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Hi, 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 >>> Taichi Kato <tkato@kusastro.kyoto-u.ac.jp> 01/29/03 01:43PM >>> 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 -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean. "The CSIR exercises no editorial control over E-mail messages and/or attachments thereto/links referred to therein originating in the organisation and the views in this message/attachments thereto are therefore not necessarily those of the CSIR and/or its employees. The sender of this e-mail is, moreover, in terms of the CSIR's Conditions of Service, subject to compliance with the CSIR's internal E-mail and Internet Policy."
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