Why Charge-Coupled Devices (CCDs) are nearly perfect astronomical detectors:
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1) Very _sensitive_.  More precisely, have high _quantum efficiency_ (QE):
        > 90% of starlight falling on CCD makes a signal!

Compare with photoelectric/TV sensors (10% QE), eye (2% QE), 
	photographic film or plates (< 1% QE).


2) Highly _linear_: if one exposes twice as long,
one gets twice as much signal, typically to within factor of 10^5.

Compare with photographic film/plates: linear only to within factor of 2.
	This is called reciprocity failure:
	if one exposes over a given length of time,
	one doesn't get much more signal.

=> CCDs are _photometrically accurate_:
	brightness measurements of stars are reliable.


3) Have high _dynamic range_, as a result of linearity:
can see both bright and faint objects in same images.

Example of where photography fails: solar eclipse pictures.

Another: space photos---almost never show stars in the background! 
	(Most taken in daylight, overpowering stars.)
  
  
4) Low noise: CCDs don't make much spurious signal, also called noise.
Most from readout or cosmic rays, both of which are easily quantified or
removed.  Thermal noise (stray electrons from thermal motion) is often
minimized by cooling, with liquid nitrogen or electrically.


5) Wide spectral range: CCDs are sensitive from soft X-rays to near-IR.
	(Most sensitive in red.)


6) All digital!  CCD images go directly into computer, no need to scan,
as with photographic film or plates.


-> DON'T call CCD images "photographs": that refers to the now-obsolescent
chemical process.  Call them IMAGES!


Disadvantages of CCDs:
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- Costly: but not really, anymore. (Digital cameras & camcorders now common)

- Small area => small field of view: 
	but not really, anymore. (Now CCD mosaics cover whole Moon!)

- Relatively insensitive in blue/near-UV:
	but not really, anymore (with new coatings)

- Readout too slow for high-speed photometry:
	but not really, anymore. (Mostly up to software.)

-> Can't count individual photons, just many at once, when read out. 

OK, got me there.  But are these on the horizon?  (See November 1999 Sky &
Telescope, p. 20:  Superconducting Tunneling Junction [STJ] devices).