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ESO X-shooter User Manual

European Organisation for Astronomical Research in the Southern Hemisphere
Organisation Européenne pour des Recherches Astronomiques dans l’Hémisphère Austral
Europäische Organisation für astronomische Forschung in der südlichen Hemisphäre
VERY LARGE TELESCOPE
X-shooter
User Manual
Doc. No.: VLT-MAN-ESO-14650-4942
Issue: P94
Date: 30.06.2014
Prepared: Christophe Martayan, originally written by Joël Vernet & Elena Mason
Name Date Signature
Approved: Andreas Kaufer, originally approved by Sandro D’Odorico
Name Date Signature
Released: Christophe Dumas
Name Date Signature
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ISSUE
DATE
SECTION/PARA.
AFFECTED
REASON/INITIATION
DOCUMENTS/REMARKS
0.1
13.01.06
All
FDR version: Table of Content prepared by Céline Péroux
0.2
14.08.08
All
PAE version prepared by Joël Vernet
1
01.03.09
All
First release prepared by Joël Vernet, with contributions by Elena Mason
2
01.07.09
All
Prepared by Joël Vernet and Elena Mason.
- Added description of IFU centring and tracking wavelength
- Updated all TSF in Sec 5.
- Added spectrograph orientation figure.
- Added description of Threshold Limited Integration in the NIR
- Added information about ghost spectra
- Added information about slit/ifu position information in acq image header.
- Updated limiting mags with measured NIR sensitivity and background between OH lines in VIS
- Updated UVB/VIS/NIR detector parameters
- Added warning about 2x2 binning mode and inter-order bck subtraction
2.1
15.01.2010
Section 5
- Templates name changed from SHOOT to XSHOOTER; default parameters and hidden parameters.
Sections 2.4.3 and 3.3.1
-... plus sparse minor corrections.
86.1
09.02.2010
None
cmmModule creation
CHANGE RECORD
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87
25.08.2010
26.08.2010
All
CMa, sections 2.4.7 and
2.4.8, 2.4.9, 4.4, 4.7, 4.8 added. Sections 2.2.1, 3.2,
4.5.2, 4.6, 5.1.1, 5.1.2 modified. Figure added in
5.1.1, Table 11 updated, old Table 3 removed. + modified structure of the sections
88
27.02.2011
01.03.2011
03.03.2011
07.03.2011
28.03.2011
20.06.2011
Modified sects. 1.3, 2.2.1.3,
2.2.4.2, 2.3.2, 2.4, 2.4.3,
2.4.4, 2.4.6.1, 2.4.7, 3, 3.2,
3.3.1, 3.3.2, 3.3.3, 3.3.4,
4.1.1, 5, 5.1, 5.6.1, 5.6.2,
5.7, 5.9, 6.1.3, 6.2.3 New Sects 2.4.10, 2.4.11,
2.4.12, 2.4.13, 2.4.13,
2.4.14, 3.4, 3.4.1, 3.4.2,
3.4.3, 3.4.4, 3.4.5, 3.4.6 Modified tables: 2, 7, 11, 12, 66 New tables: 10, 13 Modified figures: 10, 15 New figures: 5, 11 New subsections 2.4.6, 6.1.3
CMa, update wrt the performances, new identified problems and status + description of the current ones. New items in the FAQ, new calibration plan, new section about the observation strategy. Figures updated to be more clear and useful. NIR 1.5” slit removed. Intervention of July 2011 briefly described +additional corrections of figures and sections according to IOT comments. Very minor changes. New templates added + minor corrections
89
04.08.2011
30.11.2011
Modified Sections: 2.1,
2.2.1.4, 2.2.4.2, 2.2.4.5,
2.3.2, 2.4.6, 2.4.9, 2.4.13,
2.4.15, 3.4.1, 5.1, 5.5, 5.7,
6.1.5, 6.2.3. New sections: 2.2.4.3, 2.4.7 Modified tables: 1, 9, 12, 13, 16, 72 New tables: 3, 4, 10
CMa, major modifications wrt the new slits in the NIR + new slits with K-band blocking filter added and background performances + the new TCCD performances + the new calibration plan + correction of typos and clarification of different points (attached calibrations, known problems, etc), weblinks modified.
Modifications regarding phase2 + changes for the acquisition+setup+readout+ wiping overheads. + additional information regarding integration times for the TCCD.
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90
20.02.2012
03.04.2012
Modified sections: 2.2.1.4,
2.2.4.5, 2.4.3, 3.4.3, 4.1.2,
5.1, 5.4, Table 16 revised Clarification of 2.2.4.3 (new
NIR slits) New 6.1.2 for better explanation of slit orientation and offsets.
DIT of 1800s with JH slits, TCCD limiting magnitudes + direct acquisition. Telluric std star observations,
How to minimize the overheads and optimize the integration times. Calibration plan revised.
