CCDs: made up of an array of pixels (picture elements),
read out like a bucket brigade.

(CCD picture here)











Other detectors:
---------------

Eye:	like any camera, with a lens in front, and a detector (the retina)
---	in back.

2 types of cells in retina:

1) Rods:  center of field-of-view: for reading, high resolution
2) Cones: around edge of vision field: larger, less resolution,
	but more sensitive (for seeing lions, cars at night)

=> Use averted vision on faint objects: look a little to the side.


Color vision: become color-blind at night; 
	brightest stars _can_ activate it, in a dark sky.


(Sky & Telescope article in 1994 on astronomy-optimized eyes)



Photography:
-----------

- Now largely obsolete, thanks to CCDs

- Can cover wide area: Schmidt telescopes 
  (but now so can CCDs)

- Cheap (sometimes)

-> Archives valuable: Harvard and Sonneberg (former E. Germany)


Photomultiplier tubes/photoelectric photometers/TV cameras:
----------------------------------------------------------
not as sensitive or linear as CCDs, now mostly only special uses 
(e.g. finders)

========================================================================

Instruments: what goes between detector and telescope
-----------
- spectrographs
- cameras (for direct imaging and with CCDs, photometry)
- photometers (now done by CCDs)
- others, e.g. polarimeters