INSTITUTO DE ASTROFISICA DE CANARIAS
38200 La Laguna (Tenerife) - ESPAÑA - Phone (922)605200 - Fax (922)605210
February 15, 2006 Project Ref.:
DRAFT
PROJECT / DESTINATION:
EMIR
TITLE:
EMIR INSTRUMENT DESCRIPTION
AUTHOR LIST
Name Function
Francisco Garzón Principal Investigator
APPROVAL CONTROL
Control Name Function
Revised by: Francisco Garzón Vicente Sánchez José Javier Díaz
Principal Investigator
Head of the EMIR Mechanics Group
Head of the EMIR Control Group
Approved by: Jesús Patrón Project Manager
Authorised by: Jesús Patrón Project Manager
DOCUMENT CHANGE RECORD
Issue Date Change Description
1 18/06/2001 First Issue 2 11/06/2002 Revision
3 21/02/2003 Inclusion of the instrumental sections 4 01/12/2005
11/01/2006
15/02/2006
Update of the instrumental sections Updated section 6.4.1. Control Software Updated section 6.4.2. Electronics Added new reference documents Updated section 6.3 Mechanical Design
SUMMARY
This document is intended to provide a general description of the EMIR instrument and its capabilities, from a scientific point of view. It will also give support to the top level requirement list.
It is primarily aimed at giving an overview of the main scientific needs, which will yield all together to a conceptual definition of the proposed EMIR instrument. The second part of this document makes an overview of the proposed instrumental layout, which covers the scientific needs summarised in the first sections. This instrumental description is up to date at the time of the writing.
TABLE OF CONTENTS
AUTHOR LIST ...2
APPROVAL CONTROL ...2
DOCUMENT CHANGE RECORD ...2
SUMMARY ...3
TABLE OF CONTENTS...4
LIST OF ABBREVIATIONS...5
1. INTRODUCTION ...7
2. GENERAL DESCRIPTION ...7
2.1 EMIR AND OTHER EXISTING AND PLANNED INSTRUMENTS ON LARGE TELESCOPES...8
3. SCIENCE WITH EMIR ...10
3.1 THE GOYA PROJECT...10
3.1.1 Summary ...10
3.1.2 High redshift galaxies: a case study for EMIR...11
3.1.3 Scientific rationale behind GOYA...12
3.1.4 Major scientific goals ...14
4. TOP LEVEL REQUIREMENTS SPECIFICATION...17
5. EMIR EXPECTED PERFORMANCE...19
5.1 LIMITING MAGNITUDES...19
5.2 SIGNAL-TO-NOISE DEPENDENCE ON SLIT WIDTH...21
5.3 OH AVOIDANCE...21
5.4 DETECTOR SATURATION...22
6. INSTRUMENT OVERVIEW ...23
6.1 THE EMIR CONSORTIUM...23
6.2 OPTICAL LAYOUT...24
6.3 MECHANICAL DESIGN...26
6.4 CONTROL AND ELECTRONICS...31
6.4.1 Control Software...31
6.4.2 Electronics ...31
ANNEXES...37
A. LIST OF REFERENCE DOCUMENTS...37
LIST OF ABBREVIATIONS
CSU Configurable Slit Unit
DQE Detector Quantum Efficiency DRP Data Reduction pipeline DRS Data Reduction System DTU Detector Translation Unit ECS EMIR Control System
EMIR NIR Multiobject Spectrograph (Espectrógrafo Multiobjeto Infra–Rojo) ETC Exposure Time Calculator
FOV Field of View
FWHM Full Width at Half Maximum GUI Graphic Unit Interface
IAC Instituto de Astrofísica de Canarias IFS Integral Field Spectroscopy
IFU Integral Field Unit
MIFU Multiple Integral Field Unit IDT Instrument Definition Team
IR Infra Red
IS Image Simulator
LAOMP Laboratoire d’Astrophysique Observatoire Midi-Pyrénées LAM Laboratoire d’Astrophysique de Marseille
MO Multi-Object
MOS Multi-Object Spectroscopy
MSSM EMIR Multi Slit Spectroscopic Mode.
ORM Observatorio del Roque de los Muchachos PI Principal Investigator
PD Preliminary Design
PDR Preliminary Design Review PM Project Manager
PSF Point Spread Function RON Read Out Noise
RUP Rational Unified Process S/N Signal to Noise ratio
SED Spectral Energy Distribution SICS Science Instrument Control System TBC To Be Confirmed
TBD To Be Defined
UCM Universidad Complutense de Madrid WFIM EMIR Wide Field Imaging Mode.
1. INTRODUCTION
The new generation of large-aperture optical-infrared telescopes currently being built by European and American countries, by peering ever deeper into the Universe, hold the promise of providing, for the first time, a direct view of the processes that shaped the formation stars, galaxies and the Universe. A collective instrumentation effort is underway to allow these new infrastructures to be used to their full potential. The scientific capabilities of the new telescopes are thought to be enormous, not only because of the larger photon- collecting area, but especially because of the new instruments, which, due to major technological advances, are orders of magnitude more efficient than their current-day counterparts, and which are able to systematically explore wavelength ranges which until now were barely explored. Key to these observing goals is the study of faint stellar and galaxian populations in survey and non–survey mode.
The Observatorio Roque de los Muchachos, operated by the Instituto de Astrofísica de Canarias (IAC) on the island of La Palma, is the site of the 10.4 meter Gran Telescopio Canarias (GTC) due for first light in 2006. GTC will be the largest aperture European telescope in the Northern Hemisphere. Since mid 2000, a partnership of Spanish and French research institutions (Instituto de Astrofísica de Canarias–IAC, Universidad Complutense de Madrid–UCM, LAOMP, LAM) is working on the design and construction of EMIR, an advanced spectrograph for GTC.
2. GENERAL DESCRIPTION
EMIR (Espectrógrafo Multiobjeto Infrarrojo) is a common-user, wide-field, near-infrared camera-spectrograph operating in the near-infrared (NIR) wavelengths 0.9–2.5µm, using cryogenic multi-slit masks. Top level specifications are listed in Table 2. EMIR will provide GTC with imaging, long-slit and multi-object spectroscopic capabilities. The EMIR consortium, led by the IAC, includes world-class experts with a strong track record in the development of innovative telescopes and IR instruments for large telescopes. In March 1999, GRANTECAN S.A. selected EMIR as one of GTC’s first-generation science instruments, which changes to second generation instrument in early 2000. After completing a drastic simplification phase aimed at ensuring the feasibility and availability of the instrument at GTC, EMIR is currently at the middle of its construction phase, once passed succesfully has currently the PDR in April 2003. The instrument is fully funded to completion by GRANTECAN and the Spanish Plan Nacional de Astronomía y Astrofísica (National Plan for Research in Astronomy).
