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AM BE R Cons or tiu m
UNSA UniversitИ de Nice Sophia Antipolis OCA Observatoire de la CТte d'Azur LAOG Laboratoire d'Astrophysique Observatoire de Grenoble MPIfR Max Planck Institute fЭr Radioastronomie OAA Osservatorio Astrofisica di Arcetri

VERY LARGE TELESCOPE

AMBER Spectrograph Lower Level Specifications
Doc. No.: VLT-SPE-AMB-15830-20000-0001 Issue : 2.0 Date : 28/01/02

Prepared:

S. Lagarde .............................................................
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Approved: .............................................................
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Released: .............................................................
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Change record
Issue
1.0

Date
06/03/2000

Section/Page affected
All

Reason/ Initiation/Remarks
First issue

2.0

02/11/2000

Chapter 3

Specification on the number and the sizes of the slits of the INW Specification on the number and the shapes of the pupil masks Only three dispersion elements are necessary All reference about the interface between SPG and the other sub-systems are described in the "AMBER interface control document"

Chapter 4

Chapter 6 All

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Table Of Contents
1 INTRODUCTION ............................................................. 6 1.1 1.2 1.3 1.4 2 PURPOSE ..................................................................................................................... ................................... APPLICABLE DOCUMENTS ................................................................................................................................. REFERENCE DOCUMENTS .................................................................................................................................. ABBREVIATIONS AND ACRONYMS..................................................................................................................... 6 6 6 6

GENERAL OVERVIEW ......................................................... 7 2.1 GENERAL FUNCTIONS ........................................................................................................................................ 2.2 GENERAL SPECIFICATIONS ................................................................................................................................ 2.2.1 Spectral coverage ................................................................................................................................... 2.2.2 Spectral resolution .................................................................................................................................. 2.2.3 Cryostat specification ............................................................................................................................. 2.3 GENERAL INTERFACE ............................................................................................................................... ......... 2.3.1 Beam configuration at the spectrograph entrance.................................................................................. 2.3.2 Detector size ........................................................................................................................................... 2.4 SPECTROGRAPH GENERAL SCHEMA AND MODULES FUNCTIONS ......................................................................... 7 7 7 7 7 8 8 8 8

3

IMAGING AND STOPPING DEVICE MODULE ...................................... 10 3.1 3.2
INTRODUCTION ................................................................................................................................................ SPECIFICATIONS...............................................................................................................................................

10 10

4

FILTERS AND PUPILS MASKS MODULE ......................................... 11 4.1 4.2
INTRODUCTION ................................................................................................................................................ SPECIFICATIONS...............................................................................................................................................

11 11

5

PHOTOMETRIC-INTERFEROMETRIC DEVICE MODULE ............................... 14 5.1 5.2
INTRODUCTION ................................................................................................................................................ SPECIFICATIONS...............................................................................................................................................

14 14

6

DISPERSION UNIT MODULE .................................................. 15 6.1 6.2
INTRODUCTION ................................................................................................................................................ SPECIFICATIONS...............................................................................................................................................

15 15

7

CHAMBER MODULE .......................................................... 15 7.1 7.2
INTRODUCTION ................................................................................................................................................ SPECIFICATIONS...............................................................................................................................................

15 15

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1 INTRODUCTION
1.1 PURPOSE

This document defines the specifications for the spectrograph. 1 .2 APPLICABLE DOCUMENTS

[1] VLT-ICD-ESO-15000-1826 Interface Control Document between VLTI and its Instruments [2] VLT-ICD-ESO-15000-1918 Functional Description of the VLTI

1 .3

REFERENCE DOCUMENTS

[3] VLT-PLA-AMB-15830-0001 AMBER Product tree [4] VLT-TRE-AMB-15830-0001 AMBER Instrument Analysis Report [5] VLT-SPE-AMB-15830-0004 AMBER Instrument Conceptual Design [6] VLT-TRE-AMB-15830-20000-0001 AMBER Spectrograph Design Report [7] VLT-ICD-AMB-15830-0001 AMBER Interface Control Document 1 .4 ABBREVIATIONS AND ACRONYMS

