ESPRESSO. Preliminary Specifications for the Dichroic. VLT-SPE-ESP , Issue 5.0 October 17 th, 2013

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1 Centro de Astrofísica da Universidade do Porto Universidade de Lisboa, CAAUL and LOLS INAF, Osservatorio Astronomico di Trieste INAF, Osservatorio Astronomico di Brera Observatory of the University of Geneva Physics Institute, University of Bern Instituto de Astrofísica de Canarias European Southern Observatory ESPRESSO Preliminary Specifications for the Dichroic VLT-SPE-ESP , Issue 5.0 October 17 th, 2013 Prepared Ana B. Fragoso October 17, 2013 Name Date Signature Approved Name Date Signature Released F. Pepe Name Date Signature

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3 VLT-SPE-ESP , Issue 5.0 3/20 Change Record Issue/Rev. Date Section/Page affected Reason/Remarks /03/2013 First version /03/2013 Coating on the DC front face / 12 Coating on the DC rear face/ 12 Shape and size / 13 Transmission for the red range is specified Wavelength range is modified Misprint /07/2013 Optical figure / 10 Clear Aperture/12 Coating / 12 Wedge/13 Refractive Index/14 Environmental Requirements/15 Packaging, Handling, Storage and Transportation Requirements/16 Radii of curvature changed CA changed Details in coating design Wedge is included in new design Data of index is included /10/2013 Coating on the DC rear face/12 Material/14 Glass homogeneity/15 Homogeneity is relaxed /10/2013 Applicable Documents/7 Reference Documents/7 Table of Contents Chapter 1. Introduction Scope of the Document Documents Applicable Documents Reference Documents Acronyms and Abbreviations Acronyms...7 Chapter 2. Functional description...9 Chapter 3. Preliminary Specifications Functional Requirements Optical Requirements Optical figure Surface form tolerance of the optical surfaces Surface shape accuracy Surface slope error... 11

4 4/20 ESPRESSO Project Surface micro-roughness Surface Imperfections Coating Coating on the DC front face Transition profile (coating on front face) Coating on the DC rear face Resistance to cleaning Physical Requirements Clear Aperture Shape and size Central thickness Wedge Conic bevel finishing Material Refractive Index Bubble classification Striae grade Glass homogeneity Stress birrefringence Mechanical Performance Requirements Long term stability Reliability Requirement Lifetime guideline Environmental Requirements Packaging, Handling, Storage and Transportation Requirements Transport obligations Handling and storage requirements...16 Chapter 4. Cleanliness Requirements Cleanliness...17 Chapter 5. Appendix A Appendix B Verification at factory...17 : Descriptive drawing...19 : Opto-mechanical Concept...20

5 VLT-SPE-ESP , Issue 5.0 5/20 List of Figures Figure 1. Spectrograph optical layout (TBU)...9 Figure 2. Footprint onto DM (Blue Arm). Colour rays by wavelengths...9 Figure 3. Footprint onto DM (Red Arm). Colour rays by wavelengths...10 Figure 4. DC wedge. Vectors for each surface are indicated in red. YZ view...13 Figure 5. Direction of the reference axis...14 Figure 6. Mount concept...20 List of Tables Table 1. Refractive Index...14 Table 2. Environmental conditions (TBC)...16 Table 3. Handling and Storage Conditions at IAC facilities...16 Table 4. Handling and Storage Conditions at Geneve...16 Table 5. Handling and Storage Conditions at Paranal Observatory...17 Table 6. Verification Matrix...18

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7 VLT-SPE-ESP , Issue 5.0 7/20 Chapter 1. Introduction 1.1 Scope of the Document Manufacturing specifications for the ESPRESO Dichroic are established in that document. They are mainly following the ISO Standards. If some other Standard were used, it will be explicitly mentioned. Comments from possible suppliers and expert people have been taken into account in the specification of the element. 1.2 Documents Applicable Documents AD-1 ESPRESSO DC Drawing VLT-DWG-ESP SP B Reference Documents RD Acronyms and Abbreviations Acronyms AD Applicable Document ADC Atmospheric Dispersion Corrector AOI Angle Of Incidence APSU Anamorphic Pupil Slicer Unit AR Anti-Reflective BC Blue Camera BDS Blue Detector System BTM Blue Transfer Mirror BXD Blue Cross Disperser CA Clear Aperture CCD Charge Coupled Device CCL Combined Coudé Laboratory CIU Calibration Injection Unit CR Coudé Room CT Coudé Train CU Calibration Unit DC Dichroic EE Exit End EG Echelle Grating EPT ESPRESSO Project Team at the IAC

