Astronomical Astronomical InstrumentationInstrumentation

Light DetectorsLight Detectors

EyeEye• Use avertedUse averted

vision to seevision to see

Faintest objectsFaintest objects

Only the brightestOnly the brightest

stars show colorstars show color

with the naked eyewith the naked eye

Naked eye limiting magnitudeNaked eye limiting magnitude

A difference of a factor of 100 in A difference of a factor of 100 in brightness corresponds to a brightness corresponds to a difference of 5 magnitudesdifference of 5 magnitudes

Limit usually taken as around 6.0, Limit usually taken as around 6.0, but by taking special precautions but by taking special precautions some have seen to magnitude 7 or some have seen to magnitude 7 or fainter with the naked eyefainter with the naked eye

PhotographyPhotography

1852 daguerreotype1852 daguerreotype Daguerreotypes and Daguerreotypes and

wet plate photographs wet plate photographs were very insensitive were very insensitive to light and were to light and were inconvenient to useinconvenient to use

Dry plates developed Dry plates developed in 1870sin 1870s

Harvard College ObservatoryHarvard College Observatory

Collection of some Collection of some 500,000 500,000 photographic photographic plates taken plates taken between 1880s between 1880s and 1980s and 1980s

Provide a record of Provide a record of changes in the skychanges in the sky

Different photographic emulsions were sensitive to Different photographic emulsions were sensitive to different wavelengths of lightdifferent wavelengths of light

Early emulsions were mostly Early emulsions were mostly sensitive to blue and ultraviolet lightsensitive to blue and ultraviolet light

Early photographic magnitudesEarly photographic magnitudes• ““pg” blue pg” blue • ““pv” or “pvis” yellowpv” or “pvis” yellow

Kodak emulsions blue to redKodak emulsions blue to red

Oh, John, George doesn’t eat flannel Oh, John, George doesn’t eat flannel underwear nor milk zebrasunderwear nor milk zebras

OJGDEFUNMZOJGDEFUNMZ

Not made anymoreNot made anymore

National Geographic Palomar Sky National Geographic Palomar Sky SurveySurvey

1950-57103aO103aF

POSS IIPOSS II

1980s and 1990s1980s and 1990s• IIIaJIIIaJ• IIIaFIIIaF• Finer emulsion than in POSS IFiner emulsion than in POSS I

UK Schmidt telescope in AustraliaUK Schmidt telescope in Australia

Southern counterpart to POSSSouthern counterpart to POSS• IIIaJ and IIIaFIIIaJ and IIIaF

Digitized versions of these surveys are Digitized versions of these surveys are downloadabledownloadable

Photoelectric PhotometryPhotoelectric Photometry

AdvantagesAdvantages• LinearLinear• Higher quantum Higher quantum

efficiency than efficiency than photographyphotography

Stebbins 1910Stebbins 1910

1930s: Photomultiplier tubes1930s: Photomultiplier tubes

IP21 workhorse IP21 workhorse photomultiplier photomultiplier tube after WW IItube after WW II

One star at a time One star at a time photometryphotometry

1950s: UBV filter system1950s: UBV filter system

U 365nm FWHM = 68nmU 365nm FWHM = 68nm B 440nm 98B 440nm 98 V 550nm 89V 550nm 89

B-V = color index (bigger means B-V = color index (bigger means redder)redder)

1980s: CCD detectors (charge-1980s: CCD detectors (charge-coupled devices)coupled devices)

Array detectors for Array detectors for visible to about visible to about 1000 nm1000 nm

Combines high Combines high quantum efficiency quantum efficiency and imaging and imaging capabilitycapability

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

CCD ChipsCCD Chips

CCD in use at the 24-inch on CCD in use at the 24-inch on campuscampus

Apogee Ap47p 1024x1024 CCDApogee Ap47p 1024x1024 CCD• Thermoelectrically cooledThermoelectrically cooled• Designed for observing fainter objectsDesigned for observing fainter objects

SLOAN Digital Sky SurveySLOAN Digital Sky Survey

SLOAN Digital Sky SurveySLOAN Digital Sky Survey

Infrared ObservingInfrared Observing

Has also gone to Has also gone to area arraysarea arrays

Spartan Infrared Spartan Infrared imager (1-2.5 imager (1-2.5 microns)microns)• 4 2048x2048 4 2048x2048

HgCdTe detectors HgCdTe detectors (mercury cadmium (mercury cadmium telleride)telleride)

Inexpensive planetary imagersInexpensive planetary imagers

Celestron Celestron NexImageNexImage

Meade AutostarMeade Autostar

Basic CCD observingBasic CCD observing

CCD images should be calibratedCCD images should be calibrated• BiasBias• Dark frameDark frame• Flat fieldFlat field

Bias FrameBias Frame

Sets the zero-point Sets the zero-point of the CCD outputof the CCD output

Essentially a 0 Essentially a 0 second exposure second exposure with the shutter with the shutter closedclosed

Should be Should be subtracted from all subtracted from all imagesimages

Dark FramesDark Frames

Even if the shutter is closed, every Even if the shutter is closed, every CCD image will have some signalCCD image will have some signal

The warmer the CCD, the bigger this The warmer the CCD, the bigger this “dark signal” is“dark signal” is

To minimize this we can cool the To minimize this we can cool the CCD, either electrically or CCD, either electrically or cryogenicallycryogenically

Dark FramesDark Frames

Take one or more Take one or more exposures with the exposures with the shutter closedshutter closed

Ideally about as Ideally about as long as the real long as the real exposures and at exposures and at the same CCD the same CCD temperaturetemperature

Subtract these Subtract these dark framesdark frames

Flat fieldsFlat fields

Not every pixel on the CCD will be Not every pixel on the CCD will be equally sensitive to lightequally sensitive to light

With the same filter you will use for With the same filter you will use for observing, taken an exposure of a observing, taken an exposure of a uniform light source, such as the uniform light source, such as the twilight skytwilight sky

Flat fieldsFlat fields

Flat field images Flat field images should be divided should be divided into the object into the object imageimage

The Night Sky LiveThe Night Sky Livehttp://nightskylive.net/main.htmlhttp://nightskylive.net/main.html

StardialStardial

StardialStardial

TT Mon