[vsnet-chat 5930] Re: [AAVSO-DIS] Reporting observations: Multi-letter codes

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

Re: [AAVSO-DIS] Reporting observations: Multi-letter codes

> I have been pushing the adoption of the terms "CR" and "CV"
> for use with unfiltered CCD systems that are R-like and V-like
> respectively, and which then use Rc and V magnitudes for the
> comparison stars/zeropoint.  The logical extensions would be
> "CI" and "CB", though as Kato-san mentions, there would be
> very few occasions where those terms would be useful.

   You need to be aware of the need for determining the zero point.  Even
if the effective wavelength (on some CCD system) of the target object is
close to that of B or I(c), the effective wavelength of the comparison
star is usually far from them (probably either close to Rc or V).
Then how do you adjust the zero point?  The adoption of "CI" or "CB" system
is thus only hypothetical extentions, rather than practical.
(Regardless of this, I wonder which combination of an unfiltered CCD and
an object could reproduce a B-like response?)

   So please don't make a confusion between the system used to determine
the zero point, and the system representing the approximate response to
a specific object.  Representing these two different "systems" in a
single suffix is already a source of confusion.  Since we usually have no
color information on unfiltered photometry, the second system is usually
less rigorously defined.  The first system (zero-point definition) can
be more reasonably defined: this is one of the reasons why I recommend
against the usages of superflous extentions based on the assumption of
the second system.

> You know my opinion.  However, if you *are* doing unfiltered
> photometry, you need to pass on as much information about how
> the observation was "calibrated" as possible, which is why
> terms like CR and CV have relevance.

   Speaking of calibration, it is already obvious a single suffix is
not sufficient to describe the system.  A system response function would
be adequate; I agree that an abbreviated specification of the response
function would be useful.  A set of number output (e.g. photon number)
and the response function is the key information to describe the
observation.  A convolution (i.e. determination of the effective
wavelength) requires other knowledge of the object, and is usually
a matter of to interpretation of the nature of the source.  In principle,
reporting raw observations should be as free as possible from these
interpretations -- that is to avoid using a concept or a definition
based on convolution with the source spectrum.

   From a different viewpoint (as I wrote before), I don't recommend to
use Cx (x = passband), since there is sufficient evidence of confusion
that "one can get x-band photometry using x-band comparison star magnitude".
There is also a confusion between "CV" (unfiltered CCD observations with
V-band comparison stars, if I correctly interpret Henden's concept) and
V-filtered CCD observations.  Also some people use "CU", with which they
mean "CCD unfiltered".  How can one know that "CV" and "CU" are not a
same kind of extentions??  Since such confusions during observation
are often irrecoverable, a potential source of confusion should be
minimized.

Regards,
Taichi Kato

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp