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: 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 SydneyE-mail: gfl@physics.usyd.edu.au

Nicolas Martin Institute: Observatoire de StrasbourgE-mail: martin@astro.u-strasbg.fr

Mustapha Mouchine Institute: John Moore’s Univeristy, Liverpool E-mail: mm@astro.livjm.ac.uk

Nial TanvirInstitute: University of Hartfordshre

E-mail: nrt@ast.cam.ac.uk4. 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?