Phase 2: minor modifications, re-writing sentences + new draws+ contacts added at the beginning (already present in other pages) Other minor adjustments of the tables and links.
90/91
08.08.2012
No ADCs mode: sect. 2.2.2, updates of sects. 2.4.2,
24.13-1.4.15, 3.1, 3.4.3, 5.7,
5.9
Adding a new section about the observations without ADCs (2.2.2). Updates of sections for the observations in slit with disabled ADCs + more infos for the IFU. Updates wrt the telluric std star policy starting in P91.
91
09.10.2012
Transmission curve of the K­band blocking filter added. Telluric std star policy updated for P91.
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91/92
10.02.2013
Section 3.2 split in 2: 3.2.1
3.2.2 New section 3.3 New section 1.6
Sects 3.2.1/3.2.2: main acq loop and 3.2.2 blind offset precision
--­New section 3.3 about examples of OBs preparation with p2pp3 especially regarding the acqs (direct or blind offsets)
--­new section 1.6 regarding the acknowledgements
--­warning about the snapshots during the acquisitions offsets that will not be saved anymore, only last snapshots end of acquisition kept.
--­warning about the exposure times of all calibration frames that will be revised.
--­warning about the wavelength calibration at night that should be performed with 2dmap template instead of ARC.
P92
Change of format .doc to .odt, allowed 2dmap wave calibrations at night, Move of XSHOOTER from UT2 to UT3
minor changes in various sections
P93
Back to format .doc Introduction of the
XSHOOTER imaging mode (new sect 4), comments in various sections
Minor changes every where
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P94
26.02.2014
Minor changes, references to the imaging mode user manual added. New table about the limiting magnitude for a S/N=10 in sec 2.2.1.4. Some details provided for the dichroic dip oscillation, corrected cross-references.
30.06.2014
All
CMA: Merging imaging mode manual with main manual as per ESO standard.
Correction of some language issues, obsolete sections removed or reorganized. Radial velocity accuracy added, telluric lines correction tool reference added, updates of references and features
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TABLE OF CONTENTS
1. Introduction ...................................................................................................................11
1.1 Scope ...................................................................................................................12
1.2 X-shooter in a nutshell ..........................................................................................12
1.3 Shortcuts to most relevant facts for proposal preparation ......................................12
1.4 List of Abbreviations & Acronyms ..........................................................................13
1.5 Reference Documents ..........................................................................................14
1.6 Acknowledgements ...............................................................................................15
1.7 Contact .................................................................................................................15
1.8 News .....................................................................................................................16
2. Technical description of the instrument .........................................................................17
2.1 Overview of the opto-mechanical design ...............................................................18
2.2 Description of the instrument sub-systems ............................................................18
2.2.1 The Backbone ...............................................................................................19
2.2.1.1 The Instrument Shutter and The calibration unit ........................................19
2.2.1.2 The Acquisition and Guiding slide ..............................................................20
2.2.1.3 The IFU .....................................................................................................21
2.2.1.4 The Acquisition and Guiding Camera ........................................................23
2.2.1.5 The dichroic box ........................................................................................24
2.2.1.6 The flexure compensation tip-tilt mirrors ....................................................24
2.2.1.7 The Focal Reducer and Atmospheric Dispersion Correctors .....................25
2.2.2 ADCs problems and disabled ADCs observing mode in SLIT and IFU ..........26
2.2.3 Detector QE curves .......................................................................................34
2.2.4 The UVB spectrograph ..................................................................................34
2.2.4.1 Slit carriage ...................................................................................................34
2.2.4.2 Optical layout ................................................................................................35
2.2.4.3 Detector ........................................................................................................36
2.2.5 The VIS spectrograph ...................................................................................38
2.2.5.1 Slit carriage ...................................................................................................38
2.2.5.2 Optical layout ................................................................................................38
2.2.5.3 Detector ........................................................................................................38
2.2.6 The NIR spectrograph ...................................................................................39
2.2.6.1 Pre-slit optics and entrance window ..............................................................39
2.2.6.2 Slit wheels .....................................................................................................39
2.2.6.3 NIR Backgrounds ..........................................................................................43
2.2.6.4 Optical layout ................................................................................................46
2.2.6.5 Detector ........................................................................................................47
2.3 Spectral format, resolution and overall performances ...........................................50
2.3.1 Spectral format ..............................................................................................50
2.3.2 Spectral resolution and sampling ...................................................................51
2.3.3 Overall sensitivity ..........................................................................................52
2.4 Instrument features and known problems to be aware of ......................................54
2.4.1 UVB and VIS detectors sequential readout ...................................................54
2.4.2 Effects of atmospheric dispersion ..................................................................54
2.4.3 Remanence ...................................................................................................54
2.4.4 Ghosts ...........................................................................................................55
2.4.5 Inter-order background ..................................................................................56
2.4.6 NIR frames with the K-band blocking filter features .......................................56
2.4.7 Instrument stability ........................................................................................58
2.4.7.1 Backbone flexures ................................ .........................................................58
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2.4.7.2 Spectrograph flexures ...................................................................................58
2.4.8 Radial velocity accuracy ................................................................................58
2.4.9 NIR 11th order vignetting (K band) ................................................................ .58
