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X-shooter DRS project. Paolo Goldoni, APC-SAp/CEA. VLT 2nd Generation Instruments Meeting - MPE Munich 18/04/2007. “Call for proposals for 2nd Generation VLT Instruments” ( ). R~ 10 4 wide-band visible-NIR high-throughput Spectrometer.

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X-shooter DRS project

Paolo Goldoni, APC-SAp/CEA

VLT 2nd Generation Instruments Meeting - MPE Munich 18/04/2007


“Call for proposals for 2nd Generation VLT

Instruments” (

R~ 104 wide-band visible-NIR high-throughput Spectrometer

The main goal is to get maximum detectivity on stellar or small emission-line objects,

while covering the largest possible wavelength range (ideally 0.32 to 2.4 mm) in a single observation, presumably leading to a multiple arm ("x-shooter") system. A particularly important requirement is the ability to get spectrographic data on unpredictable/fast varying objects like supernova explosions or gamma ray burst optical counterparts, for the latter if possible in a matter of minutes….

Goal of the instrument: Single object observations at the sky limit

consortium and schedule
Consortium and Schedule

ESO approval 12/2003

PDR 13/12/2004

FDR 07/07/2006

PAE 04/2008

PAC 04/2009

X-shooter will be released to the community in April 2009


3 arms, single object spectrograph

1 IFU (1.8" x 4") reformatted into a 12 arcsec slit.

instrument characteristics
Instrument characteristics

Wavelength range: 300 nm to 2.5 mm

Prism cross-dispersedechelle (slit length12”)

Spectral resolution:6,000 to 12,000 for 0.6” slit or IFU

Calibration unit, A&G unit, ADC for UVB and VIS arms

Detectors:2Kx4K 15 mm CCDs(UVB and VIS arms);2Kx1K segment of a 2K x 2K 18mmHawaii2 RGMBE(NIR arm)

Limiting magnitudes (1h, S/N=10, slit width=1”):

U=21.9 B=22.0 V=21.7 R=21.7 I=21.3 Z=20.3

J=20.7 H=20.9 K’=19 K=18.9


Simulated image, VIS arm

Difficulties: Spectral range, Curved orders, tilted lines (e.g. need a new method for

optimal extraction). Moreover Staring mode preferred in UVB, ON/OFF

(or dithering) required in NIR.


X-shooter DRS Project Products

The DRS for X-shooter shall produce a wavelength calibrated, sky subtracted,

flux calibrated, merged, 1D spectrum of the observed target both in the slit and

IFU modes of the instrument for the three separated arms, together with the

corresponding sky and noise spectra.

1D mode: optimal extraction of the sky-subtracted spectrum of the stellar like

targets. The 2D (the second dimension being the coordinated along the

slit) extracted and wavelength calibrated spectra shall also be made available .

IFU Mode: The DRS product shall be the wavelength calibrated spectra in a

reconstructed 3D data cube ( x,y coordinates on the sky and wavelength)

Team: P. Goldoni (APC/CEA), M. Horrobin (UvA), F. Royer (OPM),

G. Blanc (APC), L. Guglielmi (APC), R. Haigron (OPM) + ESO P. Bristow,

A. Modigliani, J.M. Larsen


Project Realisation

Written in C using wherever possible CPL subroutines, on ESO CVS ==> Fast Integration !!

Borrowing from UVES subroutines to speed up the development & standardize pipeline

Monthly progress teleconf with ESO (Instrument Scientist, PI,


Intermediate Releases of the software

Time gain from standardization ==> advanced features


X-shooter DRS Flowchart (staring)






See Goldoni et al. Proc. SPIE 2006


Advanced Features I: Physical Model

Highly precise wavelength

calibration using Physical

Modelling. Projected

accuracy of Wavelength

Calibration : <0.2 pix.

Same method is being

used in CRIRES DRS

P. Ballester, M. Rosa: Modeling

Echelle spectrographs (A&AS

126, 563, 1997)

Bristow, Kerber, Rosa: four

papers in HST Calibration

Workshop, 2006



Advanced Features II: Single Frame Sky Subtraction

Because of the tilt, each pixel samples a slightly different part of the wavelength space, on the left a single row of data, on the right a sampling of all data rows.

Supersampling used in SDSS pipeline Kelson (2003) PASP 115, 688



Advanced Features III:

Single Frame Cosmic Rays Subtraction

Used in HST/ACS images and in SDSS pipelines van Dokkum (2001) PASP 113, 1420


Advanced Features IV: Optimal Extraction

Optimal Extraction using the method of Marsh (1989) PASP 101, 1032

(similar method in EspaDons/CFHT)


Advanced Features V:

Improved Spectrophotometric Calibration

Situation:current methods for spectro-photometric calibration in the NIR no

more precise than 20-30% because a proper set of NIR spectro-photometric

standards simply does not exist at the moment.

Extend the wavelength coverage of the well established UV/Optical

spectro-photometric standards (Oke 1990, Hamuy et al. 1992,1994)

into the near-IR with SINFONI observations.

•Rely on the 2 HST Primary Standards(WD):

–robust reference: flux measured outside the atmosphere between 115 and

1800 nm (accuracy better than 1%, Bohlin 2007)

–Interpolate between flux measurement windows using state-of-the-art stellar

atmosphere models (TMAP) and derive and absolute flux table for each Secondary

Standard across the whole wavelength range

Proposal being carried out, first data being analyzed.

See Vernet et al. Proc. Cal2007


Integral Field Unit

Realised by GEPI-Observatoire de Paris (Girafe):


IFU Image Reconstruction

Calibration across slices

Calibration along slices