/~fsvs/

          They examine stars much fainter than you are likely to
          observe, but the distribution of types of variable stars
          they find may still prove useful.


  If you do read through these (or other) sources of information on
variability, and create summary tables of information, would you mind
telling us about them via the TASS E-mail list?

                                                   Michael Richmond