The following abbreviations and acronyms are used in this document: AMBER ANS DET OPM OSI SPE TBD TBC VLT VLTI Astronomical Multi-Beam Recombiner Anamorphosis System Detector module Optics and Mechanics module OPM-SPE Interface module Spectrograph module To Be Defined To be confirmed Very Large Telescope Very Large Telescope Interferometer

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2 GENERAL OVERVIEW
2 .1 GENERAL FUNCTIONS

The general functions of the spectrograph are the following : · spectral dispersion · thermal flux reduction · photometry calibration · beams focalization 2.2 GENERAL SPECIFICATIONS

2.2.1 Spectral coverage · · · · J Band : 0 1.25 µm 0.3 µm H Band : 0 1.65 µm 0.35 µm K Band : 0 2.2 µm 0.4 µm Total coverage 1.1 / 2.4 µm

2.2.2 Spectral resolution · No dispersion · R 35 · R 1000 · R 10000 These spectral resolutions are given for observations in the K Band. 2.2.3 Cryostat specification Cryostat Specification < 77°K Liquid Nitrogen 16 h (minimum) / 24 h (goal)

Temperature Cooling Autonomy

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2.3

GENERAL INTERFACE

2.3.1 Beam configuration at the spectrograph entrance The beam configuration at the spectrograph entrance is given in the "AMBER Interface Control Document" [7] 2.3.2 Detector size The camera is a 512 x 512 HAWAII array. The pixel size (p) is 18.5 µm x 18.5 µm. 2.4 SPECTROGRAPH GENERAL SCHEMA AND MODULES FUNCTIONS

Figure 1 shows a conceptual design of the spectrograph which contains six functional units :

INW FOC COL PMW SPF

SPG-ISD

SPG-FPM

SPG-IPS

SPG-DIU SPG-CHA

Figure 1 : conceptual design of the spectrograph of AMBER The cooling system Acronym : AMB-SPG-CSY Composition : - Cryostat (CRY) - CCT (Cryostat ConTrol ) Functions : - Spectrograph optics cooling - Thermal flux reduction The imaging and stopping device Acronym : AMB-SPG-ISD Composition : - Focalization mirror (FOC) - Input wheel with cold stop and diaphragm (INW) - Collimator (COL)

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Functions : - Calibration (Dark) thanks to the obturation position - Thermal flux reduction thanks to the cold stop position - Technical operation thanks to the wide diaphragm position The filters and pupils masks module Acronym : AMB-SPG-FPM Composition : - A pupils masks motorized wheel (PMW) - A fixed spectral filter (SPF) Functions : - Thermal flux reduction The interferometric photometric splitter Acronym : AMB-SPE-IPS Composition : - Beam splitter (BES) - Photometric Deflection Device (PDD) Functions : - Photometric calibration - Images positions on the detector The dispersion unit Acronym : AMB-SPE-DIU Composition : - Grating high resolution (GHR) - Grating low resolution (LHR) - Prism (PRI) - Motorized wheel (WHE) Functions : - Spectral dispersion - Spectral selection - Spectral modulation The chamber Acronym : AMB-SPE-CHA Composition : - First conic mirror (FCM) - Second conic mirror (SCM) - Flat mirror (FMR) Functions : - Beam focalization

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3 IMAGING AND STOPPING DEVICE MODULE
3 .1 INTRODUCTION

The two sources of noise are the detector read-out noise and the photon noise due to the thermal background which comes from the blackbody emission from the fiber heads. The following figures [4] show that the thermal noise can be greater than the RON especially for long time exposures and for H-Band and K-Band observations. It means that a cold stop is necessary to reduce the thermal noise. Thermal noise for UT H-Band 1000 100 (e-) 10 0.1 0.01 0.01 No filter R=35 R=100 R=1000 0.1 1 10 100 1000 0.01 0.1 1 10 100 1000 K-Band

Exposure time (s) Figure 2 : Thermal noise per spectral channel for UT in K-Band and H-Band without image cold stop (dotted line) and with image cold stop for different dispersion (solid line). The read-out noise is shown by a dashed line.

