What is the download time of a CCD readout?

Hi Ed,

For download time, the pixel size is irrelevant.  2kx2k = 4 million pixels / 500,000 pixels/sec = 8 seconds. This is probably a lower limit; other overhead will increase the read time somewhat.

CCDs read out through usually 1 node (some science cameras utilize up to 4 nodes, one on each corner, but I haven't seen that in the commercial market).  This gives the vendor the opportunity to optimize the signal chain.  One way they decrease noise is to read out slowly.  At USNO-Flagstaff, we typically read out at 100KHz, so that the 2kx2k sensor there took 40 seconds to read.  Lots of dead time!  However, our exposures were almost always in the multi-minute range, and the dead time was not as important.  If you were working on a target that required 1 second exposures, then 40 seconds (or 8 seconds in your case) dominates, and you get very little on-sky time.  Note also that most CCD cameras require a mechanical shutter, and to eliminate shutter vignetting, exposures typically shouldn't be shorter than ~5 seconds without an extra correction step.

CCDs are useful in several parts of the parameter space.  If you need big pixels (say 4m focal length), then the 13.5 or 15 micron native pixels of E2V sensors is nice.  If you need even bigger pixels, then the noiseless binning feature of CCDs means lower readnoise than CMOS.  If you need the best red response, the deep-depletion CCDs will yield the highest QE.  The AR coating on the front of the sensor can be customized to yield the highest QE in the wavelength region of choice.  Most professional observatories use CCDs because their telescopes are long focal length and their instruments are expensive, so they choose the most optimal solution for their projects.

For most amateur projects today, though, the current CMOS sensors like the IMX455 or IMX571 are better than CCDs.  They read out faster, are lower noise, have more and smaller pixels (great for sub-2m focal lengths), and are way cheaper.  You have multiple vendors to choose from.

Arne