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Re: [CCD-astrometry-photometry] Photometry without filters Dear Bill and all, > 1. Do one has to use exactly the same sets of reference stars to > measure a target star/asteroid? Not necesarily. Especially when a variable star has a large amplitude, we usually need to change comparison stars in order to avoid saturation. The most important thing is how to establish a link between different comparison stars. This can be usually done with intermediate exposure times which enable photometry of both bright and faint stars. The colors in reference stars may be different, which may cause a subtle zero-point problem, if the observations are not done in the same condition. Choosing comparison stars with neutral colors (e.g. avoid B-V > +1.x etc.) would improve this situation. > 2. Does the reference stars selected has to be within 1 to 2 mag. of > the target stars? Not necessarily. In case you need a higher precision, rather use a bright comparison star. A independent check for a possible non-linear problem (both from electronics and measuring software) by measuring faint secondary comparison stars using the bright comparison star. A small non-linear slow drift could be experimentally removed by fitting a smooth function to the observed (faint star)-(bright star) light curve. (This technoque could be used to remove secondary color-term problem in broad band, esp. unfiltered, photometry at varying airmass/extinction). > 3. For a 8" scope with 60 second expsoure, is it possible to detect a > 1% variation in brightness for targets brighter than say mag. 13? Possible. In case you don't need 0.01 mag accuracy for every *single* image, this limitation can be much lowered. If you want to detect 0.01 mag periodic variation, you can even go fainter magnitudes. > 4. Is it absolutely necessary to use some kind of filter? This is what I wrote on "Photometry without filters" a moment before. The VSNET Collaboration (http://www.kusastro.kyoto-u.ac.jp/vsnet/) also has sufficiently succesful experience with this kind of observation. See, e.g. WZ Sge 2001 campaign http://www.kusastro.kyoto-u.ac.jp/vsnet/DNe/wzsge01.html was mostly conducted with unfiltered CCDs. > 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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