EMIR will provide the GTC user community with key new observing capabilities. It is expected that it will be the first cryogenic multi-object spectrograph (MOS) on a 10m class telescope, hence it will be able to observe in the K band at 2.2µm. Similar NIR MOSs existing or planned for other telescopes are not cooled and reach out to 1.8µm only (§2.1).
Extending MOS capabilities to 2.2µm is the natural next step in MOS design. EMIR will open, for the first time, the study of the nature of galaxies at redshifts beyond z=2 with unprecedented depth and area. At these redshifts, the well-studied visible rest-frame of galaxies, in particular the strong Hα line, is shifted to the K band (see Figure 1), allowing key diagnostics of the star formation history of the Universe. EMIR will allow to bridge between the extensive studies at lower redshifts carried out in the nineties on 4m class telescopes and those above z=6 planned for the near future using the far infrared and millimetric wavelengths. EMIR will also provide a bridge between current spectroscopic
capabilities and those that will become available when the James Webb Space Telescope (JWST) becomes operational late in the next decade.
The EMIR design was largely determined by the requirements of its main scientific driver, the study of distant, faint galaxies. Being a common-user instrument, however, it has been designed to meet many of the broader astronomical community. It is therefore a versatile instrument that will accomplish a wide variety of scientific projects in extragalactic, stellar and Solar System astronomy. The description of the top level scientific requirements, as well as the complete list of physical and functional requirements, can be found in RD. 1.
The construction of EMIR pushes the challenges of large-telescope instrumentation to new limits. The GTC 10m aperture translates into a physically large focal surface. Matching the images given by the telescope to the small size of current detectors requires large optics with fast cameras. Large, heavy optics need advanced mechanical design and modelling to bring flexure down to acceptable levels. To work in the region beyond 1.8µm, the EMIR optical system and mechanical structure will be cooled down to cryogenic temperatures.
Temperature stability and cycle-time requirements pose stringent demands on the design and performance of the instrument’s cryogenic system. A key module of EMIR is a cryogenic slit unit (CSU), fully integrated in the instrument main vessel, to allow remotely configurable multislit pattern for GTC’s intended queue observing, without warming up the spectrograph.
All the aforementioned aspects need development effort, as the technology is not available or it is not scalable from existing solutions. We see the development of a documented, robust processing pipeline as an integral part of the service provided by a modern research infrastructure and include such software effort in the developments needed for a successful operation of EMIR.
2.1 EMIR and other existing and planned instruments on large telescopes.
Multi-object spectrographs are planned by the four large telescope projects in the world (VLT, Gemini, Keck, Subaru). Operating in the visible range are GMOS in Gemini, LRIS and DEIMOS in Keck, VIMOS and GIRAFFE in the VLT. Emphasis in the new instrumentation is now shifting to NIR MOSs in order to exploit the superior performance of large telescopes in the IR, and open several new science windows, despite of the much higher technical difficulty of working at cryogenic temperatures. To date, planned NIR MOSs in other large telescopes cover the non-cryogenic 1–1.8µm range (NIRMOS –recently discarded because of the forthcoming venue of KMOS– and FLAMES/AUSTRALIS in the VLT, FMOS in Subaru) and allow to measure the important Hα line for objects at z<2. EMIR will extend this coverage to z >2, thus opening an observing niche for GTC observers. Only FLAMINGOS, currently mounted in GEMINI S, will offer MOS performance in the K band, prior to EMIR but at lower spectral resolution and smaller FOV. EMIR also compares well with other instruments in the non-cryogenic IR. EMIR has lower multiplex advantage than the fibre-based FLAMES/AUSTRALIS, but has higher sensitivity, owing to the larger collecting area of GTC and the high throughput of EMIR. Technologically, EMIR will provide innovative European-based solutions for large instrumentation operating at cryogenic temperatures, advanced optical design, mechanical and thermal control, and cryogenic mask- exchange mechanisms.
Among the large variety of observing instrument already in use, in development of planned for large telescopes, NIR spectrographs and cameras operating in the range between 1 and 5
µm are of interest for this project. Their main features are listed in Table 1. This is a long list, giving idea of the interest in the astronomical community for such instrument. We have only listed the characteristics which define grosso modo the instrument and which are of interest to know the state of the art in the development of NIR spectrographs for large telescopes.
Table 1:Main features of NIR spectrographs comparables to EMIR
Tel. Instrument Range
(µm) Status Obs. Modes FOV
(arcmin) ΡΡΡΡ Detector
KECK NIRC 0.8–5.5 In use Imaging
LS Spec.
38.4”2
5.3”2 60–120 InSb SBRC NIRSPEC 0.9–5.4 In use Imaging (AO)
LS Spec. < 0.7 25000 Aladdin NIRC-2 0.9–5.3 In use Imaging (AO)
LS Spec. <❏40” 164 Aladdin MOSFIRE 0.9–2.5 Feas. Study Imaging
MOS Hawaii-2
GEMINI NIRI 1–5 In use Imaging (AO)
LS Spec. ❏2 < 1000 Aladdin NIFS 0.9–2.5 Fabrication. AO 3D spec. ❏3” < 5000 Hawaii-2
GNIRS 1–2.5
2.5–5 plan.
In use, commision
LS Spec.
IFU Spec. ❏2.5 <18000 Aladdin FLAMINGOS2 0.9–2.5 Fabrication. Imaging
MOS Ø6.2 < 3000 Hawaii-2 SUBARU CIAO 0.9–2.5 In use Imaging
LS Spec. ❏22” ~200 Hawaii-2
IRCS 1–5 In use Imaging (AO)
LS Spec. <❏1 < 1000 Aladdin
MOIRCS 0.9–2.5 Fabr. Imaging
MOS 4 x 7 < 2000 2*Hawaii-2
VLT ISAAC 1–5 In use Imaging
LS Spec. ❏1.5–2.5 < 3000 Hawaii Aladdin CRIRES 1–5 Fabrication. LS Spec. <❏1 < 100000 3*Aladdin SINFONI/
SPIFFI 1–2.5 Fabrication Imaging (AO)
IFS <❏8” < 3500 Hawaii
NACO 1–5 In use AO Image
LS Spec. <❏5” < 1000 Aladdin KMOS 0.9–2.5 Feas. Study Imaging
MIFU.
< Ø7.2
patrol FOV < 4000 Hawaii-2
LBT LUCIFER 0.9–2.5 Fabrication
Imaging LS Spec.
MOS.
❏4 < 10000 Hawaii-2
GTC EMIR 0.9–2.5 Fabrication
Imaging LS Spec.
MOS
❏6 <5000 Hawaii-2
Besides the MOS capability, EMIR provides an efficient imager that compares well even to dedicated imagers built for other telescopes (see the EMIR main performances in Table 2).
