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

[Jeff Hopkins]

Hello Geir,

I don't understand what you are doing, but normally differential 
photometry uses one comparison star with a given program star. If 
multiple program stars are in the same field for a CCD exposure, 
there may be multiple comparison stars of similar spectral type for 
each corresponding program star. The only purpose of a check star is 
as a safety in case the comparison star happens to be found to be 
variable.

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.

You do not mean absolute magnitude. Absolute magnitude is the 
magnitude of a star at a standard distance of 10 parsecs. What you 
measure is an instrumental magnitude which you then reduce to a 
standard and produce an extra-atmospheric magnitude. The magnitudes 
listed in reference material for stars is standardized 
extra-atmospheric magnitude. When you are done with your data 
reduction, that is what you should have produced.

Jeff

At 16:43 -0700 1/27/05, Geir Klingenberg wrote:
>Hi Jeff,
>
>That is all true but I was thinking check star, not comp star. The
>check star is not used to say anything about the actual magnitude of
>the target, its only purpose is to give me a standard deviation, and
>this standard deviation can be quoted as the uncertainty of the
>target. The standard deviation is calculated from the series of K - C
>values where K is the instrumental mag of the check star and C is the
>instrumental mag of the comp star(s). I believe that this std will not
>depend on the color of the stars, and will give a realistic estimate
>of the uncertainty of the target measurements as long as the check
>star and the target are close in magnitude in whatever filter is used,
>but someone please correct me if I am wrong.
>
>Of course, this is not the whole story since uncertainty in the
>transformation from differential mag to absolute mag is not included.
>For this uncertainty of the comp star catalogue values are relevant,
>and, if the measurements are transformed to a standard system the
>uncertainty in the transformation coefficients are also relevant. (One
>thing to keep in mind is that if transformations are NOT performed
>this should give a higher error than if transformations ARE
>performed).
>
>I don't know the best way to do this though, I only state the standard
>deviation of K - C as described above as my measurement error, and I
>believe most others do as well.
>
>I tend to I think of precision and accuracy in a graphical manner; if
>we do a time series of two constant stars and plot their diff. mags we
>see the measurements scattered about a straight line. This scattering
>is what I think of as precision, and the std quantifies it (this is
>about repeatability of the measurements). When we transform to
>absolute magnitudes we shift the average up or down, and I think of
>accuracy as how well we hit the true magnitude.
>
>I believe precision is most important for time series of the AAVSO
>campaign kind as it says something about our confident in the shape of
>the light curve.
>
>Geir Klingenberg

-- 
Jeff Hopkins
HPO SOFT
http://www.hposoft.com/Astro/astro.html

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