3.2

SPECIFICATIONS

The imaging and stopping device is composed by two off-axis parabolic mirrors (the focalization mirror and the collimator) with a focal length equal to 700 mm [6]. The chamber focal length is 350 mm (see chapter 7), so the size of the image at the focal plane of this mirrors is two times larger than at the detector level. A motorized wheel is inserted at the focal plan of these mirrors. This wheel contains 5 positions : · Position 1 : a single mode cold stop slit · Position 2 : a multi mode narrow slit · Position 3 : a multi mode large slit · Position 4 : a wide diaphragm · Position 5 : an obscuration position

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The size and the accuracy of positioning of each positions of this input wheel are summarized in the following table : single mode cold stop slit 2 multi mode narrow slit 4 multi wide obscuration mode diaphragm position large slit

Position

Size Width A

(in equivalent 16 >512 >512 detector pixel unit) >19 mm >19 mm (in µm or mm) 74 µm 148 µm 592 µm 118 118 118 118 >512 Length (in equivalent detector pixel unit) B >19 mm (in µm or mm) 4366 µm 4366 µm 4366 µm 4366 µm Size 5 A (in µm) Accuracy 10 B (in µm) 10 Positioning A (in µm) Accuracy 100 B (in µm) 300 Focus (in µm) Orientation (in °) 0.25 Table 1 : Size and accuracy of positioning of each positions of the input wheel of the spectrograph

4 FILTERS AND PUPILS MASKS MODULE
4 .1 INTRODUCTION

The main function of the filter and the pupils masks is to reduce the background noise due to the thermal emission of the warm optics outside the cryostat. Some other filters placed before the spectrograph cryostat allow also to avoid the superposition of the spectral orders in the case of a spectral dispersion with a grating. 4.2 SPECIFICATIONS

The specification for the spectral filter is to eliminate the wavelengths higher than 2.4 µm The pupils masks wheel at the pupil plan of the spectrograph must be motorized to select 8 positions :

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Position Pupils masks Position 1 3T K Spatial Filter Position 2 3T KHJ Spatial Filter Position 3 2T K Spatial Filter Position 4 2T KHJ Spatial Filter Position 5 3T Wide Field Position 6 2T Wide Field Position 7 Technical position Position 8 Free position Table 2 : Pupils masks corresponding to the height positions of the PMW Pupils masks shapes for each position

Position 2 Position 3 : Position 4 : Position 5 : Position 6 : Position 7 : Position 1 3T KHJ SF 2T K SF 2T KHJ SF 3T WF 2T WF Tech pos. 3T K SF Figure 3 : Pupils masks shapes corresponding to seven positions of the PMW Pupils masks dimensions In order to relax the accuracy of reproducibility of the wheel, the masks are oversized by a factor 1.05. It generates an increase of the thermal noise leading to a loss of 0.06 magnitude in the K Band. Mask Y Pupil Size in (mm) DX DY K Band 2.7 42 H Band 2.05 32 J Band 1.6 25 Table 3 : Pupil mask dimensions for the spatial filter mode in different spectral bands. X

DY Figure 4 : Pupil masks dimension. The size of the mask is 1.05 time greater than the pupil. DX

Size in Wide Tech Table 4 for the

(mm) DX DY Field 1.3 20 pos. 22 22 : Pupil mask dimensions wide field mode and the technical mode

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Pupils masks localization

Y

Figure 5 : Positions of the centers of each pupil for the spatial filter mode. The reference origin O represents the middle of the furthest spatial filter beams in K band.

O

X

XK3 XH3 XJ3 XJ2 XH2 XK2

XJ1 XH1

XK1

XK1 XH1 XJ1 XK2 XH2 XJ2 XJ3 XH3 XK3 Distance in (mm) 5 2.917 1.458 -1.667 -2.083 -2.375 -4.292 -4.583 -5 Table 5 : Position of the center of each pupil mask to the reference origin (middle of the 2 furthest beams in K band) for the spatial filter mode

Y

Figure 6 : Positions of the centers of each pupil for the spatial filter mode. The reference origin O represents the middle of the furthest spatial filter beams in K band.

