[vsnet-chat 5971] Re: [AAVSO-DIS] CCD-V Vs visual observations

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

Re: [vsnet-chat 5968] Re: [AAVSO-DIS] CCD-V Vs visual observations

> I'm gonna go out on a limb here and say: CCD observations that have 
> large errors are (most likely) systematic. That is, a reproducible 
> event or error is involved, such as a bad comp star, bad image 
> reduction, etc. In theory, if one makes a note of all related criteria, 
> one can always re-reduce a CCD observation based on better information 
> and revise the estimate accurately. CCD observations are, in this way, 
> not subjective. You can screw up your process but it is hard to 
> introduce subjective error.

   There are two main sources of errors: random errors and systematic
errors.  CCDs are likely superior in terms of random errors, if the other
conditions are the same.  Or one can use the terms like internal errors
and external errors.  CCDs are superior in reducing internal errors
(with which we can make fine time-series analysis of low-amplitude
variables).  However, external errors are more difficult to reduce, and
in order to do so, one will require high quality control (e.g. checking
consistencies between different nights and images), which does not seem
to be reasonably performed by every CCD observer.  This would be one of
reasons why CCD observations of individual variable stars just as in the
procedure as in visual observations have not been surprisingly accurate
as is expected from small internal errors in time-series photometry.

   The other thing is more human.  It seems to me that the fraction of
careless mistakes is not reduced in CCD observations (or even more
abundant in CCD reports).  These careless mistakes include incorrect
identification of comparison stars or even objects, bad calculation,
poor use of reduction software, error in copying data from the software
output, date error, and many, many.  It sometimes looks like that CCD
observers tend to think there primary aim is to make observation, but not
to make accurate reports.  Some people don't care if the variable is
calculated to be several magnitudes different on subsequent nights,
and don't care if they confuse a 10-th mag star with a third mag star.
It may have partly due to the fact that CCD observation and reduction
are usually more time-consuming than visual observation, and people tend
to make mistakes and dismiss doubts in fatigue.

   Not all CCD observers store their CCD images; some people indeed delete
the images after sending reports.  Not all errors in CCD observations
can be re-reduced as in theory.

Regards,
Taichi Kato

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