astronomical instrumentation. light detectors eye eye use averteduse averted vision to see faintest...
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Astronomical Astronomical InstrumentationInstrumentation
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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
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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
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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
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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
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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
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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
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National Geographic Palomar Sky National Geographic Palomar Sky SurveySurvey
1950-57103aO103aF
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POSS IIPOSS II
1980s and 1990s1980s and 1990s• IIIaJIIIaJ• IIIaFIIIaF• Finer emulsion than in POSS IFiner emulsion than in POSS I
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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
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Photoelectric PhotometryPhotoelectric Photometry
AdvantagesAdvantages• LinearLinear• Higher quantum Higher quantum
efficiency than efficiency than photographyphotography
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Stebbins 1910Stebbins 1910
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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
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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)
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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
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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
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CCD ChipsCCD Chips
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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
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SLOAN Digital Sky SurveySLOAN Digital Sky Survey
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SLOAN Digital Sky SurveySLOAN Digital Sky Survey
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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)
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Inexpensive planetary imagersInexpensive planetary imagers
Celestron Celestron NexImageNexImage
Meade AutostarMeade Autostar
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Basic CCD observingBasic CCD observing
CCD images should be calibratedCCD images should be calibrated• BiasBias• Dark frameDark frame• Flat fieldFlat field
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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
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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
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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
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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
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Flat fieldsFlat fields
Flat field images Flat field images should be divided should be divided into the object into the object imageimage
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The Night Sky LiveThe Night Sky Livehttp://nightskylive.net/main.htmlhttp://nightskylive.net/main.html
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StardialStardial
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StardialStardial
TT Mon