[Aavso-photometry] Uncertainty

[bailyhill at aol.com]

Hello Richard;

I like the idea of calculating the std dev of the check star, 
especially when the check star it is close in magnitude to the 
variable.  However, lets assume that the variable is really changing.  
Including its std dev with the comp star does not tell us much about 
our process.  It is fair to compare the std dev of a check star and the 
variable (assuming once again that they are about the same magnitude), 
to see if the variable is changing relative to the check.

Rather than include the std dev of the variable, I use the std dev of 
several other check stars to monitor the quality of the photometry.  
Realize that this calculation tells us much about the scatter in the 
measurement.  That is the precision of the measurement or the 
repeatability of the measurment.  It tells us nothing about the 
accuracy of the measurement.

Clear Skies
Gary



-----Original Message-----
From: Richard Harvan <tkdvarstar at yahoo.com>
To: AAVSO Photometry <aavso-photometry at mira.aavso.org>
Sent: Mon, 19 Nov 2007 17:55:29 -0800 (PST)
Subject: [Aavso-photometry] Uncertainty






Dear All,
           I would like to get your opinion on my calculation of 
uncertainty.
The procedure I am writing about calculates the standard deviation of 
the
variable and the check star.  These values are then combined in 
quadrature with
the stated uncertainty of the photometry of the comp star and check 
star.  Is
this a reasonable method or am I going overboard.
                     I saw in a textbook that the uncertainty of an 
average was
the standard deviation divided by the square root of the sample size.  
When I
applied this to my data the calculated value was much less than the 
stated
uncertainty.  This seemed too good to be true, so I have doubts about 
applying
this to my measurements.  What is your opinion?
  Thanks for your help in advance.

  Rich Harvan


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