Its FOV of 6’x6’ is significantly larger than those of NIRI in Gemini (1.8’), NIRC in Keck (38”), IRCS in Subaru (1’) or ISAAC in VLT (2.5’). The small fields of some of the earlier instruments stem from tailoring the optics to smaller detectors that were available in the
beginning of the decade. Note that, from the 256x256 detector of NIRC to the 2Kx2K detector of EMIR, a factor of 64 in area coverage has been gained in less than 10 years. In EMIR, the FOV is limited by optics rather than by detector size, its design being poised to become the deepest imaging survey instrument in the K band.
Table 2:Top level instrument specifications of EMIR Wavelength range
Optimization Observing modes Top priority mode Spectral resolution Spectral coverage Array format Scale at detector OH suppression Image quality
0.9-2.5 microns 1.0-2.5 microns
Multi-object spectroscopy Wide-field Imaging
K band Multi-object spectroscopy
5000,4250,4000 (JHK) for 0.6'' (3-pixel) slit width 7500,6375,6000 (JHK) for 0.4'' (2-pixel) slit width (TBD) One observing window (Z, J, H or K) per single exposure 2048 x 2048 HgCdTe (Rockwell-Hawaii)
0.2 arcsec / pixel In software
θ80< 0.3 arcsec
Multi-object mode Slit area
Sensitivity
6' x 4', equipped with ~ 50 slitlets (~0.6” x 7.2”) Klim >20.5, t=4hrs, S/N=5 per FWHM (continuum)
Image mode FOV
Sensitivity
6' x 6'
Klim~22.8, t=1hr, S/N=5, in 0.6 arcsec aperture Klim~23.6, t=4hr, S/N=5, in 0.6 arcsec aperture
3. SCIENCE WITH EMIR
A near infrared camera spectrometer would be a work horse instrument on any telescope as it covers a huge range of astronomical fields, from solar system to cosmology. However, when the larger collecting area of the GTC is coupled with the wide field of view of EMIR and with a spectral resolution two to three times more than is currently available on most comparable instruments, the result will be science that cannot be easily done on other facilities. One such science project is GOYA (formerly known as COSMOS), which has been linked with EMIR since its start. GOYA is a very challenging project even on a 10m class telescope, but which makes clear demands upon the instrument. Hence many of the top level requirements for EMIR can be traced directly to the needs of GOYA. In the following subsections, the GOYA project, which has been selected as the main scientific driver for the design and construction of EMIR, are summarized with the goal of retrieving the top level user requirements from which EMIR was conceived.
3.1 The GOYA project
3.1.1 Summary
GOYA is an ambitious scientific program to be carried out using EMIR at the GTC. The main goal is to provide a comprehensive study of the galaxy population at z > 2, which corresponds to the epoch of maximum star formation in the history of the universe. At these
redshifts, the optical window -the region of the spectrum that has been most widely studied in nearby galaxies- is shifted into the near infrared. Using the unique multiobject capability of EMIR over the entire 1-2.5µm range, GOYA will be the first major survey to provide accurate measurements of the kinematics, ages, metallicities, dust content, and star formation rates for thousands of galaxies beyond z ≈ 2. This data set will be used to:
i) characterize the nature of the distant galaxy population
ii) identify their local counterparts and assess the amount of galaxy evolution; and iii) iquantify their contribution to the global star formation history of the universe.
The GOYA survey aims to be the bridge between similar extensive studies at lower redshifts in the optical and near infrared, and those at z >6 planned for the near future using in the far infrared and millimeter wavelengths. The ultimate goal of all these studies is to unveil the star formation history of the universe since the epoch of galaxy formation till the present.
3.1.2 High redshift galaxies: a case study for EMIR
A near-infrared, multi-object spectrograph on a 10m-class telescope, such as EMIR on the GTC, is the sine qua non instrumentation for studying galaxies at z ≥2.
• Why K? Because at z ≥2, the Hα emission line is redshifted into the K band.
Observation of Hα is key for the scientific goals of our program since: i) it is the highest S/N feature of the spectrum in star-forming galaxies and thus, ideal for measuring their internal kinematics (velocity widths and rotations); ii) it will allow us to determine the Hα luminosity (the best SFR tracer); and iii) we will be able to correct for the presence of dust via the Balmer decrements technique. Note that we will also be able to measure [OII]3727 at z ≤ 5.7 (see Figure 1a). [OII]3727 is also a very strong feature that can be used to measure internal kinematics, and as a reasonably good SFR indicator. Thus covering the K-band will allow to measure the star-formation history of the universe since the highest redshifts where radio-galaxies and even normal galaxies are now being found (e.g., z = 5.6, Spinrad et al. 1998).
• Why a multiobject spectrograph? Because sample size is critical for surveys and population studies, and also when searching for rare objects such as primeval galaxies at the highest possible redshifts. The expected number density of LR≥L* star-forming galaxies at z ≥ 2 is ~104 per square degree (Lowenthal et al. 1997). In principle, a multiobject spectrograph covering the entire unvignetted field of view for the Nasmyth focus of the GTC (i.e., ~8'×8'), may offer up to a gain factor ~200 over a traditional single-slit spectrograph.
(a) (b)
Figure 1: (a) Observed wavelengths of the most representative emission and absorption lines in normal galaxy spectra at various redshifts. (b) K-band apparent magnitude for a typical
early-type (solid line) and late-type (dotted line) L* galaxy (MK = –23.1 + 5 log h) as a function of redshift (Ho = 50 km/s/Mpc, qo = 0.05).
• Why a 10m telescope? Because galaxies at z ≥ 2 are very faint. In Figure 1b we show the apparent K-band magnitude corresponding to an unevolved L* galaxy, as a function of redshift for the two extreme cases in galaxy spectral types. In order to observe a typical unevolved L* such as the Milky Way at z ≈2 we will need to reach K ≈ 20-21.
Figure 2. Evolution of the global star formation rate density of the universe with redhift.
3.1.3 Scientific rationale behind GOYA
The evolution of the star formation history of the universe has become one of the key problems in cosmology at the dawn of the XXI century. Recent observations indicate that the peak of the star formation activity is probably in the redshift range 1 ≤ z ≤ 3, i.e., when the universe was ≈10–40% of its current age (Lilly et al. 1995; Ellis et al. 1996; Cowie et al.
1996; Madau et al. 1996; see Figure 2). Theoretical studies suggest this is likely to be the main epoch of star-formation for normal galaxies fainter than L* (Lacey et al. 1993; Cole et al. 1994). However, there may be a wide variety of physical phenomena at play behind the observed increase in the global star formation rate of the universe with redshift, including the passive evolution of stars, the fading of galaxies after the first starburst, the amount of dust,
the successive mergers and interactions, or the link to a central AGN. The first step to understand the star formation history of the universe is to understand the star formation history of normal galaxies. Essential to this goal are redshift surveys beyond z=1 and detailed studies of the physical properties of large numbers of high redshift galaxies.
A very successful technique to find galaxies at z ≥ 2 is to use a combination of broad-band ultraviolet and optical filters to select out objects by virtue of the very significant break the Lyman limit imposes upon their far-UV continuum (Steidel et al. 1996; Lowenthal et al.
