[Aavso-photometry] Errors and error reporting for CCD observations (fwd)

[Aaron Price]

 Let's move this to aavso-photometry. :)

---------- Forwarded message ----------
Date: Fri, 30 Jul 2004 10:47:57 -0700 (PDT)
From: Lew Cook <lcoo at yahoo.com>
To: aavso-discussion at mira.aavso.org
Subject: [AAVSO-DIS] Errors and error reporting for CCD observations

There are many ways to skin a cat. Reported CCD
observations contain numerous sources of errors. For
unfiltered work, and to a lesser extent filtered work, lack
of good transformation coefficients from V magnitude to CCD
magnitude is a prime source of systematic error.

Transformation coefficients themselves are a source of
error. CCD magnitude is transformed to an instrumental
magnitude from V magnitudes using the formula

CCD mag = V + Tc*(B-V) + offset

I have begun a preliminary study if my G16 camera and found
that different fields give different transformation
coefficients, Tc, for the same camera and telescope
combination.

I suspect the amount of interstellar reddening affects the
coefficient more than any other factor. For example, NGC
7790 gave me a coefficient of 0.46 while the field of CG
Cep gave 0.67. The stars at CG Cep are all quite red,
suggesting a lot of absorption. If the field I am working
in has high absorption and the stars are reddened, then I
will be systematically off when I use the transformation
coefficient from NGC 7790.

So, as Arne has repeatedly drummed into our heads,
unfiltered data has problems that filtered data does not.
Then again you get more light, and noise is reduced. Yes,
filtered data is best, but that loses a magnitude at least.
You can compensate with longer exposures, but there you
lose time resolution for rapid variations. It is like
asking a lawyer for advice: "On one hand ... but one the
other ... "

CCD Cameras and telescopes will have different
transformation coefficients. You cannot directly compare
one observer+camera+telescope with another using unfiltered
data even when transformed. There WILL be differences,
systematic, between them. Some observers do not know the
transformation coefficients and ignore the correction. You
have a mixed bag of data sets and an intelligent analysis
will apply corrections to the data sets as a set to correct
systematic errors.

The best practical way to get a good light curve is having
2 observers observe the variable at the same time.  Find
the difference empirically and apply it as a correction to
get a consistent light curve.

Another source of errors is statistical noise. Those who
report only it as their error are not reporting the whole
story, but it may be the best data they have, or the
easiest to get ahold of.

Yet another source of error is imperfect flats. I have
trouble getting flats that are truly flat better than 0.01
magnitude from corner to corner on my frames. I flat one
flat against another and look at the differences. Even if
you have perfect guiding, that could introduce a systematic
error of 0.005 magnitude from the center of the field to an
edge. My drive is imperfect and the field wanders from the
east edge to the west edge of the chip (if I let is), so
there is some systematic error there.

I report the standard deviation of the check star
magnitudes throughout the night where the check star is
about the same magnitude as the variable. If the variable
is a bit brighter than the check star, I still report the
SD of the check star. I try not to claim higher percision
than I have, however when the variable is varying wildly,
that number will be high when the variable is bright and
low when dim. Of course, I could just leave that field
blank, but I'd rather report something.

Yes, there are differences between observers in
(especially) unfiltered CCD data sets, but anyone analyzing
the data cannot take the magnitudes as gospel as reported.
You have got to use your head, just as with visual observations.

=====
Regards, Lew  - from CBA Concord -
http://www.geocities.com/lcoo/cbacal.htm
My other observing site is CBA Pahala
http://www.geocities.com/lcoo/pahala.htm
Get Donna's Book  "GHOLSON ROAD: Revolutionaries and Texas Rangers"
at
 http://www.authorhouse.com/BookStore/ItemDetail.aspx?bookid=18043
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