Astronomical Instrumentation

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

Naked eye limiting magnitude A difference of a factor of 100 in brightness corresponds to a difference of 5 magnitudes 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 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 1852 daguerreotype 1852 daguerreotype Daguerreotypes and wet plate photographs were very insensitive to light and were inconvenient to use Daguerreotypes and wet plate photographs were very insensitive to light and were inconvenient to use Dry plates developed in 1870s Dry plates developed in 1870s

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

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

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

National Geographic Palomar Sky Survey aO 103aF

POSS II 1980s and 1990s 1980s and 1990s IIIaJIIIaJ IIIaFIIIaF Finer emulsion than in POSS IFiner emulsion than in POSS I

UK Schmidt telescope in Australia Southern counterpart to POSS Southern counterpart to POSS IIIaJ and IIIaFIIIaJ and IIIaF Digitized versions of these surveys are downloadable

Photoelectric Photometry Advantages Advantages LinearLinear Higher quantum efficiency than photographyHigher quantum efficiency than photography

Stebbins 1910

1930s: Photomultiplier tubes IP21 workhorse photomultiplier tube after WW II IP21 workhorse photomultiplier tube after WW II One star at a time photometry One star at a time photometry

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

1980s: CCD detectors (charge- coupled devices) Array detectors for visible to about 1000 nm Array detectors for visible to about 1000 nm Combines high quantum efficiency and imaging capability 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 Apogee Ap47p 1024x1024 CCD Apogee Ap47p 1024x1024 CCD Thermoelectrically cooledThermoelectrically cooled Designed for observing fainter objectsDesigned for observing fainter objects

SLOAN Digital Sky Survey

Infrared Observing Has also gone to area arrays Has also gone to area arrays Spartan Infrared imager (1-2.5 microns) Spartan Infrared imager (1-2.5 microns) x2048 HgCdTe detectors (mercury cadmium telleride)4 2048x2048 HgCdTe detectors (mercury cadmium telleride)

Inexpensive planetary imagers Celestron NexImage Celestron NexImage Meade Autostar Meade Autostar

Basic CCD observing CCD images should be calibrated CCD images should be calibrated BiasBias Dark frameDark frame Flat fieldFlat field

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

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

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

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

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

The Night Sky Live

Stardial

Stardial TT Mon