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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