Astronomical Instrumentation Light Detectors n Eye Use averted

Astronomical Instrumentation

Light Detectors n Eye • Use averted vision to see Faintest objects Only the brightest stars show color with the naked eye

Naked eye limiting magnitude n n A difference of a factor of 100 in brightness corresponds to a difference of 5 magnitudes Limit usually taken as around 6. 0, but by taking special precautions some have seen to magnitude 7 or fainter with the naked eye

Photography n n n 1852 daguerreotype Daguerreotypes and wet plate photographs were very insensitive to light and were inconvenient to use Dry plates developed in 1870 s

Harvard College Observatory n n Collection of some 500, 000 photographic plates taken between 1880 s and 1980 s Provide a record of changes in the sky

Different photographic emulsions were sensitive to different wavelengths of light n n Early emulsions were mostly sensitive to blue and ultraviolet light Early photographic magnitudes • “pg” blue • “pv” or “pvis” yellow

Kodak emulsions blue to red n Oh, John, George doesn’t eat flannel underwear nor milk zebras n OJGDEFUNMZ n Not made anymore

National Geographic Palomar Sky Survey 1950 -57 103 a. O 103 a. F

POSS II n 1980 s and 1990 s • IIIa. J • IIIa. F • Finer emulsion than in POSS I

UK Schmidt telescope in Australia n Southern counterpart to POSS • IIIa. J and IIIa. F Digitized versions of these surveys are downloadable

Photoelectric Photometry n Advantages • Linear • Higher quantum efficiency than photography

Stebbins 1910

1930 s: Photomultiplier tubes n n IP 21 workhorse photomultiplier tube after WW II One star at a time photometry

1950 s: UBV filter system n n U 365 nm FWHM = 68 nm B 440 nm 98 V 550 nm 89 B-V = color index (bigger means redder)

1980 s: CCD detectors (chargecoupled devices) n n Array detectors for visible to about 1000 nm Combines high quantum efficiency and imaging capability

CCDs were originally much smaller than photographic plates, but they are getting bigger

CCD Chips

CCD in use at the 24 -inch on campus n Apogee Ap 47 p 1024 x 1024 CCD • Thermoelectrically cooled • Designed for observing fainter objects

SLOAN Digital Sky Survey

SLOAN Digital Sky Survey

Infrared Observing n n Has also gone to area arrays Spartan Infrared imager (1 -2. 5 microns) • 4 2048 x 2048 Hg. Cd. Te detectors (mercury cadmium telleride)

Inexpensive planetary imagers n n Celestron Nex. Image Meade Autostar

Basic CCD observing n CCD images should be calibrated • Bias • Dark frame • Flat field

Bias Frame n n n Sets the zero-point of the CCD output Essentially a 0 second exposure with the shutter closed Should be subtracted from all images

Dark Frames n n n Even if the shutter is closed, every CCD image will have some signal The warmer the CCD, the bigger this “dark signal” is To minimize this we can cool the CCD, either electrically or cryogenically

Dark Frames n n n Take one or more exposures with the shutter closed Ideally about as long as the real exposures and at the same CCD temperature Subtract these dark frames

Flat fields n n Not every pixel on the CCD will be equally sensitive to light With the same filter you will use for observing, taken an exposure of a uniform light source, such as the twilight sky

Flat fields n Flat field images should be divided into the object image

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Stardial

Stardial TT Mon