2.4.10 VIS CCD pick-up noise ..................................................................................59
2.4.11 NIR –IFU parasitic reflections ........................................................................59
2.4.12 UVB/VIS ADCs problem ................................................................................60
2.4.13 TCCD features ..............................................................................................60
3. Observing with X-shooter .............................................................................................61
3.1 Observing modes and basic choices .....................................................................61
3.2 Target acquisition ..................................................................................................62
3.2.1 Acquisition loop ................................................................ .............................62
3.2.2 Blind offset precisions ...................................................................................63
3.3 Examples of OBs preparations/acquisitions with p2pp3 ........................................64
3.3.1 Direct acquisition ...........................................................................................64
3.3.2 Blind offset acquisition ...................................................................................70
3.4 Spectroscopic observations ..................................................................................72
3.4.1 Overview and important remarks ...................................................................72
3.4.1.1 Observing modes ..........................................................................................72
3.4.1.2 Effect of atmospheric dispersion....................................................................72
3.4.1.3 Exposure time in the NIR arm .......................................................................72
3.4.2 Staring (SLIT and IFU) ..................................................................................73
3.4.3 Staring synchronized (SLIT and IFU) ............................................................73
3.4.4 Nodding along the slit (SLIT only) ..................................................................74
3.4.5 Fixed offset to sky (SLIT and IFU) .................................................................75
3.4.6 Generic offset (SLIT and IFU) .......................................................................75
3.5 Observation strategy, summary, and tricks ...........................................................76
3.5.1 Instrument setup ...........................................................................................76
3.5.2 Observation strategy .....................................................................................77
3.5.3 Telluric standard stars and telluric lines correction (see also Sect.6.6.1) .......79
3.5.4 Observing bright objects, limiting magnitudes, and the diaphragm mode ......79
3.5.5 Readout times in the UVB and VIS arms: minimization of overheads ............80
4. The XSHOOTER imaging mode ...................................................................................81
5. Instrument and telescope overheads ............................................................................96
5.1.1 Summary of telescope and instrument overheads .........................................96
5.1.2 Execution time computation and how to minimize the overheads ..................97
6. Calibrating and reducing X-shooter data .......................................................................99
6.1 X-shooter calibration plan .....................................................................................99
6.2 Wavelength and spatial scale calibration............................................................. 102
6.3 Flat-field and Wavelength calibrations ................................................................. 103
6.4 Spectroscopic skyflats......................................................................................... 104
6.5 Attached calibrations ........................................................................................... 105
6.6 Spectrophotometric calibration ............................................................................ 105
6.6.1 Telluric absorption correction ...................................................................... 105
6.6.2 Absolute flux calibration .............................................................................. 107
6.7 The X-shooter pipeline ........................................................................................ 108
6.8 Examples of observations with X-shooter............................................................ 109
6.9 Frequently Asked Questions ............................................................................... 109
7. Reference material ..................................................................................................... 111
7.1 Templates reference ........................................................................................... 111
7.1.1 Orientation and conventions ........................................................................ 111
7.1.2 Examples of position angles and offsets ...................................................... 113
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7.1.3 Acquisition templates .................................................................................. 115
Slit acquisition templates ........................................................................................ 115
IFU acquisition templates ........................................................................................ 117
7.1.4 Flexure compensation templates that can be used in OBs .......................... 119
7.1.5 Science templates ....................................................................................... 119
Slit observations ..................................................................................................... 119
IFU observations ..................................................................................................... 124
7.1.6 Night-time Calibration Templates ................................................................ 127
Spectro-photometric Standard Stars ....................................................................... 127
Telluric standards ................................................................................................... 132
Attached night calibrations: must be taken after a science template ....................... 135
Arcs multi-pinhole: 2d wave maps (wavelength calibration) .................................... 138
7.1.7 Daytime Calibration templates ..................................................................... 140
Slit and IFU arc lamp calibrations (resolution, tilt) ................................................... 140
Flatfield (pixel response, orders localization) .......................................................... 141
Format check (1st guess of wavelength solution) ..................................................... 144
Order definition (1st guess of order localization) ...................................................... 144
Arcs multi-pinhole: 2d wave maps (wavelength calibration) .................................... 145
Detector calibrations ............................................................................................... 146
7.1.8 Imaging mode templates manual ................................................................. 149
7.2 Slit masks ........................................................................................................... 155
7.2.1 UVB ............................................................................................................ 155
7.2.2 VIS .............................................................................................................. 155
7.2.3 NIR .............................................................................................................. 156
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Figure 1: 3D CAD view of the X-shooter spectrograph at the Cassegrain focus of one of the VLT Unit Telescopes.
