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In Reply to: Can anyone explain me how the digital works at low light condition? posted by Romy on October 01, 2001 at 09:02:56:
But, basically, there's an error (noise) ever time the CCD is read out, due to the charge on the electron. this noise grows as square root of the number of reads. There's also a cumulitive noise effect due to thermal electrons from the CCD. There are also a couple of semiconductor effects due to surface crystal structure problems, etc, but I've not used CCD detectors now in about 20 years at the gut level, so I don't recall all the effects, but what it comes down to is that while film is "stable" to the tune of worrying about reciprocity, CCD's have noise that adds as a function of time.When photons are very scarce, this noise can be the primary "signal", which isn't very useful, I agree.
Now, SOME CCD's, used with liquid gases to cool them, do incredibly well in the 10-hour exposure department, but we're talking handheld cameras, right? :)
JJ
Follow Ups:
Well, we are talking about $5K-$10K handheld cameras or about $10K-$15K digital backs for 2/1/4. Is I realized those “pro” solution still do not address the problem. Are you saying that still there is the technologies that could “handle it”? At which level they start? Do those guy who shot the astronomical object still use the silver film?Romy the Cat
Romy,The folks who make digital "backs" for telescopes have pretty muched solved the problem. Also try the Canon D-30 as it uses CMOS and not CCD. The D-30 has long exposure "reduce noise" mode. Hope this helps.
Enjoy the Music,
Steven R. Rochlin
CCD's have taken over in spectroscopy, and are used as photon counters chilled to at least liquid nitrogen temperature. The trick being that as photon counters the noise can be ignored, since it's pulse counting. It seems (it's reported, I haven't my own hands on one) that there is better capture effiency with CCD's, perhaps layer-tuned or the like, I dunno.But for handheld things, I think you can see the problems.
I'm not sure at what speed the crossover between CCD's and film happens at, but it will undoubtedly go in the CCD's favor as better devices are made, with less surface defects, etc.
There isn't a whole lot of film usage left in MOST kinds of astronomy (imaging being the one that still to some extent uses it) any more, because CCD's and other detectors work better for most of the specialized applications.
BUT, I think that for the time being, we'll see better long-term results from film, simply because the CCD's in most photographic camera applications aren't running in photon-counting mode, and there will still be a constant noise growth with time that you don't get with film.
JJ
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