1997). This technique can be further improved and extended to even higher redshifts by including deep infrared photometry (see Figure 3, a and b). Optical spectroscopy at Keck using LRIS has confirmed these galaxies to be at high redshifts and to have spectral characterictics very similar to those of nearby extreme star-forming galaxies. Providing the all-important spectroscopic follow-up can be undertaken, this technique can be reliably used to identify many hundreds of truly high-redshift galaxies, thereby putting the detailed study of their fundamental properties on a firm statistical basis.
Figure 3: (a) UBVIJHK images of 6 high redshift candidates (at the center . (b) Model spectral energy distributions derived from the broad-band colors for the same objects, and
their best redshift estimates (Lanzetta et al. 1998).
To date, spectroscopy of z ≈ 3 galaxies has been confined to optical wavelengths, thereby providing rest-frame coverage of only the far-UV portion of their spectral energy distributions, which reflects the physical conditions dominated by only the very massive stars (Figure 4). The rest-frame optical region, however, contains light contributions from both gaseous emission regions and a broader range of stars, thereby providing information on the on-going rate of star formation (from Hα or [OII]3727 emission), the recent rate of star formation (from Balmer line absorption), as well as abundances (for both the gaseous and stellar components), overall stellar content, amount of dust (from Balmer decrements) and internal dynamics/masses (from the broadening of the absorption/emission features). The rest-frame optical region is clearly fundamental if physical studies of galaxies are to be comprehensive. In the context of high redshift galaxies, this dictates the absolute requirement for intermediate resolution spectroscopy capability at near-infrared wavelengths.
Figure 4. Average UV spectra of 12 star-forming galaxies at z≈3 (Lowenthal et al. 1997) Various major infrared spectroscopic surveys are currently being planned using new instrumentation on 10-m class telescopes (e.g., see the scientific justification for NIRMOS and AUSTRALIS). However, the spectrographs designed for these surveys will reach to only 1.8µm. Thus their capability to carry out comprehensive studies of the entire rest-frame optical region is restricted to galaxies at redshifts z ≤ 2, since the key Hα emission line is shifted beyond 1.8µm at z >1.8. GOYA will be the first such survey to provide a systematic study of the rest-frame optical galaxy properties beyond z ≈ 2. The extension of these studies to the highest possible redshifts is fundamental to understand the complete evolutionary sequence that led to the galaxies that we know today. New space missions and ground-based antennas, such as SIRTF or FIRST from NASA-ESA, and the LMT from UMass-INAOE, are being planned to study the star formation at z >6. The GOYA survey will build the bridge between the near-IR distant galaxy surveys at z ≤ 2, and those at z >6 using longer wavelengths.
3.1.4 Major scientific goals
We intend to use EMIR to carry out a comprehensive study of the rest-frame optical properties of thousands of galaxies at z ≥2, including: kinematics, dust content, star formation rate, metallicity, age, luminosity and mass functions, merging rate, clustering, and large scale-structure. The fundamental questions that we are attempting to answer include:
• What is the nature of the galaxy population at z ≥2? What are their local counterparts? At z≈2 the 1–2.5 µm range translates into rest-frame ≈3300–8300Å , the region of the spectrum that has been most widely studied in nearby galaxies. GOYA will address these questions by comparing directly the properties of the high redshift galaxies with those of the nearby population in the same parameter space, including emission line ratios (e.g., [OII]3727/[OIII]5007, [OIII]5007/Hβ, [NII]/Hα), SFRs (from both Hα and [OII]3727 emission), metallicities (from various line strength indices), dust content (using Balmer decrements), and internal kinematics (from Hα velocity widths, σ). This procedure avoids the uncertainties in the calibrations and biases that affect similar studies of high-redshift galaxies in the rest-frame UV. A similar approach has been successfully used to identify unambiguously the local counterparts of compact emission-line galaxies at z≈1. For instance, Figure 5a and b show MB- OIII/Hβ and L(Hβ)-σ diagrams for a sample of emission-line galaxies at z≈1 (Guzmán et al. 1997). These two diagrams allow to discriminate among various types of active galaxies and star-forming systems. The high blue luminosities, small velocity widths and high OIII/Hβ ratios of these distant
galaxies are consistent with their being low-mass (M≤ 1010M
) extreme star-forming systems similar to nearby HII galaxies.
Figure 5: (a) MB vs. [OIII]/Hβ. Local galaxies: DANS=Dwarf Amorphous Nuclear Starbursts; SBN=Starburst Nuclei; Sy2:=Seyfert 2 galaxies; HII=HII galaxies. Dashed lines
represent the approximate location of spiral galaxies. Star forming galaxies at z≤1 are represented by the squares. (b): Velocity width versus observed Hβ luminosity. Crosses: local
HII galaxies. Stars: star forming galaxies at z ≤ 1. Squares: star forming galaxies at z=3.
• How have galaxies at z≥2 evolved? Are they the progenitors of today's quiescent stellar systems? Galaxies at z>2 are being observed when the universe was ≤20% of its current age. According to theoretical models of galaxy formation, this early epoch corresponds to the era of primeval galaxies, i.e., the progenitors of today’s grand-design spiral galaxies and massive ellipticals, which are undergoing the first major burst of star formation. A key diagnostic diagram to assess the amount of evolution experienced by these galaxies with look-back time is the luminosity-linewidth relation (Tully-Fisher for spirals, Faber-Jackson or the Fundamental Plane for ellipticals). In Figure 6, we show the Tully-Fisher relation for a sample of field spirals at z≈0.8 (Vogt et al. 1997). The comparison with a similar relations for nearby spirals implies that these systems have experienced little evolution in the last 8 Gyrs (the observed increase of ≈1 mag in luminosity is consistent with passive evolution), and suggests that their epoch of formation is indeed beyond z>2. Note that a key new aspect common to these studies at high redshift, including GOYA, is to measure galaxy internal motions, which are not affected by the luminosity evolution of the stellar population.
• Another key diagnostic to study the evolution of the stellar population in elliptical galaxies is to measure line strength indices such as the λ4000 A break and Mg2. In particular, the sudden onset of absorption features bluewards of λ4000Α is specially sensitive to the stellar effective temperature and, therefore, to the mean age of a composite stellar population. Recently, an empirical calibration of the break in terms of the main stellar parameters has been derived by Gorgas et al. (1998), allowing new accurate predictions of its amplitude for composite populations. Compared to other features usually used in the blue spectral region, the break has a clear advantage: due to its wide sidebands in can be measured with good precision in relatively low signal-to- noise ratio spectra. Using the recipes from Cardiel et al (1998) it can be shown that the relative error of the break is:
1 2 -1
=0.1 (1+z) SN(Å)
ε
γ ⋅ − ⋅where z and SN(Å) are the redshift and the signal–to–noise ratio per Α. For instance, for galaxies at z=2 the break can be measured in the J–band with a 10% uncertainty in spectra with SN(Å) ≈0.6, and this required minimum SN decreases to 0.4 for the K–band.
