[Aavso-photometry] When to submit "Fainter Than"versusactualnumbers.

[Arne Henden]

Jeff Hopkins wrote:
> I recommend reading Arne Henden's "Astronomical Photometry." Chapter 3 
> is devoted to statistics. Standard deviation is an indication of 
> accuracy or how good the data is. It is based on a series of data points 
> (program star magnitudes). Your idea of scatter about a straight line is 
> wrong. In your example time is a factor (you are plotting magnitude vs. 
> time), but standard deviation calculations are time independent. The 
> standard deviation is a representation of the deviation from the mean of 
> a sample of observations. To use other star data to determine the 
> program star's standard deviation should not be done. Only compute your 
> program star's standard deviation from program star magnitudes or 
> differential magnitude with respect to a known comparison star. Also, 
> you really need to understand the difference between precision and 
> accuracy.
> 
The problem here is that the variable star, well, varies.  So it is
difficult to determine a standard deviation for its magnitude during a
time series.  That is why people are using a known constant star of
roughly equivalent magnitude as a surrogate.
   The CCD photometric procedure is just different than PEP.  For PEP,
you often use a set of measures, such as CVCVCVCK (C=comp, V=variable,
K=check) and consider it a single datapoint.  This is because there
are a number of PEP error sources, such as how well you center each
star in the diaphragm, how much the sky transparency changes during
the set, etc.  For CCD, all stars are measured simultaneously, and
centering errors are nonexistent.  You gain in data quality; you lose
in not having that standard deviation error estimate.  For a single
frame, you can use Poisson error as a lower limit to the true error.
For multiple frames, such as a time series, you can use the surrogate
star standard deviation from its mean magnitude as a good estimate
of the true error in the variable.
   I think we are talking apples and oranges here.
Arne