Astronomical
Instrumentation
Light Detectors

Eye
• Use averted
vision to see
Faintest objects
Only the brightest
stars show color
with the naked eye
Naked eye limiting magnitude
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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
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1852 daguerreotype
Daguerreotypes and
wet plate photographs
were very insensitive
to light and were
inconvenient to use
Dry plates developed
in 1870s
Harvard College Observatory


Collection of some
500,000
photographic
plates taken
between 1880s and
1980s
Provide a record of
changes in the sky
Different photographic emulsions were sensitive to
different wavelengths of light

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Early emulsions were mostly
sensitive to blue and ultraviolet light
Early photographic magnitudes
• “pg” blue
• “pv” or “pvis” yellow
Kodak emulsions blue to red

Oh, John, George doesn’t eat flannel
underwear nor milk zebras

OJGDEFUNMZ

Not made anymore
National Geographic Palomar Sky
Survey
1950-57
103aO
103aF
POSS II

1980s and 1990s
• IIIaJ
• IIIaF
• Finer emulsion than in POSS I
UK Schmidt telescope in Australia

Southern counterpart to POSS
• IIIaJ and IIIaF
Digitized versions of these surveys are
downloadable
Photoelectric Photometry

Advantages
• Linear
• Higher quantum
efficiency than
photography
Stebbins 1910
1930s: Photomultiplier tubes
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IP21 workhorse
photomultiplier
tube after WW II
One star at a time
photometry
1950s: UBV filter system

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U 365nm FWHM = 68nm
B 440nm
98
V 550nm
89
B-V = color index (bigger means
redder)
1980s: CCD detectors (chargecoupled devices)


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

Apogee Ap47p 1024x1024 CCD
• Thermoelectrically cooled
• Designed for observing fainter objects
SLOAN Digital Sky Survey
SLOAN Digital Sky Survey
Infrared Observing


Has also gone to
area arrays
Spartan Infrared
imager (1-2.5
microns)
• 4 2048x2048
HgCdTe detectors
(mercury cadmium
telleride)
Inexpensive planetary imagers


Celestron
NexImage
Meade Autostar
Basic CCD observing

CCD images should be calibrated
• Bias
• Dark frame
• Flat field
Bias Frame

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
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

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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

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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

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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

Flat field images
should be divided
into the object
image
The Night Sky Live
http://nightskylive.net/main.html
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