[vsnet-chat 1746] Re: BVRI Photometry

[aah@nofs.navy.mil]

Kato-san makes a good point:  using comparison stars that
are really faint can lead to systematic errors if, for example,
the sky background is not treated properly.  Like him,
I recommend that you use brighter comparison stars if at
all possible (and if you have lots of choice, I select
comparison stars that are equal in brightness and color
to the variable).  However, if you are in a situation like
Stan, where there are no bright comparison stars in the
CCD field of view, then I would much rather use the
faint-star ensemble method for getting differential
photometry rather than all-sky photometry with comparison
stars in other areas of the sky, if you are
trying to get high accuracy.

An interesting side-point:  since you would normally
expose your frames so that the variable is close to
saturation to give as much signal/noise as possible in
the fainter comparisons, you actually get better results
near the variable's minimum brightness, where the exposures
are longer and therefore you have more signal/noise in
the comparisons.  Likewise, when the variable is bright,
you often have trouble adjusting the exposure with
changing seeing such that the variable is not overexposed.
This method is not for the weak-of-heart!

Dan asked if you could just observe in one filter since
the object is to check for period changes.  As Brian
mentioned, Cepheids do have different times-of-maxima
depending on wavelength.  This is a small effect and
is not important if you are using, say, a CCD with a V filter
to compare with previous PEP V-filter data.  So the answer
is 'yes': one filter, especially V, would be sufficient.
However, observing with a second filter once you have spent the
time moving to a field is a small penalty, and then you
have sufficient information to report data on the
standard Johnson-Cousins system.  Also, most of the bright
Cepheids have little or no R&I data (they were observed back in
the PEP UBV days), and getting the red-filter light curves
can have some useful scientific benefits.  Stan has a filter
wheel and standard BVRI filters, so I would recommend more
than one filter for his project.

Arne

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