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Your Observing Challenge:White Dwarfs in Open Star Clusters
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Star clusters are collections of stars born nearly-simultaneously. Once we know their ages, we can determine how different stars
change with time.
Messier 35 and NGC 2158 (CFHT)
47 Tucanae (HST)
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Models tell us how the appearance of stars should change with time.
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The ages of star clusters are usually determined by the “main sequence turn-off”
in a color-magnitude diagram.
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The models that give white dwarf ages use different physics from the models that give us main sequence ages, giving us a sanity check!
von Hippel (2005)
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Your Observing Challenge:
Take pictures of a star cluster with our 0.8-meter telescope and find candidate white dwarfs that might be useful for studying
ages of the cluster.
First, let’s examine your primary tools: the CCD camera and ImageJ
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Light is the only probe we have for objects outside the
solar system.
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Telescopes are funnels that collect large amounts of
light from stars, but must send light somewhere.
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Several devices are capable of collecting light and
storing information about it.
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The first modern astronomers used their eyes,
pencil and papers and brains.
William & Caroline Herschel
Heber CurtisMaria Mitchell
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Eyes are very inefficient and cannot integrate.
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Eyes can only do comparisons between
objects, and have trouble making quantitative
measurements.
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The brain is not the most reliable image processing
software.
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Around the turn of the century, photographic plates
became popular.
John Draper 1839Henry Draper 1880David Malin 1979
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Plates can integrate and are modestly efficient.
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Plates store images accurately for long periods
of time.
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CCDs are the modern astronomer’s weapon of
choice for observing.
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CCDs can integrate for long times at nearly 100%
efficiency.
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The images are read into a computer and stored on
disk.
Dr. Jana Pittichova
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CCDs pictures can be added together, letting you see
fainter.
Single Stack of 5
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CCDs convert light to electrical signals.
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CCDs are based on Einstein’s photoelectric effect.
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CCDs are made of individual pixels, each of which works
independently.
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The electrons knocked out of silicon are held in a well until
the exposure is finished.
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QuickTime™ and aGIF decompressor
are needed to see this picture.
After the exposure, circuitry counts the number of
electrons and reports it to the computer.
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If the signal is too high, the circuitry can’t count the electrons – the pixel is
“saturated.”
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Colors are measured by taking images through colored glass filters.
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CCDs only detect the number of photons, not their color.
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Filters allow only one color of light through.
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Measuring brightness through different filters
gives us color.
From Hubblesite.org
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There are dozens of flavors of filters; the most common are the
Johnson system
U = ultraviolet
B = blue
V = green
R = orange/red
I = very red (not infrared) Wavelength
blue red
Th
rou
gh
pu
t (%
)
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CCDs are not perfect, so astronomers must take
calibration data every night.
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These calibrations take care of most defects.
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Images of “standard stars” allow images to be put on an
absolute scale.
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Different telescopes and instruments have different
throughput.
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Dust and haze change the amount of light reaching the
ground.
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Calibration using standard stars lets us compare data from different telescopes
and nights.
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With cameras using one or more CCDs, astronomers
have been study very large areas of sky.
Kitt Peak Mosaic
Keck Observatory LRIS
Steward Observatory 90Prime
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Astronomy requires the collecting of light for later
study.
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Astronomers need cameras that take long exposures and
are efficient.
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CCDs fit the bill -- provided the right calibrations are
taken.