X-shooter DRS project
Paolo Goldoni,
VLT 2nd Generation Instruments Meeting - MPE Munich 18/04/2007
“Call for proposals for 2nd Generation VLT
Instruments” (http://www.eso.org/instruments/vlt2ndgenins.html)
R~ 104 wide-band visible-NIR high-throughput
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
11 Institutes in 5 countries:strong financial contribution
from consortium
Consortium and Schedule
-Instrument Assembly
PI: S. D’Odorico
-Backbone unit
- UVB spectrograph
PI: P. Kjaergaard
NIR spectrograph
Support to DRS (NIR)
PI:L. Kaper
VIS Spectrograph
Observing Software
PI: R. Pallavicini
Data Reduction
Software (APC/GEPI)
PI: F. Hammer
Co PI: P. Binetruy
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
Wavelength range: 300 nm to 2.5 mm
Prism cross-dispersed echelle (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 18mm Hawaii2 RG MBE (NIR
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
X-shooter Functional Scheme
Integral Field Unit
Realised by GEPI-Observatoire de Paris (Girafe):
IFU Image Reconstruction
Calibration across slices
Calibration along slices

Paolo Goldoni