Cours M1
Signal et Bruit en Astronomie
Rodrigo Ibata
demandes de temps télescope...Date: September 21, 2006Category: Structure and Dynamics of Galaxies
Proposal: F1738CFHTOBSERVING TIME REQUEST
Semester: 2006A Agency: France
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 32Phone: +33 3 90 24 23 91E-mail: [email protected]
3. Co-Investigators:Scott Chapman Institute: Caltech
E-mail: [email protected]
Annette Ferguson Institute: Royal Observatory Edinburgh E-mail: [email protected]
Michael Irwin Institute: Institute of Astronomy, Cmabridge E-mail: [email protected]
Geraint Lewis Institute: University of SydneyE-mail: [email protected]
Nicolas Martin Institute: Observatoire de StrasbourgE-mail: [email protected]
Mustapha Mouchine Institute: John Moore’s Univeristy, Liverpool E-mail: [email protected]
Nial TanvirInstitute: University of Hartfordshre
E-mail: [email protected]. 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, low-surface
brightness, extended disk-like structure, a previously unknown component of galaxies. This would have
profound implications for our understanding of galaxy formation. However, if this structure is not present, we
will be able to interpret the Milky Way and M31 detections as being due to their peculiar accretion history;
the requested MegaCam data will then serve as an excellent probe of the halo component of M81, improving
significantly on extant studies of the large-scale structure of the halos of giant spirals.
5. Summary of the Observing Run Requested:Instrument Detector Moon (d)
Filters
Grisms
MegaPrime N/A 6
g, i
Time Req. Service/Queue?Queue Mode
Image QualityOpt. LST
Min. LSTMax. LST
8 hours Queue Regular 0.65” < IQ < 0.80” 10:00 05:00 15:00
6a. Is this a joint proposal? NO 6b. If yes, total number of nights or hours requested from all agencies? —
7a. Is this a Thesis Project? NO 7b. If yes, indicate supervisor: —
8. Special instrument or telescope requirements:9. Scheduling constraints:
demandes de temps télescope...
9. Justificationof 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 (8484–9001A, R=16200),
31 minutes are necessaryto 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 observatio
ns. 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.
Calibration Request:
Standard Calibration
10.Report on the use of ESO facilities during the last 2 years
No observing time was previously allocated.
11.Applicant’s publications related
to the subject of this applicationduring the last 2 years
Martin N., Ibata R., Bellazzini M., Irwin M., Lewis G., Dehnen W., 2004a, MNRAS 398, 12: A dwarf galaxy
remnant in Canis Major: the fossil of an in-plane accretion onto the Milky Way.
Martin N., Ibata R., Conn B., Lewis G., Bellazzini M., Irwin M., McConnachie A., 2004b, MNRAS 355, L33:
Why the Canis Major overdensity is not due to the Warp: analysis of its radial profile and velocities.
Martin N., Ibata R., Conn B., Irwin M., Lewis G., 2005a, PASA, submitted: Correcting the influence of an
asymmetric line spread function in the 2-degree Field spectrograph.
Martin N., Ibata R., Conn B., Lewis G., Bellazzini M., Irwin M., 2005b, MNRAS, submitted: A radial velocity
survey of low Galactic latitude structures: I. Kinematics of the Canis Major dwarf galaxy.
Lewis G., Ibata R., Irwin M., Martin N., Bellazzini M., Conn B., 2004, PASA 21, 371: The Canis Major dwarf
galaxy.
Bellazzini M., Ibata R., Monaco L., Martin N., Irwin M., Lewis G, 2004, MNRAS 354, 1263: The Moon behind
the finger: detectionof the Canis Major galaxy in the background of Galactic open clusters.
Bellazzini M., Ibata R., Martin N., Lewis G, Conn B., Irwin M., 2005, MNRAS submitted: The core of the
Canis Major structure as traced by Red Clump stars.
- 6 -
Presque tous les spectrographes...
(spectre “echelle” du Soleil)plus rouge
plus bleu
et les cameras...
Filtre
...utilisent les cameras CCD...
Les cameras CCD
Très efficaces!!!
Les cameras CCD
Puce silicium
Propriétés des CCDs• CCDs optiques faites sur puce de Silicium
• ΔE = 1.2 eV entre bandes de valence et conduction
• h ν / λ > ΔE ⇒ λ < 1 μm
• CCDs “minces” et “épaisses”
• Grand avantage: détecteurs à réponse linéaire et très sensibles
• Problèmes:– bruit de lecture / temps de lecture
– difficile de fabriquer des grands détecteurs– pixels “chauds”
– “fringing”– “traps” (pièges)– “dead pixels” et “dead columns”
– saturation (typiquement à valeurs de 216=65536)– “video pattern noise”
– “charge transfer efficiency”
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 CCDraw = (obj + sky x (1 + fringe)) x QE + dark + flash + bias
Correction for Zero exposure Additive Systematics:
<dark> = dark + flash + bias
“Overscan” region to remove DC bias variations:
raw - <dark> - (overscn - <overscn>) = obj +sky x (1+fringe)) x QE
Correction for Multiplicative Spatial Systematics (flat-fielding):
<flat> = QE + dark + flash + bias
const x (raw - <dark>) / (<flat> - <dark>) = obj + sky x (1 + fringe)
Correction for Additive Spatial Systematics:
<sky> = 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 / m2
• Fλ(mag=0) = 5.48 x 106 /(λ[Å]) ph/s/Å/cm2
• Fλ ≈ 1000 x 10-0.4 V ph/s/Å/cm2 (en V)
Brillance du ciel (mag arcsec2)
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
PleineLune
18.8 18.5 18.9 18.2 13.7 13.7 12.5
Les CCDs comptent des photons
⇒ Distribution de Poisson• f(k, λ) = e-λ λk / k!
• moyenne = λ• variance = λ
k
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?