8 8/20 ESPRESSO Project ESO European Southern Observatory ESPRESSO Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations ExpM Exposure Meter FEU Front End Unit FL Fiber Link, Field Lens FPCS Fabry-Perot Calibration Source FSU Field Stabilization Unit FT Feed Through GP Gerardo Periscope IAC Instituto de Astrofísica de Canarias IE Input End LDLS Laser Driven Light Source LFC Laser Frequency Comb MC Main Collimator OB Optical Bench PSU Pupil Stabilization Unit RC Red Camera RD Reference Document RDS Red Detector System RFM Red Folding Mirror RFU Refocusing Unit RTM Red Transfer Mirror RXD Red Cross Disperser SP Spectrograph SU Calibration Selector Unit TBC To Be Confirmed TBD To Be Defined/To Be Developed TBU To Be Updated ThAr Thorium Argon TS Toggling System, Thermal System UT Unit Telescope VLT Very Large Telescope VV Vacuum Vessel XD Cross Disperser

9 VLT-SPE-ESP , Issue 5.0 9/20 Chapter 2. Functional description ESPRESSO spectrograph optical design is based onto a white-pupil asymmetric configuration. A large off-axis parabolic mirror acts as main collimator and is used in double-pass: to illuminate a large mosaic echelle grating (ESPRESSO main disperser) and to collect dispersed light towards transfer optics. Once the light is dispersed by the echelle, a dichroic beam-splitter mirror, located near an intermediate focal plane, splits light into two optimized arms: Blue and Red. Each arm consists of re-imaging optics, specific cross-disperser VPH grism, and a refractive lens camera. The spectrograph optical layout is shown in figure below just as to give an idea of the orientation and position of the elements. The sketch is not representative of final dimensions or sizes. DC g Figure 1. Spectrograph optical layout (TBU) Scale: Millimeters Footprint onto DC is shown in figure below. Aperture Full X Width : Aperture Full Y Height: Figure 2. Footprint onto DM (Blue Arm). Colour rays by wavelengths

10 ESPRESSO Project Scale: Millimeters 10/20 Aperture Full X Width : Aperture Full Y Height: Figure 3. Footprint onto DM (Red Arm). Colour rays by wavelengths Chapter 3. Preliminary Specifications 3.1 Functional Requirements ESPRESSO Dichroic shall reflect wavelength in the blue range (from 380 nm to 515 nm) and transmit the red range (from 535 nm to 780 nm). It will be provided without any kind of cell or mounting. The corresponding Positioning Mechanism will be designed and provided by IAC (see the concept in Appendix B). Interface area with the mechanical parts (see drawing in Appendix A) shall be completely free of coating or any other material. 3.2 Optical Requirements Source: ESPRESSO Optical Group Optical figure The optical figure shall be: Surface Front face Back face 1 Test wavelength: nm Radii of Curvature Infinity mm Tolerance ± 1 fringes1 ± 50 mm Flat Spherical

11 VLT-SPE-ESP , Issue / Surface form tolerance of the optical surfaces The irregularities of the optical surface to which the requirements shall apply includes irregularities due to the following sources: Residuals of the polishing process. Gravitational deformation (see orientation in Figure 1). Thermal deformation within nominal environment conditions (see section 3.6 ) and for testing temperature to operation temperature. Temporal drift Surface shape accuracy The effect of the irregularity of the optical surface of DC on the reflected and transmitted wavefront shall be: < 150 nm P-V, over sub-apertures of 90 x 60 mm. < 40 nm rms, over sub-apertures of 90 x 60 mm Surface slope error 2 Surface slope error is specified for both optical surfaces. Maximum slope shall be: < 1.3 arcsec P-V, up to a spatial frequency of 2 mm. < 0.25 arcsec rms, up to a spatial frequency of 2 mm. NOTE: This requirement means that considering an integration lenght of 2 mm, the maximum P- V value of the slope error shall be < 1.3. arcsec (angle) and the rms value of the slope error shall be < 0.25 arcsec (angle). It complements the surface form tolerance requirement, being the first one for midspatial frequency errors Surface micro-roughness The optical surface shall be polished to a residual surface roughness of 2nm (rms) or better over the whole CA. The way in which the Micro-roughness is measured, shall be proposed by Contractor and approved by EPT. NOTE: Surface roughness of the non-optical surfaces shall be considered and agreed with EPT so as to assure the suitability for the Positioning Mechanism Surface Imperfections According to MIL-C-48497, the surface imperfections of the optical surfaces shall be 40/20. 2 Surface slope error is specified to control the non-uniform deviations in midspatial frequencies