Table 1: X-shooter characteristics and observing capabilities
Wavelength range
300-2500 nm split in 3 arms
UV-blue arm
Range: 300-550 nm in 12 orders Resolution: 5100 (1" slit) Slit width: 0.5”, 0.8”, 1.0”, 1.3”, 1.6”, 5.0” Detector: 4k x 2k E2V CCD
Visual-red arm
Range: 550-1000 nm in 14 orders Resolution: 8800 (0.9" slit) Slit width: 0.4”, 0.7”, 0.9”, 1.2”, 1.5”, 5.0” Detector: 4k x 2k MIT/LL CCD
Near-IR arm
Range: 1000-2500 nm in 16 orders Resolution: 5100 (0.9" slit)
Slit width: 0.4”, 0.6”, 0.9”, 1.2”, 1”, 5.0”,
0.6”JH, 0.9”JH
Detector: 2k x 1k Hawaii 2RG
Slit length
11” (SLIT) or 12.6” (IFU)
Beam separation
Two high efficiency dichroics
Atmospheric dispersion compensation
In the UV-Blue and Visual-red arms Disabled on Aug. 1st ,2012
Integral field unit Acquisition and guiding camera
1.8" x 4" reformatted into 0.6" x 12"
1.5’x1.5’ +Johnson and SDSS filters
1. Introduction
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Table 2: collaborating institutes and their contributions
Collaborating institutes
Contribution
Copenhagen University Observatory
Backbone unit, UVB spectrograph, Mechanical design and FEA, Control electronics
ESO
Project Management and Systems Engineering, Detectors, final system integration, commissioning, logistics, Data Reduction Software
Paris-Meudon Observatory, Paris VII University
Integral Field Unit, Data Reduction Software
INAF - Observatories of Brera, Catania, Trieste and Palermo
UVB and VIS spectrograph, Instrument Control Software, optomechanical design.
Astron, Universities of Amsterdam and Nijmegen
NIR spectrograph, contribution to Data Reduction Software
1.1 Scope
The X-shooter User Manual provides extensive information on the technical characteristics of the instrument, its performances, observing and calibration procedures and data reduction.
1.2 X-shooter in a nutshell
X-shooter is a single target spectrograph for the Cassegrain focus of one of the VLT UTs covering in a single exposure the spectral range from the UV to the K band. The spectral format is fixed. The instrument is designed to maximize the sensitivity in the spectral range through the splitting in three arms with optimized optics, coatings, dispersive elements and detectors. It operates at intermediate resolutions (R=4000-18000, depending on wavelength and slit width) sufficient to address quantitatively a vast number of astrophysical applications while working in a background-limited S/N regime in the regions of the spectrum free from strong atmospheric emission and absorption lines. A 3D CAD view of the instrument attached to the telescope is shown on Figure 1. Main instrument characteristics are summarized in Table 1. A Consortium involving institutes from Denmark, Italy, The Netherlands, France and ESO built x-shooter. Name of the institutes and their respective contributions are given in Table 2.
1.3 Shortcuts to most relevant facts for proposal preparation
The fixed spectral format of X-shooter: see Table 11 on page 49
Spectral resolution as a function of slit width: see Table 12 on page 51  Information on the IFU: see Section 2.2.1.3  Information on limiting magnitudes in the continuum: see Section 2.3.3 on page 52  Information on observing modes: see section 3.1 on page 61  Observing strategy and sky subtraction: see Section 3.3 on page 64  Overhead computation: see Section 4 on page 81
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A&G/AG
Acquisition and Guiding
ADC
Atmospheric Dispersion Compensator
AFC
Active Flexure Compensation
DCS
Detector Control Software
DEC DFS
Declination Data Flow System
DIT
Detector Integration Time
ESO
European Southern Observatory
ETC FDR
Exposure Time Calculator Final Design Review
FF
Flat Field
GUI
Graphical User Interface
ICS
Instrument Control Software
IFU
Integral Field Unit
ISF
Instrument Summary File
IWS
Instrument Workstation
LCU
Local Control Unit
N/A
Not Applicable
OB PAE
Observing Block Preliminary Acceptance Europe
P2PP
Phase 2 Proposal Preparation
RA RMS RON SM TBC
Right Ascension Root Mean Square Readout Noise Service Mode To Be Clarified
TCCD QE
Technical CCD Quantum Efficiency
SNR
Signal to Noise Ratio
TBD
To Be Defined
TCS
Telescope Control Software
TLI
Threshold Limited Integration
TSF
Template Signature File
VLT VM
Very Large Telescope Visitor Mode
WCS ZP
World Coordinate System Zeropoint
1.4 List of Abbreviations & Acronyms
This document employs several abbreviations and acronyms to refer concisely to an item, after it has been introduced. The following list is aimed to help the reader in recalling the extended meaning of each short expression:
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1.5 Reference Documents
1. X-shooter Calibration plan, v1.0, XSH-PLA-ESO-12000-0088
2. X-shooter Templates Reference Manual, v0.2, XSH-MAN-ITA-8000-0031
3. X-shooter technical note about the 11th order vignetting in K band
4. X-shooter A&A article: Vernet et al. 2011A&A...536A.105V
