Cours M1 Signal et Bruit en Astronomie Rodrigo Ibata - Observatoire

Cours M1
Signal et Bruit en Astronomie
Rodrigo Ibata

Date: September 21, 2006Category: Structure and Dynamics of Galaxies Proposal: F1738
CFHT
OBSERVING TIME REQUEST
Semester: 2006A Agency: France
demandes de temps télescope...
1. Title of the Program (may be made publicly available for accepted proposals):
The extended disks of galaxies: a new galactic component?
2. Principal Investigator: Rodrigo Ibata
Postal address: Observatoire de Strasbourg, 11, rue de l’Universite, F-67000 Strasbourg, France
Fax: +33 3 90 24 24 32 Phone: +33 3 90 24 23 91E-mail: ibata@astro.u-strasbg.fr
3. Co-Investigators:
Scott Chapman Institute: Caltech E-mail: schapman@astro.caltech.edu
Annette Ferguson Institute: Royal Observatory Edinburgh E-mail: ferguson@roe.ac.uk
Michael Irwin Institute: Institute of Astronomy, Cmabridge E-mail: mike@ast.cam.ac.uk
Geraint Lewis Institute: University of Sydney E-mail: gfl@physics.usyd.edu.au
Nicolas Martin Institute: Observatoire de Strasbourg E-mail: martin@astro.u-strasbg.fr
Mustapha Mouchine Institute: John Moore’s Univeristy, Liverpool E-mail: mm@astro.livjm.ac.uk
Nial Tanvir Institute: University of Hartfordshre E-mail: nrt@ast.cam.ac.uk
4. Summary of the Program (may be made publicly available for accepted proposals):
We propose to use MegaCam to observe M81, the nearest giant spiral galaxy beyond the Local Group, reaching
∼ 2 magnitudes below the red giant branch (RGB) tip to probe the stellar populations beyond the end of the
thin disk. If this galaxy is similar to the Milky Way and M31, we will uncover an inhomogenous,
brightness, extended disk-like structure, a previously unknown componen
profound implications for our understanding of galaxy f
will be able to interpret the Milky
the requested

demandes de temps télescope...
9. Justification of requested observing time and lunar phase
Lunar Phase Justification: Grey time or bright time is adequate to observe these relatively bright sources.
Time Justification: (including seeing overhead) Using the Version 2.9.4 ETC for FLAMES-GIRAFFE, when
observing the CaII triplet of a template K7V star at I ∼ 16.5 with the HR21 setting (8484–9001˚A, R=16200),
31 minutes are necessary to reach S/N= 20 in normal observing conditions (seeing 0. ′′ 8; airmass 1.3, 10 days from
new moon). This is sufficient to measure velocities to an accuracy of 2 km/s and determine the intrinsic velocity
dispersion of the CMa population as well as derive metallicities from the Ca triplet observations. Including the
overheads, this means that the single MEDUSA configuration requested for eight fields correspond to 0.7 h of
exposure for each field, including the overhead. The three last fields we wish to observe will require a second
MEDUSA configuration, and will therefore need 1.5 h of exposure time.
Thus, the total observation time adds up to 10.0 hours.
alibration

Presque tous les spectrographes...

plus bleu
(spectre “echelle” du Soleil)
plus rouge

Filtre
et les cameras...

...utilisent les cameras CCD...

Très efficaces!!!
Les cameras CCD

Les cameras CCD
Puce silicium

Calibration des images CCD
bias
flat

Défauts typiques
Lignes mortes

Défauts des CCDs
Poussière

Rayons cosmiques

Rayons cosmiques

Défauts typiques

Saturation et diffraction

“Fringing”

PSF (point spread function)
• PSF idéal est un disque d’Airy:
• Les obstructions dans l’instrument et
l’optique rendent la PSF plus complexe
• résolution angulaire:

“Seeing” aux télescopes ESO

Réduction des données CCD
raw = (obj + sky x (1 + fringe)) x QE + dark + flash + bias
Correction for Zero exposure Additive Systematics:
= dark + flash + bias
“Overscan” region to remove DC bias variations:
raw - - (overscn - ) = obj +sky x (1+fringe)) x QE
Correction for Multiplicative Spatial Systematics (flat-fielding):
= QE + dark + flash + bias
const x (raw - ) / ( - ) = obj + sky x (1 + fringe)
Correction for Additive Spatial Systematics:
= sky x (1+fringe)

Définition du système de magnitudes AB:
• Fν(mag=0) = 3631 Jy (pour toutes les ν)
• = 3631 x 10 -26 W / Hz / m 2
• Fλ(mag=0) = 5.48 x 10 6 /(λ[Å]) ph/s/Å/cm 2
• Fλ ≈ 1000 x 10 -0.4 V ph/s/Å/cm 2 (en V)

Brillance du ciel (mag arcsec 2 )
U B V R I J H K
Ciel obscur 22.8 22.5 21.5 20.8 19.3
6 nuits 21.3 20.8 20.4 19.2
Pleine
Lune
18.8 18.5 18.9 18.2 13.7 13.7 12.5

Calcul du Signal sur Bruit
• Autres:
– Bruit de lecture =
– Bruit de “flat-fielding” = facteur x Nsig
– Bruit de “fringing” = facteur x Nciel

Exemples:
• Quelle est la magnitude limite de l’oeil
humain? Ou de ma camera numérique?
• Combien de temps faut-il
– pour attendre la magnitude V=25 en
photométrie avec le VLT?
– pour mesurer les spectres des étoiles rouges
de magnitude I=21 dans la galaxie M31, avec
le télescope Keck, avec une résolution de
R=10000?