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> For blue stars, incl CVs, the effective wavelength of this observing > window of most CCD cameras is near that of the R band. Therefore R > magnitudes of reference stars (with non-red colours) in the field can be > used. Such observations are usually reported as CR. > If this is applied to yellow-orange stars (G-K) one will end up with a > waveband close to I. I magnitudes of similarly coloured reference stars > could be used for them. Observations are reported as CI.Please don't be confused. For most of stars with usual colors, the effective wavelengths of the most CCDs have close to that of Rc. Effective wavelengths are integrated quntities, and little vary around the mean wavelengths of the original CCD/filter passbands. Your explanation only applies to extremely red objects which virtually emit no light shorter than 650 nm.
Regards, Taichi Kato(vsnet-chat 5927)
I must admit that my scalefactor was a bit out. It should have read
like:
"If this is applied to yellow-orange stars (G-K) one will end up with a
waveband somewhat closer to I, and for very red stars close and even
beyound (?) the I band."
As effective wavelengths do change more considerably with unfiltered CCDs, red stars and reddened regions are photometrically 'out' for unfiltered CCD cameras.
I did make the mistake once.
Regards, Berto(vsnet-chat 5928)
G-K stars have peak emissions not far from Rc peak sensitivity. So you will get effective wavelength fairly close to that of Rc passband. For redder stars, your new statement is basically correct, but considering the large variety of spectral energy distributions of these extremely red objects, I doubt any practical meaning of a system like "CI".
Regards, Taichi Kato(vsnet-chat 5929)
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. One such case is similar to Berto's example: if you were observing a Mira unfiltered, and had a very red comparison star as well, then the effective wavelength of your unfiltered system moves towards the red and using the Ic magnitude of the comp star and reporting "CI" might make sense. Another might be where you use a "blue blocking" filter or an "IR blocking" filter for some reason, still giving a much wider bandpass for your system than even a Johnson/Cousins filter, but one that has been moved redwards or bluewards respectively.
The problem, as has been mentioned over and over again, is that unfiltered magnitudes from different CCDs/telescopes/locations are really hard to combine except under special circumstances. Working with CVs, where they usually have nearly zero color and don't change color with time, is one such case, which is why CBA does so well. If you are observing by yourself, and creating light curves exclusively from one CCD system, you can often work with other classes of stars for getting periods and basic light curve shapes. However, filtered observations are definitely preferred whenever possible.
Other than this, I won't get pulled into yet another discussion of the relative merits of filtered vs. unfiltered photometry. 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.
Arne
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.
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-chat 5931)
I am not in agreement with what Taichi writes.
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.
Unfiltered CCD measurements of non-red stars are likely to correspond to the R region. The extension R (as in CR) then specifies (that, and also) that differential measurements were made to R magnitudes of reference stars in the field. That's were the zeropoint is fixed.
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.
I definitely support Arne in the use of CR as opposed to simply stick to C which could mean everything.
Regards, Berto(vsnet-chat 5932)
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.
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.
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
Berto,
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
Arne,
I have explained almost everything in a reply to Berto's message. I presume that even you don't dare to refer to standard V-filtered observation of extremely red objects with a different notation, even if the effective wavelength is far from that of the V band. Think of an object emitting only Halpha. The effective wavelength is then around 655 nm, pretty close to that of Rc. But we don't refer to these magnitude as Rc. This is because "V band" is so defined to denote any convoluted quantity of V-passband and any spectrum. The same is true for other passbands. Any variety of CCD passband is not an exception -- it is simply not a standard passband, but is a real passband.
> 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.I mean that the required system is a "function" of a CCD (and minor contribution from instumental and atmospheric effect). So if one use a specific CCD, one doesn't need to use different symbols. One doesn't need to occasionally introduce "CI" for very red objects (this is what I read from Berto's original posting).
> 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".You seem to tend to speak without evidence. The notation "CU" can be found in many places, the best examples being Tonny Vaunmunster's cataclysmic variable circulars. You may find other examples only if you have ever tried to search for them. I have (today) just received a report with a system "CV". How can one discern whether this notation was used to refer to "unfiltered CCD observation reduced on V-band system" or "CCD V-band observation", without asking to the observer? Or we should silently regard them as "unfiltered CCD observation reduced on V-band system" regardless of the original intention of the observer? This is already an evident source of confusion! Leading people like you must be more aware of the consequence of an intriduction of a new concept; it is you who need to educate observers [or possibly you need a re-education for photometry? -- even an excellent practical photometrist may not be neccesarily an expert in theoretical background], but not the database managers such as AAVSO or VSNET. For me, the introduction of "CCD V" by the AAVSO instead of simply "V" is another source of confusion; some people may regard "CV" and an abbreviated form of "CCD V".
Regards, Taichi Kato
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