5. Report about the non destructive NIR readout mode
http://www.eso.org/sci/facilities/paranal/instruments/xshooter/doc/reportNDreadoutpublic.pdf
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1.6 Acknowledgements
Please if you use XSHOOTER data, cite the following articles:
1. main article: Vernet et al., 2011A&A...536A.105V
X-shooter, the new wide band intermediate resolution spectrograph at the ESO Very Large Telescope
2. For the flux calibrations:
Vernet et al., 2010HiA....15..535V
Building-up a database of spectro-photometric standards from the UV to the NIR
Hamuy et al., 1994PASP..106..566H
Southern spectrophotometric standards, 2
3. For the pipeline and data reduction: Modigliani et al., 2010SPIE.7737E..56M
The X-shooter pipeline
4. For the Reflex interface: Freudling et al., 2013A&A...559A..96F
Automated data reduction workflows for astronomy. The ESO Reflex environment
5. For the imaging mode: Martayan et al., The Messenger, 156, June 2014
The X-shooter Imaging Mode
1.7 Contact
In case of instrument related questions, use xshooter@eso.org In case of phase1/2 related questions, use usd_xshooter@eso.org or usd-help@eso.org
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1.8 News
-In P94, XSHOOTER will probably go back to UT2. The imaging mode will again be available.
- In P93 as in P92, XSHOOTER will be available at UT3 instead of UT2. This would allow decreasing a bit the pressure factor on this instrument.
- In P93 is introduced the light imaging mode of XSHOOTER performed with the acquisition and guiding camera. At the same time only a single snapshot is taken of the last image during the acquisition loop (2 in case of blind offset before and after the blind offset). More details will come in a dedicated document.
- Note: in P92 some tests were started of a new mode that allows observing very bright
objects (even negative magnitudes). Once the tests completed this mode could eventually be offered to the community (manpower and time dependent).
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Figure 2: Schematic overview of X-shooter
2. Technical description of the instrument
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2.1 Overview of the opto-mechanical design
Figure 2 shows a schematic view of the layout of the instrument. It consists of four main components:
The backbone which is directly mounted on the Cassegrain derotator of the
telescope. It contains all pre-slit optics: the calibration unit, a slide with the 3­positions mirror and the IFU, the acquisition and guiding camera, the dichroic box which splits the light between the three arms, one piezo tip-tilt mirror for each arm to allow active compensation of backbone flexures, atmospheric dispersion compensators (ADCs) in the UVB and VIS arms and a warm optical box in the NIR arm.
The three arms are fixed format cross-dispersed échelle spectrographs that operate
in parallel. Each one has its own slit selection device.
o The UV-Blue spectrograph covers the 300 – 550 nm wavelength range with a
resolving power of 5100 (for a 1” slit)
o The Visible spectrograph covers the range 550 - 1000 nm with a resolving
power of 7500 (0.9” slit).
o The near-IR spectrograph: this arm covers the range 1000 - 2500 nm with a
resolving power of 5300 (0.9” slit). It is fully cryogenic.
2.2 Description of the instrument sub-systems
This section describes the different sub-systems of X-shooter in the order they are encountered along the optical path going from the telescope to the detectors (see
Figure 2). The functionalities of the different sub-units are explained and reference is made to their measured performance.
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Figure 3: 3D view of a cut through the backbone.
2.2.1 The Backbone
2.2.1.1 The Instrument Shutter and The calibration unit
In the converging beam coming from the telescope, the first element is the telescope entrance shutter. Then follows the Calibration Unit that allows to select a choice of flat-fielding and wavelength calibration lamps. This unit consists of a mechanical structure with calibration lamps, an integrating sphere, relay optics that simulate the f/13.6 telescope beam, and a mirror slide with 3 positions that can be inserted in the telescope beam:
one free position for a direct feed from the telescope, one mirror which reflects the light from the integrating sphere equipped with:
o Wavelength calibration Ar, Hg, Ne and Xe Penray lamps operating
simultaneously
o three flat-field halogen lamps equipped with different balancing filters to
optimize the spectral energy distribution for each arm
one mirror which reflects light from:
o a wavelength calibration hollow cathode Th-Ar lamp o a D2 lamp for flat-fielding the bluest part of the UV-Blue spectral range
A more detailed description of the functionalities of the calibration system is given in Sect. 6.