12 12/20 ESPRESSO Project Coating Source: ESPRESSO PI Coating design will be provided by the supplier to be revised and approved by the EPT before manufacturing Coating on the DC front face DC front face (flat) shall be dichroic coated ensuring: Reflectivity over 97% (goal 99%) from 380 nm to 515 nm. Transmission over 98% (goal 99%) from 535 nm to 780 nm Maximum AOI is 14 degrees. (TBC) Transition profile (coating on front face) Nominal transition wavelength is 525 nm. A sharp transition from reflection to transmission is required. Transition shall be < 10 nm (goal 6 nm) from 95%R to 0.5%R, and < 20 nm (goal 10 nm) from 98%R to 0.2%R (TBC) Ripples shall be minimized in amplitude and frequency. They shall be < ±0.5% of average efficiency (reflexion or transmission) and < 1% per Angstrom (TBC) Coating on the DC rear face DC rear face (spherical) shall be AR coated. AR coating transmission shall be over 99% (goal 99.5%) from 380 nm to 780 nm. NOTE: All coating imperfections shall be included in the allowable general surface imperfections and roughness indication Resistance to cleaning The DC coating shall not be adversely affected by materials and procedures used when cleaning the surfaces with: CO2 snow, peel off methods and wet cleaning methods. If some special procedure is required it shall be indicated by DC supplier. 3.3 Physical Requirements Clear Aperture Source: ESPRESSO Optical Group DC Clear Aperture dimensions shall be: 455 x 80 mm 2 (centered)

13 VLT-SPE-ESP , Issue / Shape and size Source: ESPRESSO Optical Group Shape and size of the optical surface shall be rectangular. Dimensions: 460 x 90 mm 2. Tolerance: ±0.010 mm Central thickness Source: ESPRESSO Optical Group DC central thickness shall be 30 mm. ±1 mm. NOTE: DC thickness shall assure the optical and mechanical requirements. EPT shall be informed if any requirement is compromised by this small thickness Wedge Source: ESPRESSO Optical Group DC shall have a wedge of 0.5 degrees in YZ section (see Figure 4). That means that the angle between the curvature radius of the rear surface and the flat surface shall be βx=95degrees (rotation around X axis). (See Figure 4 and Figure 5) Y Z Figure 4. DC wedge. Vectors for each surface are indicated in red. YZ view

14 14/20 ESPRESSO Project Y X Figure 5. Direction of the reference axis Tolerance in wedge shall be < 60 arcsec (in any direction) Conic bevel finishing Source: EPT All the edges shall have protective chamfers from 1 to 1.5 mm width; the angle to axis shall temper the transition from the optical surface to the edge surface Material Source: ESPRESSO Optical Group Ohara BSL7Y or equivalent (Schott BK7HT). NOTE: Glass selection shall be proposed by supplier and approved by EPT Refractive Index Source: ESPRESSO Optical Group. Refractive index of the glass shall be measured and reported. Data shall be taken at 22ºC at nm. and nm. (see table below) Wavelength BSL7Y Refractive Index nm nm Table 1. Refractive Index Bubble classification Source: EPT Bubble classification of Ohara BSL7Y: According to Ohara Bubble Group B.1.

