Transcript Astronomical observations

Astronomical observations
• Telescopes
• Instruments and observations
• Detectors
• Astronomical images
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 Newton’s telescope
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
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)
F
• Aperture ratio F / d
→ optical speed (flux concentration)
• Angular resolution θ = 1.22 λ / d
for a circular aperture of diameter d
d
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)
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
Telescopes - 6
Equatorial mount
In order for the telescope to keep pointing towards a celestial
object, Earth’s rotation must be compensated
→ telescope mounted on 2 axes:
– a1st axis parallel to Earth’s
rotation axix (polar axis)
– a 2d axis perpendicular to the
latter (declination axis)
→ polar axis rotates (360° per
sidereal day)
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
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 λ)
• Polarimétry: number of photons as a function of polarisation
+ Combination of ≠ techniques (ex: spectropolarimetry)
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
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 > 90%) → more than a factor 30 gain!
– linearity
Drawbacks:
– small size (a few cm2)
– sensitive to cosmic rays
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:
E
– 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
−
bande de econduction
Egap
h+ valence
bande de
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−
electrode V+
isolating layer
silicon
Detectors - 4
Working of a CCD
channel stops
(p-doped regions)
chargecollection
transfer
charge
(shutter
(shutterclosed)
open)
+
+
+
electrodes
pixel
silicon
output
amplifier
Detectors - 6
CCD sensitivity
Photons can be absorbed only if Eγ > Egap
Nγ ~ α (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
Detectors - 7
Photon absorption in silicon
Photons penetrate deeper as λ increases
Electrodes are opaque in UV
electrode V+
isolating layer
silicon
Detectors - 8
Amélioration de la sensibilité dans le bleu et l’UV
CCD amincis et illuminés par l’arrière : thinned backside
illuminated CCDs
silicium
couche isolante
électrode V+
L’indice de réfraction du Si est élevé → possibilité de réflexions
multiples aux grands λ → possibilité de franges si les surfaces ne sont
pas parfaitement planes
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
Ne
Charge transfer is also
perturbed
→ blooming
~105
0
~105
Nγ
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)
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)
Detectors - 12
Possible causes:
– slight size differences between pixels
– dust on camera lens
– non uniform lighting of the field…
ideal CCD
Observation d’un champ uniforme
actual CCD*
(*a bit exaggerated)
Detectors - 13
Interpixel nonuniformities
May depend on λ:
→ 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
Astronomical images
Instrumental profile
Image of a point source through a circular aperture
= Airy rings
  1.22
Seeing
Ground-based observations
→ atmospheric turbulence
If exposure time long enough
→ image a bit blurred

d
Δθ
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
FWHM
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)
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
→ σ = √Ne
 tot (e )  Ne  RON 2
  tot ( ADU ) 
S sky  Sobj
g
 RON
Sobj
2
Ssky
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)
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 → σ = ∞)
• subtraction of sky background
• computation of an image containing the intensity uncertainties σ