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2.2.1.2 The Acquisition and Guiding slide
Light coming either directly from the telescope or from the Calibration Unit described above reaches first the A&G slide. This structure allows putting into the beam either:
a flat 45˚ mirror with 3 positions mirror:
o acquisition and imaging: send the full 1.5’1.5’ field of view to the A&G
camera. This is the position used during all acquisition sequences;
o spectroscopic observations and monitoring: a slot lets the central 10”15” of
the field go through to the spectrographs while reflecting the peripheral field to the A&G camera. This is the position used for all science observations.
o artificial star: a 0.5” pinhole used for optical alignment and engineering
purposes;
the IFU (described in Sect. 2.2.1.3); a 50/50 pellicle beam splitter at 45˚ which is to used look down into the instrument
with the A&G camera and is exclusively used for engineering purposes.
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Figure 4: Top: view of the effect of the IFU. The central field is directly transmitted to form the central slitlet (green) while the each lateral field (in blue and red) are reflected toward a pair of spherical mirrors and realigned at the end of the central slice to form the exit slit. Bottom: The field before (left) and after the IFU (right). The IFU acts such that the lateral fields seem to rotate around a corner of their small edge. The two white slots are not real gaps but just guides to help visualize the top and the bottom of each slice in the drawing.
2.2.1.3 The IFU
The Integral Field Unit is an image slicer that re-images an input field of 4”x1.8” into a pseudo slit of 12”x0.6”. The light from the central slice is directly transmitted to the
spectrographs. The two lateral sliced fields are reflected toward the two pairs of spherical mirrors and re-aligned at both ends of the central slice in order to form the exit slit as illustrated in Figure 4. Due to these four reflections the throughput of the two lateral fields is reduced with respect to the directly transmitted central one. The measured overall efficiency of the two lateral slitlets is ~85% of the direct transmission but drops to ~50% below 400 nm due to reduced coating efficiency in the blue. An example of an IFU standard star is showed in Figure 5.
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UVB arm
VIS arm
NIR arm
Blue orders
Red orders
Red orders
Blue orders
Blue orders
Red orders
Blue
Red
Blue
Red
Red
Blue
Below is an example of IFU observation of a telluric standard star:
Figure 5: IFU telluric standard star (B-type star). One can note the three slices in each order of each arm. The telluric absorption lines are easily visible in the VIS and NIR arms. One can also note the effect of the atmospheric dispersion (change of distance between the slices between blue and red orders in UVB/VIS arms).
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U B V R I
22
22
22.5
22.5
22.5
30s
30s
20s
20s
20s
2.2.1.4 The Acquisition and Guiding Camera
The A&G camera allows to visually detecting and center objects from the U- to the z-band. This unit consists in:
a filter wheel equipped with a full UBVRI Johnson filter set and a full Sloan Digital
Sky Survey (SDDS) filter set. Transmission curves are provided in appendix Error! Reference source not found..
a Pelletier cooled, 13 µm pixel, 512512 E2V broad band coated Technical CCD57-
10 onto which the focal plane is re-imaged at f/1.91 through a focal reducer. This
setup provides a plate scale of 0.173”/pix and a field of view of 1.47’1.47’. The QE
curve of the detector is provided in appendix Error! Reference source not found.. This acquisition device –that can also be used to record images of the target field through different filters– provides a good enough sampling to centroid targets to <0.1” accuracy in all seeing conditions. The noise of the technical CCD is currently of RON of 4.1e-. The limiting magnitudes for a direct acquisition were measured for different filters under relatively bad conditions (thin cirrus, full Moon, seeing about 0.7”), see Table 3.
Table 3: Limiting magnitudes for a direct acquisition
We still have to measure their limiting magnitudes under clear conditions and in dark time. However, in case of worse weather the limiting magnitudes are smaller. We still recommend to use blind offsets in case the object is fainter than 22-22.5, especially if the weather constraints are selected for thin/thick transparency and seeing worse than
0.7”. In case of blind offsets, we recommend to select an acquisition star with a magnitude about 19 or brighter to ensure a good centering before the offsets are done. For other SDSS filters, we recommend to keep a limiting magnitude of 20 for a direct acquisition in I’ and z’ but to go up to 21 in other SDSS filters. The exact limiting magnitudes for those filters will be determined during P93.
Examples of recommended exposure times for the acquisition CCD:
Vmag=6 integration time=0.001s Vmag=7 integration time=0.005s Vmag=16-20 integration time=1 to 5s V, R mag=23 integration time=60-120s V,R mag>=24 integration time180s These integration times should suffice for doing a direct acquisition in case of clear conditions, darktime and usual seeing. However, in case of very faint objects, the blind offset could be the best solution as it could shorten the acquisition overheads.
See Sect 4 about the imaging mode that provides updated information about the AGCCD and the imaging mode facility.
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Figure 6: The combined efficiency of the two dichroic beam splitters. In blue: reflection on dichroic 1; in orange: transmission through dichroic 1 and reflection on dichroic 2; in red: transmission through dichroics 1 & 2.
