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Astronomical observations
Telescopes Instruments and observations
Detectors Astronomical images
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Telescopes
End of 16th Century the first refracting
telescopes are built in the Netherlands 1609
Galileo builds his own telecope and turns it
towards the sky 1671 Newton builds the first
reflecting telescope
Galileo observing the sky
Replica of the first Newtons telescope
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Telescopes - 2
Telescope types Refracting - based upon
lenses ? size limited to 1 m
chromatic aberrations Reflecting - based
upon mirrors ? light does not go through glass
but partial obstruction
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Telescopes - 3
Main characteristics of a telescope Diameter of
primary mirror d ? collecting surface Focal
distance F ? scale of image in focal plane
F / 206235 (in mm/arcsec if F in mm)
Aperture ratio F / d ? optical speed (flux
concentration) Angular resolution ? 1.22 ? /
d for a circular aperture of diameter
d
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Telescopes - 4
Other telescope characteristics Image quality -
angular diameter of circle in which a given
fraction of the light fom a point source is
concentrated Field - region of the focal
plane which is lit or - region of the focal
plane where image quality is adequate Focal
plane curvature (ex Schmidt telescope wide
field but curved focal plane)
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Telescopes - 5
Types of foci Several possibilities (1) detector
at prime focus (2) A secondary mirror deflects
the light beam towards another focus Newton
Cassegrain Coudé Nasmyth
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Telescopes - 6
Equatorial mount In order for the telescope to
keep pointing towards a celestial object, Earths
rotation must be compensated ? telescope mounted
on 2 axes
a1st axis parallel to Earths rotation axix
(polar axis) a 2d axis perpendicular to the
latter (declination axis) ? polar axis rotates
(360 per sidereal day)
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Telescopes - 7
Altazimutal mount Thanks to computers, one can go
back to a simpler mount a 1st vertical axis
(azimut axis) a 2d horizontal axis (elevation
axis)
Advantages simpler, more compact ?
cheaper axes parallel and perpendicular to
gravity ? more stable ? system adopted for
the large modern telescopes
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Instruments and observations
The large majority of astronomical observations
consist in analysing the photons collected by the
telescope Photometry number of photons per
unit of time in a given spectral band (?
filters) Imaging photometry number of
photons as a function of angle Spectroscopy
number of photons as a function of energy
(? of wavelength ?) Polarimétry number of
photons as a function of polarisation
Combination of ? techniques (ex
spectropolarimetry)
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Detectors
The first detector used was the human eye (or
rather its retina) Drawbacks short integration
time ( 1/15th of a second) no
reliable recording of the observation
Photographic emulsion brought a huge
progress Advantages possibility of long
integration times (several hours)
long term recording Drawbacks low efficiency
( 3 of photons are detected) non
linearity (emulsion darkening not proportional
to luminous
flux) poor
reproducibility
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Detectors - 2
Electronic detectors Many electronic detectors
start to be developed in the 70s and 80s
(Reticon, Digicon) Among them, the CCD
(Charge-Coupled Device) rapidly
emerges Advantages with respect to photographic
emulsions quantum efficiency (up to gt 90) ?
more than a factor 30 gain!
linearity Drawbacks small size (a few cm2)
sensitive to cosmic rays
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Detectors - 3
Photon detecition in a semiconductor CCDc are
based on semiconductors (generally Si) They are
characterized by a valence band and a conduction
band separated by a gap.
At absolute zero valence band is full
conduction band is empty a photon can be
absorbed and give its energy to a valence band e-
that is sent into conduction band
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Detectors - 4
Charge collection Electrons in the conduction
band are free to move inside the silicon Surface
electrodes create potential wells that attract
these free e-
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Detectors - 4
charge transfer (shutter closed)
charge collection (shutter open)
Working of a CCD



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Detectors - 6
CCD sensitivity Photons can be absorbed only if
E? gt Egap N? a (E - Egap) as long as E not too
high then saturated and goes down
Quantum efficiency percentage of incidents
photons that are detected
Quantum efficiency of a particular CCD
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Detectors - 7
Photon absorption in silicon Photons penetrate
deeper as ? increases Electrodes are opaque in
UV
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Detectors - 8
Amélioration de la sensibilité dans le bleu et
lUV CCD amincis et illuminés par larrière
thinned backside illuminated CCDs
Lindice de réfraction du Si est élevé ?
possibilité de réflexions multiples aux grands ?
? possibilité de franges si les surfaces ne sont
pas parfaitement planes
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Detectors - 9
Linearity and saturation When the potential well
is nearly full, free e- are much less attracted
by the electrodes ? non linearity followed by
saturation
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Detectors - 10
Parasite signals Dark current e- excited by
thermal effect ? cool the CCD Cosmic ray impacts
ionizing particles crossing the CCD
? a large number of e- are freed in contiguous
pixels (pinned by the shape or multiple poses)

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Detectors - 11
Bias, gain and readout noise Output amplifier ?
intrinsic internal noise (depends on electronics,
readout speed) readout noise (RON) typically a
few e- Dynamic range of CCD RON 1 , saturation
105 ? dynamic range 105
Analog digital converter (ADC) transforms
measured signal into a number (ADU Analog to
Digital Unit) Generally 16 bits (0 ? 65535)
or 32 bits Gain g Ne / NADU 1 (unit e-
/ADU) Bias additive constant to avoid negative
signals (and thus loose a bit for the sign)

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Detectors - 12
Possible causes slight size differences
between pixels dust on camera lens non
uniform lighting of the field
(a bit exaggerated)
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Detectors - 13
Interpixel nonuniformities May depend on ? ?
hard to correct in case of observations through
wide-band filters
Intrapixel nonuniformities Sensitivity maay
depend on the region of the pixel where the
photon is absorbed ? hard to correct if image not
well sampled
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Astronomical images
Instrumental profile Image of a point source
through a circular aperture Airy rings

??
Seeing Ground-based observations ?
atmospheric turbulence
If exposure time long enough ? image a bit
blurred
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Astronomical images 2
Angular resolution minimal angular distance
between two point sources of same brightness that
can be resolved FWHM ( Full Width at Half
Maximum) of a point source
By some misuse of language, one calls seeing the
FWHM of a point source observed with a
ground-based instrument Typically, seeing is 1"
(0.5" in the best sites)
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Astronomical images 3
Signal-to-noise ratio S/N ratio between signal
and its measurement uncertainty (noise) in a
pixel in an astronomical object Counting of
photons obeys Poisson statistics
? s vNe
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Astronomical images 4
Limiting magnitude magnitude of faintest
object that can be detected on a given exposure,
with a given S/N (ex S/N 3)
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Astronomical images 5
Image reduction transformation of a raw image
into a scientifically useful image (reduced
image) bias subtraction (measured on zero
exposure time images) correction of interpixel
nonuniformities (division by a uniform field
exposure flat field) detection of cosmic ray
impacts cosmetic correction (scientifically,
the information is lost in these pixels ? s
8) subtraction of sky background computation
of an image containing the intensity
uncertainties s