Thus GOYA will be the first survey able to measure the break in early-type systems as far as z=4.5, and to address whether it may be possible to find any old-stellar-population dominated galaxy at very high redshifts.
Figure 6: Blue magnitude vs. rotational velocity for a sample of spiral galaxies at z≈0.8.
• What is the real evolution of the star formation history of the universe with look- back time? The interpretation of Figure 2 should be approached with caution, given the likely differences in the calibrations for the various SFR tracers used, incompleteness of the data sets, and uncertainties in the corrections for dust. Indeed, new estimates of the global SFR rate density at high redshifts differ in more than a factor ≈4 among various researches. Most of the caveats surrounding these estimates can be avoided by measuring the SFR using Hα emission, the best SFR tracer. This is the method adopted by Gallego et al. (1995) to estimate the local value of the global SFR density, and will also be used by the COHSI project (Aragón-Salamanca et al. 1999) to derive the value at 1≤ z ≤2.
GOYA will be the first major survey to measure Hα emission at 2≤ z ≤3 and, combined with similar surveys at lower redshift, will define a homogenous, unambiguous picture of the star formation history of the universe from z≈3 till the present. In addition, since [OII]3727 is also a fair measure of the star formation rate, GOYA will be able to provide a well-calibrated measurement of the global star formation rate to z≈6 via observations of the [OII]3727 emission down to 2.5µm.
• What is the earliest epoch of galaxy formation? The quest to observe the birth and formation of normal galaxies like our own has proved to be a very elusive one... till now.
The advent of the 10m-class telescopes combined with powerful new instrumentation has yielded in only two years over 600 high redshift candidates of which ≈250 are spectroscopically confirmed galaxies at 2<z<4 (Steidel et al. 1996; Lowenthal et al.
1997; Steidel et al. 1998). The record holder for the galaxy with the highest redshift known goes to a very recent discovery of an object at z=5.34 by Spinrad et al. (1998).
This is obviously the beginning of a new era of discovery of primeval galaxies in the early universe. The new NIR MOS planned for the 10m-class telescopes will allow systematic searches for primeval galaxies at z ≤3.5, by looking for [OII]3727 from emission-line galaxies, and Ca H+K and 4000Å break from absorption-line objects over the range 0.9 to 1.8µm. GOYA will be able to extend this search to z≈5.7 —and to much
higher redshifts via observations of Lyα— by extending the wavelength coverage to 2.5µm.
• What is the merger rate at z>2? Another crucial piece in the puzzle is an understanding of how small-scale structure (companions, satellite galaxies, compact groups and similar structures) affects the processes of galaxy formation. One currently un-answered question is the evolution of the rate of both major and minor mergers between galaxies.
Studies of the number of close pairs suggest that their numbers increase rapidly (Yee &
Ellingson 1995, ApJ, 445, 37). Indeed there is clear evidence for merger signatures in the highly distorted morphologies observed in close pairs in the Hubble Deep Field South.
GOYA will be able to securely establish which apparently close pairs of galaxies will indeed merge or interact and which are chance projections or fly-by, and thus to determine how the merging rate has evolved since z=5.7.
4. TOP LEVEL REQUIREMENTS SPECIFICATION
EMIR is conceived as a common user instrument for the GTC. Its mechanical, optical, electrical and software design and interface, storage and handling equipment, spares and documents shall fulfil the GTC standards, as summarized in the ICD´s and other applicable documents.
EMIR will be mounted on one of the Nasmyth platforms of GTC, which then poses limits on the mechanical interface, maximum weight, position and maximum available space. These are defined in RD. 1 and references therein.
EMIR provides two observing modes: multi-object spectroscopy, and imaging. Multi-object spectroscopy is taken to be the top priority functionality as imposed by the main scientific project design driver, GOYA.
Observations can be done in the wavelength range 0.9 to 2.5 µm. The specific spectral domain is set by the capabilities of the selected detector, the current state of the art in NIR array technology The instrument is designed to be optimized for the wavelength range 1 to 2.5µm, i.e., in the NIR, where the quantum efficiency of the HAWAII2 reaches to its maximum.
A primary design driver has been to obtain a large FOV (6 arcmin), with a pixel scale of 0.20 arcsec/pixel. This plate scale is a trade–off between the need of a proper sampling of the typical ORM seeing figure (median value ~0.65 arcsec in V) and the full spectral coverage of each of the NIR atmospheric windows in a single exposure, accounting for the wavelength shifts due to the different target positions on the focal plane in MOS mode. The image quality over the full FOV will be controlled via the standard θ80 value. This has been set to a maximum of 0.3 arcsec in the line of not degrading the median ORM free-air seeing in excess of 10% (this last requirement alone will imply only θ80~0.6 arcsec).
Multi-object spectroscopy is provided via cryogenic multi-slit masks, which have to be compared with the alternative option of using fibres. The ratio in performances of fibres and multislits will be largely dictated by their relative accuracies of sky subtraction. Fibres present two potential disadvantages with respect to multislits. Because object and sky samples are taken on non-adjacent places on the sky, spatial variations of the sky brightness may introduce sky subtraction errors. And, because object and sky samples travel through
different parts of the optical system, flat-fielding errors are potentially more important. Slits provide a factor of ~3 lower background subtraction residuals over fibres. Background subtraction is perhaps the most critical step in the reduction of infrared data of faint sources, and ultimately plays the decisive factor in determining the limiting magnitude that can be achieved. We estimate that the better background subtraction translates into a gain of 1 mag in the expected limiting magnitude using slits, all other factors being equal. Almost all the main science programs envisioned for EMIR -whether stellar, galactic or extra-galactic- will work at, or close to, the limiting magnitude, and will greatly benefit from any faintward extension of this limit.
Another possible option for a NIR MOS would have been the choice of Integral Field Spectroscopy (IFS) with the use of Integral Field Unit (IFU), either single or multiple (MIFU). Apart from many technical considerations, IFUs are restricted to a very small FOV which are in contradiction with one of the main EMIR design drivers, as mentioned before.
The use of MIFU to increase the available FOV would have result in a prohibitive expensive, and even more complicated, instrument which we could not afford.
Spectra are taken at a spectral resolution in excess of 4000 for slit widths of 0.60 arcsec that project onto 3 pixels. This resolution allows observers to resolve the space between the OH telluric lines. Narrower slits will be possible with the technical concept used for the CSU (see 6.3). For instance, those projecting onto 2 pixels will provide resolutions over 6000. The nominal resolving power has been selected according to the needs of the GOYA project. It basically takes into account the capability of resolving the OH telluric lines, mainly in the H band. See Figure 11.