2.2.1.5 The dichroic box
Light is split and distributed to the three arms by two highly efficient dichroic beam splitters. These are the first optical elements encountered by the science light. The first dichroic at an
incidence angle of 15˚ reflects more than 98% of the light between 350 and 543 nm and transmits ~95% of the light between 600 and 2300 nm. The second dichroic, also at 15˚
incidence, has a reflectivity above 98% between 535 nm and 985 nm and transmits more than 96% of the light between 1045 and 2300 nm. The combined efficiency of the two dichroics is shown in Fig. 6: it is well above 90% over most of the spectral range.
2.2.1.6 The flexure compensation tip-tilt mirrors
Light reflected and/or transmitted by the two dichroics reaches, in each arm, a folding mirror mounted on piezo tip-tilt mount. These mirrors are used to fold the beam and correct for backbone flexure to keep the relative alignment of the three spectrograph slits within less
than 0.02” at any position of the instrument. They also compensate for shifts due to
atmospheric differential refraction between the telescope tracking wavelength (fixed at 470 nm for all SLIT X-shooter observations) and the undeviated wavelength of the two ADCs (for
UVB and VIS arms) and the middle of the atmospheric dispersion range for the NIR arm. In case of IFU observations, one can select the telescope tracking wavelength.
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2.2.1.7 The Focal Reducer and Atmospheric Dispersion Correctors
Both UVB and VIS pre-slit arms contain a focal reducer and an ADC. These focal reducer­ADCs consist of two doublets cemented onto two counter rotating double prisms. The focal reducers bring the focal ratio from f/13.41 to ~f/6.5 and provide a measured plate scale at the entrance slit of the spectrographs of 3.91”/mm in the UVB and 3.82”/mm in the VIS.
The ADCs compensate for atmospheric dispersion in order to minimize slit losses and allow orienting the slit to any position angle on the sky up to a zenith distance of 60˚. The zero­deviation wavelengths are 405 and 633 nm for the UVB and the VIS ADCs respectively. In the AUTO mode, their position is updated every 60s based on information taken from the telescope database. Unfortunately due to some problems affecting the ADCs, they have been disabled since August 1st, 2012. See the following section for more information about the observations without ADCs.
The NIR arm is not equipped with an ADC. The NIR arm tip-tilt mirror compensates for atmospheric refraction between the telescope tracking wavelength (470 nm) and 1310 nm, which corresponds to the middle of the atmospheric dispersion range for the NIR arm. This means that this wavelength is kept at the center of the NIR slit. At a zenithal distance of 60°
the length of the spectrum dispersed by the atmosphere is 0.35”, so the extremes of the spectrum can be displaced with respect to the center of the slit by up to 0.175”. If
measurement of absolute flux is an important issue, the slit should then be placed at parallactic angle.
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2.2.2 ADCs problems and disabled ADCs observing mode in SLIT and IFU
During March to July 2012 the ADCs (atmospheric dispersion correctors) for the UVB and VIS arms in X-shooter have been occasionally failing. Unfortunately recently the rate of such failures has increased until being daily, leading sometimes to data taken in sub-optimal instrument configuration, which needs to be taken into account when reducing and analyzing such observations.
There is an ongoing investigation to find the cause for the ADCs' misbehavior, but it is unlikely that the situation is back to normal for the next few months. Incorrect position of ADCs might lead to slit losses worse than if they are not used. Consequently, the ADCs were temporarily disabled (set at the non deviation position as in the IFU mode) on August 1st. A major intervention to fix the problem is currently under investigation.
In the following pages, you will find useful information characterizing the observations without working ADCs to compensate the atmospheric dispersion in UVB and VIS arms.
Measurements were performed in the various orders of the UVB/VIS arms, some comparisons are performed and the average, the min/max values and the standard deviation are provided. The slits used are 1.0”, 0.9”, 0.9” in the UVB, VIS, and NIR arms respectively.
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Atmospheric dispersion effect (no ADCs) on the position of the spectrum inside different orders depending on the airmass and the arm (UVB:top, VIS: middle, NIR:bottom). The wavelength is in Angstroms.
UVB arm
VIS arm
NIR arm
a) Atmospheric dispersion effect on the XSHOOTER spectra without ADCs
The tracking in XSHOOTER is by default 470nm, and The dispersion effect of the atmosphere on XSHOOTER spectra depends on the tracking wavelength used (by default 470nm). Therefore the current effect is shown in the following plots for the UVB, VIS, and NIR arms.
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Arm
Average
range
std
UVB
0.88
0.85-0.92
0.01
VIS
0.94
0.92-0.97
0.01
Arm, slit angle
Average
range
std
UVB parallactic
0.88
0.46-1.0
0.12
UVB perpendicular
0.46
0.10-1.0
0.33
VIS parallactic
0.92
0.86-1.0
0.03
VIS perpendicular
0.77
0.47-1.0
0.18
As consequences, in stare mode (object centered in the slit):
-if the observation is conducted at airmass 1.2 with the slit angle at parallactic angle, then the
drift between the blue and red order spectrum will be of ~1.6” in the UVB arm, ~0.6” in the
VIS arm, and ~0.2” in the NIR arm.
