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Re: [AAVSO-DIS] Why Aren't Arne's Sequences Solid Gold? Just to clarify mo thoughts: > I don't see much difference in what you say and what I say > below. Complexity includes everything, I think. Transforming with widely > different color sequences is more complex in my thinking than comparing > something quite similar. Possibly there is a better word to use than > complexity. I cannot think of anything better at the moment. So it is > easier to believe a measurement done by a simple transformation from very > similar sequences than a measurement that is made from data far from the > sequence. That is what I was trying to say. I think not too badly. It is not probably adequate to focus on what the word complexity would exactly mean, but what came to my mind is something like that: color transformations are usually done with a couple of linear equations. It is easy to imagine a situation that someone tries to extend this formulation to higher-order polynomials, introduce cross terms, non-linear function etc. without deep understanding why the usually taken linear transformation is a good approximation. To learn this, one should know how different bandpasses behave in response to idealized astronimical light sources (such as blackbody radiation, a variety of modifications), and in what cases one can reasonably transform a system into another. This is one of the difficult aspects of broad band photometry (and from my experience with students, it is indeed difficult to understand). Without correct knowledge of them, an introduction of a degree of complexity could even degrade the quality and efficiency of measurements. This is one of the points which does not seem to be adequately addressed in many of modern CCD photometry textbooks (Sterken and Manfrid's "Astronomical Photometry A Guide" would be a good supplement to learn). I have even seen an observer, who introduced complex cross-terms and experimental terms to formulate an elaborate color-transformation equations, and who tried to experimentally determine these coefficients. The logical consequence was that he needs to observe 10^N (N = large) different measurements and fill a huge matrix to statistically meaningfully determine the transformation (and even with this, there is no guaranatee that the formulation is valid for other objects!). He even would have to observe standard stars all his life long to determine the equation! Before reaching a similar conclusion, it would be more advisable to make more effort to buy or design filters which best approximate the required passbands (not restricted to UBVRcIc). Regards, Taichi Kato
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