71951Re: low noise cameras
- Nov 2, 2012ICCD and EMCCD cameras have extremely low or nonexistent read noise.
ICCD uses a photon intensifier coupled to a normal CCD and has essentially zero read noise. But ICCD has some issues with QE (most are below 20% but there are a few very expensive ones in the 50% range) and MTF is limited by intensifier micro channels. I assembled my own ICCD (ZeroCam):
EMCCD achieves a similar effect via pixel level electronic intensification (not photonic). EMCCD has generally high QE that is tempered somewhat by stochastic noise (intensification noise). And CIC is a sort-of low level effective read noise but is less than 1e-. Because the intensification is pixel based the MTF is cleaner than ICCD but EMCCDs suffer from trapped charge lag effects that result in faint vertical smears (actually every CCD suffers from this but it is completely hidden by common read noises). I recently obtained a Photometrics EMCCD but have not posted web comments yet.
When operating in zero noise mode, ICCD and EMCCD have extremely shallow photon well depths of only a few dozen or so, which means that in most cases the exposures must be extremely short, which in turn means that the readout/download times must be very fast to avoid losing photons during the download cycle. To avoid lost photons, the camera should operate in frame transfer mode so that the next exposure is taking place while the prior exp is downloading.
All of this needs to happen very quickly because exp saturation can occur within a fraction of a second. Thus most ICCD and EMCCD have low pixel counts (by today's standards) to accomodate bandwidth limitations, typically 1x1k (1 megapix). HD+ video technology will increase that size but if you like a huge image then this technology is not for you.
Such a camera will produce enormous amounts of data requiring software that is not commercially available. I write my own software for these cameras and have created stacks made from nearly a million frames. Try doing that in CCDStack or PixInsight or Registax! <g>
High speed intensified astro-imaging is a whole new ball game and is on an exciting edge of amateur endeavors. I know of only a few pioneering practitioners. If you are interested then contract me for more info. BTW, I will probably sell my ZeroCam because I simply do not have the time to operate both cameras.
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