Special care has been taken in fixing those requirements which has to do with the flexures of EMIR during observations. These mainly affect to the accuracy of the spectrum extraction, the flux stability in the selected aperture and the displacement of the spot of equal wavelength along the spatial direction of the detector. Since EMIR is an spectrograph, the more stringent requirement, as far as mechanical stability is concerned, is the image motion of the slit image on the detector plane. This has been fixed to a 10% of the standard EMIR resolution element, defined as 3 pixels, in the spectral direction. Since EMIR will be attached to the Nasmyth focus of the GTC we expect that the A&G box can cope with the repetitive shifts of the instrument while in operation. So, at this stage, we are mostly concerned with the stiffness of the instrument which affects to the accuracy within which spectral measurements can be achieved.
The EMIR Consortium has selected a remote reconfigurable system for the cold mask unit mechanism, which seems to be the most appropriate to suite the EMIR needs, but requires a careful development. This development is being carried out via a Demonstration Programme performed by an external contratctor and the IAC team, which is due for February 2006.
Following this, a procurement process for the final unit will be launched. We have only set top level user requirements that defines the precision with which the masks has to be positioned into the focal plane, and the relative accuracy between the multislits.
5. EMIR EXPECTED PERFORMANCE
The following subsections describe the expected performance of EMIR, as given by the EMIR simulator. We provide limiting magnitudes for continuum, line flux limits, exposure times for detector saturation and OH sky line suppression capabilities.
Figure 7 Limiting magnitude as a function of exposure time for the image and spectroscopic (continuum) cases, with SNR of 5. Dashed horizontal lines indicated 1 and 2 hours of integration
time.
5.1 Limiting magnitudes
In Figure 7 we show the limiting magnitudes derived for the spectroscopic (continuum) and imaging modes of EMIR in four near infrared bands, Z, J, H, and KS, for image mode and in the three reddest bands for spectroscopic mode. The limiting magnitude is defined as the magnitude measured in an aperture equal to the median seeing at the ORM site (0.65´´) with a signal-to-noise S/N=5.
Figure 8: S/N versus line flux in a typical emission-line galaxy at z=3, as seen through the different bands. The line has a width σ = 50 km/s, and the exposure time is 1 hour.
Figure 9: Simulated extracted 1D spectra of a compact extremely red galaxy at z = 1.8, with K=18, as seen through the J band of EMIR. The deepest absorption lines are clearly seen with
this resolution.
We show in Figure 8 the sensitivity of EMIR to spectral line measurements. Figure 8 shows the S/N of a spectral line of given flux after a 1 hour exposure. Fluxes are typical of emission line galaxies at z=3, and match the types of measurements targeted by the EMIR science programs. S/N is per FWHM resolution element, under the best seeing conditions, with a slit width of 0.65”. We note that S/N depends on seeing and slit width, as shown in Section 5.2 (Figure 10). Another important remark is that the noise in J and H bands is dominated by the thermal contribution from the detector (outside the sky emission lines), and that we are not in photon noise regime. This is not true in K, where the noise is dominated by ambient thermal emissivity.
The ability of EMIR to map the absorption line spectrum of distant galaxies is shown in Figure 9, which gives an extracted spectrum of a red galaxy at z=1.8. The deepest absorption lines are clearly visible (e.g. Hβ at λ=1.36 µm).
5.2 Signal-to-noise dependence on slit width
The slit width used determines both the amount of source light and the amount of sky light that reaches the detector. The EMIR simulator was used to determine the variation of S/N with aperture size, both for point sources and for extended objects. The latter were modelled with exponentially decaying brightness profiles typical of galaxies at high redshift. The results are shown in Figure 10 for two characteristic values of the seeing at La Palma. These plots were used to determine the optimum aperture size for EMIR and to constrain the pixel scale. The plots show that apertures of 0.6 arcsec are ideal for extended sources in nights of good seeing. Wider apertures are advised for worse seeing, but they have an impact on spectral resolution. The plate scale of EMIR (0.175 arcsec/pixel) was chosen to provide adequate sampling in imaging, and also as a result of optical design constraints.
Figure 10: (a) S/N vs. Slit width for a point source, and for two galaxies with effective radius of 0.2'' and 0.4 '', respectively (solid lines). The objects have K=18. The exposure time is 1
hour. The adopted seeing is 0. 3'' FWHM. (b) As before, for a seeing of 0. 8'' FWHM.
5.3 OH avoidance
Among the top level science requirements of EMIR is the ability to resolve the space between the unresolved OH sky emission lines that dominate the J and H band sky emission.
Because a spectral feature that falls on top of an OH line becomes undetectable, the ability of EMIR to find spectral lines in the spectra of astronomical targets increases with the fraction of the spectral window that is free from OH sky lines. Figure 11 shows the percentage of pixels free from OH lines as a function of spectral resolution, for the H band. At the planned spectral resolution of EMIR in the H band (R=4000, 3-pixel slit width), 85% of the window becomes usable for spectroscopic work. This figure may be used to predict the effects of narrowing or widening the slit.
Figure 11: Percentage of pixels free of sky emission lines in the H band as a function of spectral resolution.
5.4 Detector saturation
One of the user requirements for the imaging mode is a fast readout capability, in order to optimize the observations. This is motivated by the fact that the total exposure time is divided into individual short exposures, to keep the intensity levels below saturation. Indicative values for the maximum exposure times allowed in the different filters are given in Table 3.
Table 3. Estimated Maximum exposure time in the different filters to fill a 60000 e- /well.
Fixed parameters : 0".2 /pixel, effective mirror surface 7.359 x 105 cm2, ron = 10 e-, and thermal noise of 0.3 e- s-1.
Filter Sky brightness Sky flux exposure time mag/arcsec2 photons s-1 pixel-1 sec
J 16.1 720.6 83
H 14.4 3364.9 17
K 13.3 5859.7 10
K' 13.7 4546.4 13
Figure 12 displays the limiting exposure times for standard stars as a function of the seeing conditions. In good seeing conditions, stars of K=13 saturate the detector in little over 1 second, emphasizing the need for readout times well below 1 second, at least for a detector subarray.
Figure 12: Number of e/pixel as a function of exposure time, for stars with different K´
magnitudes (labeled on the curves) as observed with a seeing of 0.3” (top) and 0.8” (bottom), centered on a pixel of 0.2”. The expected flux for the sky is indicated, as well as the limiting
magnitude to fill the well (assumed 60000 electrons, dashed line).
6. INSTRUMENT OVERVIEW
6.1 The EMIR Consortium
The development of a such a complex instrument like EMIR requires a strict control and management over the responsibilities, schedules and interfaces between the different partners institutions, which will ensure the fulfillment of the assigned tasks within the available budget and time line. The organization scheme that is proposed is based in the breakdown of the whole instrument in work packages to be developed at the different institutes within a global plan for EMIR.
At the other hand, the large number of researches involved in EMIR and the associated staff in the partner institutions, together with their location in different sites make it necessary to define a framework which specifies the links between the centres and the persons, defining the tasks and responsbilities for each one of them within the global plan, and which ensures the coordination among the different parties.
