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Re: (fwd) problems with AIP4win > The comments (below) regarding the obvious difficulties and dangers of > dealing with low S/N data leads me to raise the question as to what you > consider to be the S/N value below which an expert's input is advisable? > As a relative newcomer in CCD photometry I have followed this discussion > with great interest, so all expert advise is most appreciated. The minimum S/N is about 3, if you are in persuit of short-term variations. If there is no need to search for short-term variations, you may choose a longer exposure time to get a better S/N. In practice, the exposure time for bright CVs (e.g outbursting DNe) is limited by instrumetal factors (download time etc.) rather than the S/N limit. This is not an obvious problem here unless they have deep short eclipses. My main concern is when the object started to fade, or reached near quiescence. Most observers stop observing (or asked to stop observing) because the desired exposure time would be too long. However, my solution is different. Keep the constant time resolution as required (less than a few minutes, most likely 1 minute or so) even if the object starts fading. A good software package can extract reasonable measurements from these images. It is a pity that possibly a majority of observers regard that fading dwarf novae are below their useful detection limit. I recently measured time-series photometry of a very faint dwarf nova provided to the VSNET Collaboration. The single image had an S/N only around 1. With these conditions, I have been able to extract reasonable measurements giving determination of the fading rate and an upper limit of periodic variations. So you don't too much worry about low S/N. Regards, Taichi Kato
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