O

X

XWF

3

XWF XWF
2

1

XTP

XWF1 XWF2 XWF3 XTP Distance in (mm) 5 -1.667 -5 0 Table 6 : Position of the center of each pupil mask to the Reference origin for the wide field and technical modes

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Accuracy of positioning of the pupils masks wheel In order to optimize the flux (to avoid beam obstruction), the accuracy of positioning the wheel is : Y X Accuracy of positioning +/- 500 µm +/-30 µm Table 7 : Accuracy of positioning of the filters and pupils masks wheel

5 PHOTOMETRIC-INTERFEROMETRIC DEVICE MODULE
5 .1 INTRODUCTION

In spatial filtering mode, it is necessary to monitor the photometric fluctuations of each beam to reach a good visibility accuracy. In order to simplify the interface between OPM and SPG (only one beams triplet at the entrance of the spectrograph) the separation between photometry and interferometry channels is inserted into the spectrograph cryostat, just before the grating. 5.2 SPECIFICATIONS

Flux ration between the interferometric and photometric beams A computation has been made to evaluate the best ratio between the photometric and interferometric channels [4]. 35% of flux for the photometric channel and 65% for the interferometric channel is a good trade-off. Positions and accuracy of positions of each image on the detector The four positions of each image on the detector are the following : P1 : 448 spatial direction
Figure 7 : Positions of the three photometric images and the interferometric image on the detector.

I : 320 P2 : 192 P3 : 64 spectral direction

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Position of the image Accuracy of positioning (in pixel) center (in pixel) Spatial direction Spectral direction Photometric image 1 448 3 3 Interferometric image 320 3 3 Photometric image 2 192 3 3 Photometric image 3 64 3 3 Table 8 : Position and accuracy of position of the images on the detector

6 DISPERSION UNIT MODULE
6 .1 INTRODUCTION

The general function of this module is to disperse the fringes. Additionally for the differential mode observations, it will be interesting to be able to shift the images on the detector in the spectral direction. This spectral modulation allows to obtain a measurement of the derivative of the fringe phase variation free of instrumental defects and to be only affected by photon and detector noise. 6.2 SPECIFICATIONS

Spectral resolution · Low resolution, R 35, spectral sampling 0/ 70 ( analyzed by one pixel) · Medium resolution, R 1000, spectral sampling 0/ 2000 ( analyzed by one pixel) · High resolution, R 10000, spectral sampling 0/ 20000 ( analyzed by one pixel) These spectral resolutions are given for observations in the K Band. Accuracy of positioning of the grating wheel The gratings and prism support must be motorized to be able to select the spectral resolution. The specification of the angular accuracy of these support is about 10 pixels on the detector, but if an accuracy less than 1 pixel is possible it could allow to perform the spectral modulation and to simplify the observation procedures (no spectral calibration and no modification of detector reading zone after each spectral set-up change).

7 CHAMBER MODULE
7 .1 INTRODUCTION

In order to implement the ABCD algorithm for the data reduction, it is necessary to sample the fringes produced by the more distant beams (152 mm of separation for the K Band) on 4 pixels. 7.2 SPECIFICATIONS

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To sample the fringes on 4 pixels, the chamber focal length must be at least : f = 4p / a (0/BMAX) where p is the detector pixel size (18.5 mm), a the anamorphosys factor (15.6), 0 the wavelength (for example 2.2 microns for the K band), and BMAX the maximal value of the beams separation (for example 152 mm for the K band). It should be noted that the ration 0/BMAX is the same for each spectral band. f 336 mm If the chamber focal length equals to 350 mm, the consequence on the sampling is the following : J Band 1.1 1.25 1.4 3.6 4.1 4.6 H Band 1.5 1.65 1.8 3.8 4.2 4.5 K Band 2 2.2 2.4 3.8 4.2 4.5

Wavelength (µ m) Number of pixels per fringe Table 10 : Number of pixels per fringe produced by the furthest beams for different wavelengths if the chamber focal length equals to 350 mm.

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