[vsnet-chat 698] Re: Comparison sequences

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

Brian Skiff wrote:

> Apparnetly, Richard Stanton is re-examining the color term between "visual"
> and V as part of this effort.  For the moment, a rough transformation
> is:  mv = V + 0.2(B-V) for the dark-adapted visual response.

   From my experience (on some visual databases), visual observations are
an ensemble of different degrees of dark adaptation, whether the observer
is conscious of it or not.  Typical examples of Japanese observations
consist of roughly equal numbers of well dark-adapted and non-dark-adapted
observations, whose effect can be most clearly seen in the large scatter
during the nebular stages of [O III]-strong novae.  There seem to be no
tendency of observations cluster around the dark-adapted end.  Similar
trend is also easiliy visible for extremely red variables.

   Considering these actual outputs, rather than experiment under more ideal
conditions, we should also consider many numbers of "non-dark-adapted"
observations which consist a considerable fraction of the ensemble.
Non-dark-adapted visual response being less sensitive to blue, it would
be an acceptable choice to adopt Johnson V as representing the "ensemble
average" of visual observations done under different conditions.  There
is a good reason that sequences for visual observations should also follow
this ensemble average.  Though I am conscious in very good dark adaption
red stars look slightly (~0.1-0.2 mag) fainter than Tycho V indicates,
the reverse is also true under moonlit conditions.  Considering the ever-
increasing light pollution at least for urban observers, more weight should
be given on non-dark-adapted observations.  At least it seems to be premature
to choose transformations based solely on dark-adapted responses to represent
the visual response, or the "standard" mv.

Regards,
Taichi Kato

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