We have selected for this framework the format of an instrumental Consortium, which is sketeched in Figure 13, constituted by the PI, CoIs, key technical persons and Associated Sientist. This is the forum where all the people involved in the EMIR development will find the common place to control and monitor the run of the development and to take the appropiate actions to ensure its completion. A central task of this Consortium in the post PD phase will be to organize the scientific exploitation of the instrument, in both guaranteed and open time, disseminating EMIR within the user community and promoting its use. A full description of the WBS and the EMIR Consortium organization can be found in RD. 2.
Figure 13: Organigram of the EMIR Consortium.
6.2 Optical layout
The optical concept of EMIR has been studied in many approaches in order to have a good balance between the performance of the instrument, the technical risks and the global price.
The EMIR requirements make the optical concept extremely challenging, and the design approaches have tried to minimize the trade off between requirements and technical solutions. The optical design is described in full in RD. 3 and RD. 4 to which the reader is referred.
As a result of the EMIR Consortium Meetings held at the IAC on November 21st, 2000, and on May 16th, 2001, the selected optical concept for the EMIR instrument was the one that uses of pseudo grisms as dispersive element. The main advantages of a pseudo grism design are:
• The optical design is simpler (the distance of the pupil to the camera is almost 1/3 of that required in the case of reflection gratings). The lenses of the camera are smaller, easy to design and can be more efficient.
• The location of the cold stop in the instrument that will baffle the instrument in both modes (WFIM and MSSM) is more natural.
• The size of the pupil in the instrument and the distance to the camera can be reduced, so the lenses of the camera are smaller than in the case of using reflective gratings.
Figure 14 Sketch of the design for the dispersive component.
The parameter that drives mostly the design is the size of the required FOV in both imaging (6 by 6 arcmin) and spectroscopic (6 by 4 arcmin) modes. Requirements such the spectral resolution and operation temperature of the instrument and material availability are also important and have special role in the final design.
Figure 15 Unfolded layout of the EMIR optical design in imaging and spectroscopy modes.
A main feature in the optical design of an infrared camera is its cold aperture stop to reduce the background noise. The GTC entrance pupil or aperture stop is the secondary mirror, as in any infrared telescope. The size of the cold stop is mainly defined by the required spectral resolution (R ≈ 4000) using grisms between 0.90 and 2.50 µm. We choose a nominal collimated beam of 100 mm diameter, to achieve a resolution of 4250 with a 40º ZnSe (n ≈ 2.45) grism, with a slit width of 0.6 arcsec. The proposed dispersion component has a novel design and is based on a pseudo grism proposed by LAM whose diffractive component has been developed by Jobin Yvon (France). It consist in a high efficiency diffractive pattern, which is engraved on a fused silica substrate. To manufacture the grating first a mask is recorded holographically in photoresist and then the mask is transferred into the substrate by ion etching techniques. The difficulty arises from the fact that it is important the modulation depth to get a high efficiency in transmission mode. The transmission grating is sandwiched by two symmetric prisms to tune the undeviated wavelength and to set the spectral resolution in the instrument. An schematic of the device is shown in Figure 14.
A refractive collimator is designed to form an image of the secondary mirror (cold stop) at 200 mm (nominal) beyond its last element and produces a collimated beam where the pseudo grisms are inserted. As mentioned, the nominal size of the cold stop is 100 mm diameter. The
design has been carried out in steps, starting with the collimator that must provide a parallel beam and a pupil image of 100 mm diameter. The design of this subsystem was driven to get a good full field polychromatic pupil image, and not much attention in the colour correction on the final image. The availability of materials, specially for the first lens has been a limitation for the design as is a matter of concern because its size. The implementation of the camera was done once its performance alone was much better (in terms of polychromatic spot size) than the final EMIR requirement over a wide field and with a larger stop size than the final one in the instrument and paying attention to the telecentricity of the image as well as the free space after the last lens to have enough room to place the filters.
In Figure 15 the unfolded layout of the EMIR optical design in both WFIM and MSSM modes are presented. Details are to be found in RD. 3 and RD.4.
6.3 Mechanical design
This design is described and discussed completely at RD. 5. EMIR will be attached to the mechanical rotator and platform of the Nasmyth- focus, as depicted in Figure 16. The instrument is supported from the rotator by the Nasmyth Rotator Adapter (NRA) and from the platform by the Nasmyth Platform Adpater (NPA). This attachment causes the whole instrument to rotate together with the Nasmyth rotator. The NPA helps holding the instrument, providing additional rigidity to minimize the effects of flexure and extra support for the huge EMIR mass (in excess of 4Tons).
Figure 16 General view of the instrument in the Nasmyth platform, left panel, and a 3D view of EMIR, right panel
The mechanical layout of the instrument has been derived from the optical design, taking into account the Nasmyth space envelope. Two flats mirrors have been added to bend the beam and a cold optical bench has been optimized to fulfil the image stability error budget. Then, a mechanical concept has been developed for each subsystem.
Figure 17 3D view of main EMIR cold subsystems looked from the telescope location.
When the light beam enters the cryostat through the Cryostat Window it first passes through the Radiation Baffle which rejects back out stray light, thus limiting the incoming radiation.
The beam then encounters the Configurable Slit Unit, a remotely controlled mechanism capable of generating different patterns of multislits, long slits and free aperture at the focal plane. One step beyond, the light crosses the first Collimator Barrel, and then enters through the Periscope where the beam is folded twice by means of flat mirrors. Right at the exit of the Periscope, the second collimator barrel precede the Grisms Wheel, which carries multiple dispersive elements that can be remotely selected and introduced in the light path. Then the Camera Barrel guides the light to the Filters Wheel, finally reaching the Detector Assembly which is mounted onto an XYZ mechanism, the Detector Translation Unit.
Figure 18 Cold shields covering the cold bench and cold subsystems of EMIR.
The structural mainframe of the cold part of the instrument is the Optical Bench (see Figure 17) which holds and cools the optomechanical components of EMIR; a geometrically complex and passive Cold Shields (see Figure 18) is used to reject unwanted thermal radiation. The Optical Bench is supported and thermally isolated from the warm Vacuum Chamber by means of six isostatically mounted Radial Support Trusses and three Axial Support Trusses. The cryo–coolers attached to the Vacuum Chamber are thermally linked to the cold structure, and keep it at cryogenic temperature. There is also an additional cooling system consisting of Liquid Nitrogen Tanks attached to the optical bench, that can be filled and evacuated from the outside via special pipeworks. These will be used for the precooling of the instrument.
