[Aavso-photometry] Uncertainty

[Richard Harvan]

Arne,
          I took 12 images containing TT ARI, the 11.050 and 11.075 comparison stars, and performed the calibration with darks and flats.  
          I then performed the aperature photometry, using the 11.050 star as my reference.  When I obtained a measured magnitude of 11.050 of this star, I then measured the magitudes of TT ARI and the 11.075 comp star.  This was done with all 12 images, giving 12 values for TT ARI and the 11.075 comp star.  The value of the 11.050 comp star was always 11.050.  I obtained the standard deviation for the twelve TT ARI values from the Excel function STDEV, however I calculated the standard deviation of the twelve 11.075 comp star values manually using 11.075 as the mean in the calculations.  Perhaps this was not the way to do this calculation for the 11.075 comp star, however I thought the variance from this value was more useful than the variance from the mean of the measurements.  I then squared the calculated TT ARI and 11.075 standard deviation values, the 11.050 error vaues from the VSP chart (0.010), and the 11.075 error value from the VSP chart (0.049), summed these 
 four values, and took the square root of the sum.  It is my intention to state the uncertainty of the measurement as this value (square root of the sum).  
         This is not what I entered into WebObs.  I intend on modifying that observation, depending on the results of this discussion.
   
  Rich Harvan

Arne Henden <arne at aavso.org> wrote:
  On 11/19/07, Richard Harvan wrote:
> 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.
>
I'm a bit confused, so I need more information. My assumption from above is:
(1) you take a series of images
(2) you aperture-photometer the variable, the comparison, and the
check on all frames.
Then what steps are you taking? I don't understand "standard
deviation of the variable
and the check star." I can understand standard deviation of (K-C).
Arne


       
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