Editing Observational astronomy (section)

=== Photography ===
The [[photograph]] has served a critical role in observational astronomy for over a century, but in the last 30 years it has been largely replaced for imaging applications by digital sensors such as [[charge-coupled device|CCD]]s and [[CMOS]] chips. Specialist areas of astronomy such as photometry and interferometry have utilised electronic detectors for a much longer period of time. [[Astrophotography]] uses specialised [[photographic film]] (or usually a glass plate coated with photographic [[emulsion]]), but there are a number of drawbacks, particularly a low [[quantum efficiency]], of the order of 3%, whereas CCDs can be tuned for a QE >90% in a narrow band.  Almost all modern telescope instruments are electronic arrays, and older telescopes have been either been retrofitted with these instruments or closed down.  Glass plates are still used in some applications, such as surveying,{{Citation needed|date=July 2009}} because the resolution possible with a chemical film is much higher than any electronic detector yet constructed.

==== Advantages ====
Prior to the invention of photography, all astronomy was done with the naked eye.  However, even before films became sensitive enough, scientific astronomy moved entirely to film, because of the overwhelming advantages:
* The human eye discards what it sees from split-second to split-second, but photographic film gathers more and more light for as long as the shutter is open.
* The resulting image is permanent, so many astronomers can use the same data.
* It is possible to see objects as they change over time ([[SN 1987A]] is a spectacular example).

==== Blink comparator ====
The [[blink comparator]] is an instrument that is used to compare two nearly identical photographs made of the same section of sky at different points in time. The comparator alternates illumination of the two plates, and any changes are revealed by blinking points or streaks. This instrument has been used to find [[asteroid]]s, [[comet]]s, and [[variable star]]s.
[[File:Telescope.jpg|thumb|right|160px|50 cm refracting telescope at [[Nice Observatory]]]]