--if the observation is conducted at airmass 1.6 with the slit angle at parallactic angle, then the drift between the blue and red order spectrum will be of ~3.5” in the UVB arm, ~0.8” in the VIS arm, and ~0.3” in the NIR arm.
Such kind of drifts is important to take into account in case of nodding observations to avoid too many flux losses even with the slit at the parallactic angle. It is again more important if the slit angle is different than the parallactic angle.
b) Comparison of ADCs efficiency at different slit angle.
The measure was performed at relatively high airmass (AM=1.8) and compares the flux between the slit position parallactic+90 degrees and parallactic angles (ratio flux perpendicular/flux parallactic). The average value corresponds to the average of measurements for each order, the range gives the min/max values of the ratio and the standard deviation (std) is given.
Stare mode, AM=1.8 With ADCs ratio perpendicular/parallactic
c) Comparison of observations with/without ADCs
There are 2 sets of measurements comparing the efficiency of observations with/without the ADCs for the slit angle at parallactic angle or perpendicular to it: One in stare mode at airmass =1.8 that can be compared to the subsection b. One in nodding mode at airmass=1.35.
Stare mode, AM=1.8 Ratios no ADCs/with ADCs
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Arm, slit angle
Average
range
std
UVB parallactic
0.87
0.87-0.9
0.01
UVB perpendicular
0.82
0.56-1.0
0.15
VIS parallactic
0.88
0.82-0.9
0.02
VIS perpendicular
0.81
0.66-0.99
0.11
Arm
airmass
Ratio 45/parall
Ratio 90/parall
UVB
1.10
0.98
0.83
UVB
1.51
0.84
0.63
UVB
2.20
0.31
0.18
VIS
1.10
0.87
0.80
VIS
1.51
0.92
0.72
VIS
2.20
0.63
0.31
On those plots for observations at 90 degrees of the parallactic angle, slits of 0.4” in the UVB arm (top), 0.5” in the VIS arm (bottom) and a seeing of 0.8” have been considered.
Nodding mode, AM=1.35 Ratios no ADCs/with ADCs
The measurements were performed on short integration times and if possible in stable conditions of the seeing.
d) Efficiency of observations without ADCs at different given slit angles
and airmasses
In this subsection a summary is presented first, a modeling for narrower slits is shown in second, and finally the detailed measurements corresponding to the first part are provided. We consider here the ratios of the observation at 45 degrees or 90 degrees of the parallactic angle to the parallactic angle for different airmasses. Summary:
The similar information was computed theoretically and is shown in the following plots.
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Arm
airmass
Type of ratio
average
range
std
UVB
1.10
45/parall
0.98
0.84-1.0
0.19
UVB
1.10
90/parall
0.83
0.74-1.0
0.15
UVB
1.51
45/parall
0.84
0.56-1.0
0.13
UVB
1.51
90/parall
0.63
0.25-1.0
0.26
UVB
2.20
45/parall
0.31
0.05-0.64
0.20
UVB
2.20
90/parall
0.18
0.04-0.23
0.06
VIS
1.10
45/parall
0.87
0.83-0.94
0.03
VIS
1.10
90/parall
0.80
0.75-0.92
0.04
VIS
1.51
45/parall
0.92
0.87-1.0
0.04
VIS
1.51
90/parall
0.72
0.47-1.0
0.17
VIS
2.20
45/parall
0.63
0.37-0.83
0.16
VIS
2.20
90/parall
0.31
0.28-0.34
0.02
Arm
Average
range
std
UVB
0.96
0.66-1.47
0.25
VIS
0.85
0.74-1.00
0.09
Arm
Average
range
std
UVB
1.04
0.54-1.89
0.45
VIS
0.79
0.62-1.08
0.14
e) Efficiency of observations without ADCs at given airmass and slit angle
but with different tracking wavelength
Up to now only in IFU mode, the user can choose the tracking wavelength. This option will be considered for the SLIT mode as well. In the following tables we compare the flux ratios other the orders for the observations at 470nm (default tracking wavelength) with respect to the observation at another wavelength. The observations were performed without ADCs, in nodding mode at AM=1.35.
If the user chooses the tracking wavelength equals to 600nm instead of 470nm
For the UVB arm, the ratio is higher in blue orders (~1.4) with the 470nm tracking wavelength and lower in the red orders (~0.7) compared to the 600nm tracking wavelength. This is the same evolution for the VIS arm.
Same measurements but with the tracking wavelength at 850nm instead of 470nm
For the UVB arm, the ratio is higher in blue orders (~1.9) with the 470nm tracking wavelength and lower in the red orders (~0.6) compared to the 850nm tracking wavelength. This is the same evolution for the VIS arm.
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