Front Cover
Rear Cover Support Flange
Rear Central Section Front Central Section
Figure 19 EMIR Vacuum Chamber components. Structural notation
The Vacuum Chamber consists of the Front Cover, which holds the Cryostat Window Assembl, the Front Central Section, wich holds the Cryocoolers #1 and #2; the Support Flange that interfaces the instrument to both the Nasmyth Rotator Adapter and the Nasmyth Platform Adapter, and also to the Support Trusses; the Rear Central Section which holds Cryocoolers #3 and #4, as well as the Vacuum Hardware and the Precooling System; and the Rear Cover. All these components, which can be seen in Figure 19, belong to the “warm”
part of the instrument, that is, these components remain at room temperature and atmospheric pressure. Also the vacuum chamber supports the floating shields (see Figure 20). If the Vacuum Chamber is disassembled, the cold structure kept in vacuum inside can be observed in Figure 17.
Figure 20 Floating shields
The most striking feature in the EMIR mechanical design is the choice of a fully cryogenic robotic system which can be remotely reconfigured to form the multislit pattern in the instrument focal plane. To this end, a development contract was signed with the Swiss firm CSEM during the PD phase. Furthermore, during the ADR phase, a demonstration program was signed with the Dutch company JANSSEN Precision Electronics to built a 6 bars prototype. This subsystem, whose sketch taken from the CSEM proposal is shown in Figure 21, has obvious functional and operational advantages with respect to the more classical approach of interchangeable multislit masks, at the cost of involving much higher number of cryo–actuators and so increasing the risk of failures and bad functioning.
Figure 21 Layout of the CSU concept
A large design effort during the PD&ADR phases has been invested in to the developing of a feasible concept for the CO1 lens barrel. Being EMIR a big instrument with large FOV, the main difficulty is the secure support of large and heavy lenses (up to 490mm in diameter and 22 Kgr in mass) in cold while maintaining the stringent requirements of stability and
accuracy in geometrical position and orientation. There is very little, if any, previous experience in this field which could be applied to EMIR which have implied an extra cost in design effort for the mechanical team.
Figure 22 Details of the radial and axial support scheme for the optomechanics
During the ADR phase three prototypes has been developed successfully to test the feasibility of the proposed concepts: the Collimator #1 Barrel, the Support Trusses and the Grisms Unit.
Finally, there are four main mechanisms which are being developed, the cryostat window mechanism, the wheels for the grisms and filters and the detector translation mechanisms, which includes both focusing capability and XY positioning. The first permit operate EMIR in the best way, in extreme environmental conditions, using an auxiliary window (TBD) and also protect the cryostat window from dust, moisture condensation and any impacts during operation and, handling and alignment procedures. The latter will permit active compensation of the internal flexures of the instrument, attached to the Nasmyth rotator and could also be used to implement advanced features in the observing strategy. This mechanism has been subcontracted to CSEM.
Flexures
DA Interface plate Electronic connector box
Translation stage
positioningpads
Blocking system(for manipulation) Detector wire support
Figure 23 3D view of Detector Translation Unit
1 spring loaded radial support
2 fixed radial supports
6.4 Control and Electronics
The EMIR control software and electronic system is being developed by a multi-institutional group formed by scientist and engineers from IAC, UCM and LAOMP, under the coordination of the IAC. It has been adopted a unique structure for control software and electronics aimed in the simplification of the interfaces and to make easier the flow of information in these two fields with high interaction.
The development of both, electronic hardware and control software, follow strictly the prescriptions of GRANTECAN for the development of instruments, in view of the subsequent integration on the global GTC Control System.
The full description of the proposed control system and hardware architecture can be found in RD. 6 and RD. 8.
6.4.1 Control Software
There are four main aspects in the control system that have been considered as integral part of the instruments from the beginning:
• The EMIR Coordinated Operations (ECOs), which includes the control of the instrument global configuration related with observations and calibrations. These might have to interact with the GTC control system. It is being built in cooperation by IAC and LAOMP.
• The EMIR Data Acquisition System (DAS), which drives the different detector read- out modes and controls the flow of data. It is being developed by IAC, based on a ARC/IRL (SDSU) controller.
• The EMIR Observing Programme Management Subsystem (EOPMS, formerly EOPMT), which is the master programme which monitors the EMIR performances and will ensure an adequate use of the EMIR instrument by the regular astronomers.
LAOMP is undergoing its design.
• The Data Reduction Pipeline (DRP), which includes specific filters and reduction packages for each observing mode. It is under the responsibility of the UCM.
It is also planned the cooperation of the EMIR Control group with the rest of the instrument team during the development phases when necessary. This is the case during the prototyping phase of the mechanisms.
There are also some additional tasks required to provide EMIR with the house keeping electronics in charge of managing the vacuum, cool down and warm up procedures. Also the monitoring of working magnitudes such as temperature and pressure is carried out by this additional electronics and the corresponding software.
6.4.2 Electronics
The electronics of EMIR involves mainly the Detector Electronics and testing, the mechanisms electronics and all the monitoring and housekeeping electronics.
Regarding the Detector Electronics and Testing, some tasks have been performed to provide EMIR with a complete characterization of the Detector. This effort has been also of some benefit to determine the Detector Controller requirements for the instrument.
A set of tests have yield definitive results of the detector behaviour. As a consequence of this results the first scientific grade detector received for EMIR has been replaced by the manufacturer due to the non conformities with the instrument requirements. This tests have been of interest as they have provided useful information about the detector working parameters plus extra information to determine the detector controller requirements.
The Engineering grade and the new scientific grade array have been tested in detail using a 4 channel and a 32 channel architecture. The final results obtained for the new Science Grade Array complained with the EMIR requirements and it has been accepted as the instrument detector.
The results obtained for the Engineering and the Science grade detectors are quite similar in terms of noise, dark current, relative QE, persistence etc but differ dramatically in terms of cosmetics.
Bellow are shown some results obtained for the Engineering and the Science grade array that illustrate their characteristics. A compete reference of the testing results for the Science Detector can be found in RD7.
Figure 24 Read-out noise distribution of the engineering array.
The first results obtained for the Engineering Grade Array fulfil the typical science grade detector results but only applicable to a portion of the detector. The dark current distribution is shown in Figure 24 and its cosmetic can be appreciated in Figure 25. A brief summary of these first results follows:
• Readout noise: 11e-
• Gain: 3e-/ADU
• Well depth 150.000 e-
• Dark current 0.02 e-/sec @ 77 K
• Maximum pixel rate per channel 140 kHz.
Figure 25 Sample image taken with the EMIR engineering detector array
The final detector controller architecture for EMIR is now designed according to the experience gained during the tests. It is based on an ARC/IRL controller (formerly SDSU or Leach controller) with the architecture shown in Figure 31.
The Mechanisms details are under definition but many details have been already clarified.
The use of stepper motors and the selection of the motors, driver and indexers is already almost finished. The cryostat cabling description is under development and the requirements and cable/connectors type being chosen. Minor details are still pending till a more definitive design of the mechanisms and cryostat is available.
Figure 26 Dark current for the Science Grade Array. Derived from the first images after a long period of inactivity.
Figure 27 Dark current for the Science Grade Array. Derived from the first images after a long period of activity.
Figure 28 Histogram of the Dark current for the Science Grade Array
