[vsnet-chat 5932] Re: [AAVSO-DIS] Reporting observations: Multi-lettercodes

[Arne Henden <aah@nofs.navy.mil>]

Taichi Kato wrote in response to Henden:
>>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?)
> 
   My suggestion is that you indicate "CR" when you have a CCD with R-like
unfiltered response and you use Rc magnitudes for the comparison star(s).
Likewise, you would use "CB" _if_ you had B-like unfiltered response and
B magnitudes for the comparison stars.  Mixing and matching never works,
no matter whether you are filtered or unfiltered.  As I said, use of
"CB" and "CI" would be very infrequent, but should be available as
logical extensions of the concept in case they are needed.  Regarding
the difference in effective wavelength between the variable and the
comparison star:  this is an unfortunate consequence of observing
unfiltered.  Using comparison stars that are of similar
color to the variable is always wise, and even more so when observing
unfiltered.  Red stars, especially M-class, have such non-blackbody
spectra that even standard broad-band filters like Rc and Ic are
difficult to transform properly.  My discussion only covers those objects
for which unfiltered observations are reasonable, and as I have said
earlier, I won't get drawn into unfiltered vs. filtered observations
as this is another long-winded topic.
> 
>    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.
> 
   Here I disagree as a matter of principle and reality.  If you are
so concerned as to need the full spectral response of the CCD system
and the star in question, then you should be observing filtered or
be doing spectroscopy (and be taking the atmospheric transmission
into account).  A crude adjustment onto the standard system
as per CR and CV is about all that unfiltered observers should consider.
*Not* indicating such a zeropoint means the reported magnitudes can be on
any system and nearly useless.

> ...  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.
> 
I haven't seen "CU" used commonly, so you would have to convince me
that confusion is possible.  I can't help it if people do not use
a commonly accepted notation when submitting observations; you have
to educate them.  That is not a reason to reject "CR" and "CV".
Arne

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