[vsnet-chat 5794] Re: [AAVSO-DIS] survey, pixy etc

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

Re: [vsnet-chat 5789] Re: [AAVSO-DIS] survey, pixy etc

>    While I understand the basic intent here, I don't think these two
> bandpass discriminations are of "comparable significance."  The difference
> between Rc and Rj is small, and other than a zeropoint shift,

   These bands are different, especially in their central/effective
wavelengths (this factor is more significant than the difference in
bandpasses).  In terms of their central/effective wavelengths, Rj is between
Rc and Ic, and Ic is between Rj and Ij, and neither Rc ~ Rj nor Ic ~ Ij.
One can often easily discern (espiecially for stars with unusual colors)
which one is Rj and which is Rc when there are quasi-simultaneous
observations -- this already proves the significant difference between
these bands.  If we don't make any distinction between them, reporting
R(c) magnitude using Rj filters can be justified.  Such a situation will
certainly degrade the accuracy introduced by a new system Rc.

> the two systems are reasonably close for moderate-color stars.

   For B-V = 0 stars, almost all systems give similar magnitudes :-).
Not all stars have moderate colors.

   For almost all stars, the Stroemgren y filter gives similar magnitudes
to the Johnson V band, because the difference is in bandwidth.  However,
we usually don't mix them into a single "V" system, but often make some
distinction.  Indeed, the difference can be striking in measuring
emission-line objects such as novae in nebular phase or symbiotic variables,
because the Stroemgren y band has virtually no sensitivity to [O III] lines,
while Johnson V has.  A similar situation (but a lesser degree) can be
found between Rj and Rc, especially in objects with strong Halpha emission
(Rc is more sensitive to Halpha).

> For the K bandpass, one should remember that the K window is almost as wide
> as the entire visible window.  Ks or K' is used to denote a filter
> that covers the blue end of the window, dropping the last 200nm or
> so on the red end.

   Please also consider the frequency domain, or discuss in relative
values such as (width of window)/(central wavelength).  You would see how
comparable (K-Ks)/K is to (Rj-Rc)/Rc.

Regards,
Taichi Kato

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