[vsnet-chat 5933] Re: [AAVSO-DIS] Reporting observations: Multi-letter codes

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

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

Berto,

> Actually, I find the term C without extension rather meaningless and
> those observations 'uncalibrated'. Commercial processing programs will
> measure C magnitudes of stars based on integrated pixel values without
> any reference to actual magnitudes of comparison stars. I would suggest
> the term C refers to this type of derived unfiltered CCD magnitudes.

   No, you should not report these magnitudes.  They are what are called
"instrumental magnitudes".  I presume that no one (at least in optical
photometry) has ever reported instrumental magnitudes as a final form
of reports.  (Although I think it is unnecessary to mention for you,)
always use comparison stars, or apply all-sky photometry to derive
a zero point.  This is not a problem of the system notation, but with
a problem with the operation of software (e.g. a package like IRAF also
give these instrumental magnitudes, but people are educated to correct
them to final magnitudes).  Otherwise you need invent a new concept of
"uncalibrated V magnitude" and "V magnitude calibrated on V-magnitude
comparison stars (VV?)" (or a logical extention, following Henden's
concept, like "V-band photometry calibrated on Rc-magnitude (VRc?)"
etc.  Any combination is possible, but that's simply a nonsense.

   As you know, the standard passband of V and Rc overlaps.  In some
V-band photometry with extrmely red objects, the effective wavelength may
be closer to Rc than V.  But there is not a covention to refer to these
magnitude on Rc.  The only existing concept is simply "standard V magnitude"
of extremely red objects.  The same is true for unfiltered CCD photometry.
A similar situation occurs with a very broad-band response, such as
Hipparcos photometry.  There are VT, BT and Hp magnitudes.  I haven't
ever heard of a neccesity to make different calibration and notation
(such as "HpRc" or "HpB"), even from Arne Henden, for these very broad
passbands.  Becuase they are simply well-definable "Hp magnitudes".
The situation is not different from unfiltered CCD photometry.

> I do agree that there is less practical use of CI, CV and CB, although
> they might possibly apply to CCD systems with non standard (possibly
> pre-filtered) spectral responsivities and in the case of CI even
> referring to measurements of very red objects. 

   If you use a filter, the conventional concept of "C" (unfiltered CCD)
is no longer valid.  You had better select the best matching passband
to describe the system.  (If your response is close to Ic, simply use Ic
instead of "CI" or "CIc").  A better thing is to avoid using a "very
non-standard" filter.  You may get some value with any filter and a CCD,
but they should not be reported in a standard way.  There is frequently
a question stating that "I obtained a CCD image with a filter for
photography, what magnitude system should I use to report the observation?".

> I definitely support Arne in the use of CR as opposed to simply stick
> to C which could mean everything.  

   I don't mean C should mean everything, but I mean that indiscriminate
extention of subscript system is foundationless and should be avoided.
[Arne, this is my education].

Regards,
Taichi Kato

VSNET Home Page

Return to Daisaku Nogami

---

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