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On Mon, 17 Feb 2003, Taichi Kato wrote: > > This is not possible. Studies have shown that a minimum of 90 quanta > > incident upon the fully dark adapted human eye is the threshold of > > detectability (with optimum target size and flash duration). > > These values may be dependent on experiments. I know there exist > physiological description that the threshold is much lower, even > corresponding to a few detected photons in a single retinal cell. The 90 quanta is incident on the eye externally. 10% will get thru to be detected by the rod cells. A single photon can produce a measurable cell membrane hyperpolarization, but thats done by using patch-clamping techniques in vitro on isolated cells. In vivo, it takes 5-9 quanta to produce a measurable retinal response. > > This corresponds to a magnitude of 7.6 for a Vega type star just detectable > > with a fully wide 7mm iris. Which becomes magnitude 13.4 for a 4" > > aperture. > > Won't observers be surprised to hear this limiting magnitude for a > 10 cm (4 inch) aperture? I had years-long experience with this aperture, > but the limiting magnitude 13.5 was a "usually detectable" value under > moderate conditions (in moderate citylight). I once had an experience to I would say 13.5 at 50% detectability under good dark skies, not in the city. > catch the outburst of EY Cyg in rise. According to the reports to the > AAVSO, the object was recorded below 14.0 (in average) on the same night. > I remember that the object looked "conspicuously brighter than usual". > The quoated value more looks like to me a safe limit. Its important to use accurate photometry for these determinations, rather than quicklook reports using old charts and bad comp stars! > > About 10% of those actually get through to be detected by the rod cells, > > so in fact statistically even less than 1 quanta is getting "detected"! > > In what time, in 1 second? The time-constant of the eye response may > not be always 1 sec or similar length. Furthermore, there is always > Poissonian fluctuations of photon detections. It is possible, even at > this mean photon rate, one may detect a packet arrival of photons in > a reasonable waiting time. The observer may have succeeded in training > the detection neural network to distinguish the increased occcurence of > such events against the background. The optimum time constant of the retina is around 100ms. Longer "integrations" dont help much because of temporal summation (Bloch's Law). You can try to wait for a rare string of photon hits, but those will be infrequent. When it happens, the object may appear briefly but then vanish again for a much longer period. In a realistic star field, it will be very difficult to positively identify a faint object below your limit if seeing it just 5% of the time... Observations made like this should be highly suspect. Mike Linnolt
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