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Re: [CCD-astrometry-photometry] Photometry without filters Dear all, Unfiltered CCD photometry can *work* as long as the user knows the limitation, properties of the object's emission, required science and precision, caveats, and the way of systematic correction of this method. People can fairly well perform time-series photometry of cataclysmic variables (CVs) to search for periodicities. In case you would need to know the colors, use filters. This method is not adequate for measuring objects with unusual colors, e.g. very red stars or highly reddened object (even in these cases, monitoring for outbursts or a period search are still meaningful). In order to make better precision unfiltered photometry, select comparison stars with neutral colors, or stars with similar colors with the object. This would eliminate most of atmospheric extinction problem (secondary color term). Even in peculiar sources like blazars, we may be even able to make necessary conversions when the spectrum (power index) is known or we have an applicable model. This process fairly well works when there is a theoretical model for comparison. What we need is to simply integrate the model spectrum according to the CCD response function. This procedure is exactly same as when we make comparisons with bandpass filters. Regards, Taichi Kato
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