[vsnet-chat 716] Re: on charts and sequences again

[Taichi Kato <tkato>]

Robert Fidrich wrote:

> My points on the charts and sequences:
> 
> - I am not sure, hat we sould use V sequences for visual estimations. Since 
> 
> < mv = V + 0.2(B-V) for the dark-adapted visual response. .>
> 
> the V magnitudes would be not very good in case of comparisons 
> with (B-V) > 0.5
> Since red stars usually has (B-V)>2, so in the cases the sequences the
> scatter could reach 0.5 magnitudes in visual magnitudes. So I don't think
> the V magnitude is the best choice. My opinion then: if we know the color
> index of the comparisons, we need to calculate mv using the  relations
> f(b-r), f(B-V) etc. 

   One of the problems is that any "mv" doesn't have a fixed color term.
While for dark-adapted eyes, the color term of +0.2 may be a good
approximation, for non-dark-adapted eyes, this is not a good approximation
(the color term may become negative).  In the latter cases, sequence based
on the "+0.2" color term may introduce even more errors than only using
V-band sequence.  This is one of unavoidable characteristic of eyes; if we
want to eliminate it, we would have to determine the "observed" color term
every time we wish to make visual estimates.  This is feasibile, but is
tiresome.

   The large color difference between the variable and comparison stars is
also unsually unavoidable.  For a variable star with B-V=+3.0, we can not,
in any reasonable way, eliminate the error arising from inidiviudal eye's
sensitivity, unless we can use a nonvariable B-V=+3.0 comparison star.
This is usually impossible.  For these extremely red stars, the intrinsic
scatter in human eye's response surpasses any small modification in the
comparison system.  The scatter would be much the same whatever V+k(B-V)
(abs(k) < 0.5) system would be adopted for comparison stars.  However, the
effect of different color terms to different comparison stars may be
minimized by selecting a certain range of color indices among possible
comparison stars.  The success in homogenizing visual observations in
Novae Cas cases largely owed Brian Skiff's nice selection of comparison
stars with "homegenized" colors.  The existing charts should be re-examined
from this point, since they usually contain stars of quite different colors,
and some of them are often suspected of variability, probably simply because
the star look differently under different circumstances.  The availablity
of Hipparcos and Tycho catalogues for brighter stars, and the USNO catalog
has greatly facilitated this process.

   At least from my experiment (experience), the usage of B-V selected H/T
V-magnitudes in various (probably many more than I already had) fields
has yielded quite satisfactory agreement of the eye's sensation to what
the V values displays.  The degree of satisfaction has been hardly achieved
in any previously used charts -- has gone so much as to make me regret,
when I need to recalculate archival estimates, that I have thrown amay
many of "hardly acceptable" old charts.  More practically, typical B-V=1.5-2.0
stars look 0.1-0.2 mag fainter to my eye than H/T V mag near the visibility
limit, but look brighter than V at more brighter end.

Regards,
Taichi Kato

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