15 VLT-SPE-ESP , Issue / Striae grade Source: EPT Striae grade of Ohara BSL7Y shall be A per MIL-G-174B (no visible striae) Glass homogeneity Source: EPT Optical homogeneity of Ohara BSL7Y shall be < 1 x 10-5 (TBC). Grade Special A10 in OHARA standards. It shall be guaranteed through thickness (460 x 90 face) Stress birrefringence Source: EPT Stress birefringence of Ohara BSL7Y shall be better than 10 nm/cm. 3.4 Mechanical Performance Requirements Long term stability The optical surfaces shall be stable during lifetime. If temporal drift is produced due to the coating material, the effect shall be considered as an additional source of irregularity of the optical surface, which shall be limited. 3.5 Reliability Requirement Lifetime guideline Source: EPT DC should be designed for a minimum lifetime of 15 years under the environment conditions specified in Table 2. This lifetime start at first integration on ESPRESSO spectrograph. 3.6 Environmental Requirements Source: ESPRESSO Team The DC will operate inside a vacuum chamber. It shall survive under the environmental conditions stated in Table 2 (TBC). The DC requirements shall be fulfilled under the operation conditions.

16 16/20 ESPRESSO Project Lab conditions Vacuum conditions (alignment) (operation) Survival limit Temperature +20 C +17 C +35 C Thermal variation 2 C 1 mk 1 C/h Relative humidity ~40% (controlled) N/A 0% to 100% with condensation Atmospheric pressure ~1 atm ~10-3 mbar (TBC) N/A Table 2. Environmental conditions (TBC) 3.7 Packaging, Handling, Storage and Transportation Requirements Source: EPT Transport obligations The Contractor shall be responsible, in cost and risk, to transport the equipment manufactured from its facilities to the IAC facilities. The package shall be designed to support normal air and sea transport condition. The package shall be also designed to be used for preventive maintenance tasks and in case of reparation. So, the package shall be designed to support at least 10 packing and 10 unpacking operation keeping all its performances Handling and storage requirements The design of the items and packages shall prevent them from being damaged under the conditions shown in tables below. Condition Requirement Altitude 500 m Temperature +15 C to +30 C Relative humidity 0% to 100% with condensation Atmospheric pressure ~ 1atm Gravity orientation All orientations Shock Peak acceleration 10g all axes Table 3. Handling and Storage Conditions at IAC facilities Condition Requirement Altitude 400 m Temperature +20 C ± 3 C Relative humidity 20% to 100% Atmospheric pressure ~720 mbar Gravity orientation All orientations Shock Peak acceleration 10g all axes Table 4. Handling and Storage Conditions at Geneve

17 VLT-SPE-ESP , Issue /20 Condition Requirement Altitude 2600 m Temperature -8 to 25 C Temperature gradient during night -0.4 C/h Relative humidity 5-20% Atmospheric pressure 750 mbar Gravity orientation All orientations Moderate earthquakes (some times per year) Mg < 7.75 Shock Peak acceleration 10g all axes Table 5. Handling and Storage Conditions at Paranal Observatory Chapter 4. Cleanliness Requirements 4.1 Cleanliness Source: EPT DC shall be delivered clean: surface cleanliness (according to MIL-STD-1246). The optical surface shall be able to be cleaned at the surface cleanliness level cited above, without damaging the coating applied. The contractor shall provide a procedure for cleaning the grisms. Chapter 5. Verification at factory Source: EPT Verification shall be accomplished by one or more of the following verification methods: Test (T): When requirements have to be verified by measuring product performance and functionality. The analysis of data derived from test shall be considered an integral part of the test. Demonstration (D): Can be considered as test where qualitative operational performance and requirements are demonstrated. Analysis (A): When verification is achieved by performing theoretical o empirical evaluation by accepted techniques, the method shall be referred to as Analysis. An example is the modelling and computational simulation. Inspection (I): When verification is achieved by visual determination of physical characteristics (such as construction features, hardware conformance to document drawings, etc) the method shall be referred to as Inspection. The Verification Matrix (VM) shows the methods that shall be used to accept each one of the critical requirements. NV means No Verification is needed.

18 18/20 ESPRESSO Project Code Header Ver. Method Remarks 3.1 Functional Requirements I 3.2 Optical Requirements Optical figure T Surface form tolerance of the optical T surfaces Surface shape accuracy T Surface slope error T Surface micro-roughness T Surface Imperfections T Coating T Coating on the DC front face T Coating on the DC rear face T Transition profile T Resistance to cleaning T 3.3 Physical Requirements Clear Aperture T Shape and size T Central thickness T Wedge T Conic bevel finishing I Material T Refractive Index T Bubble classification T Striae grade T Glass homogeneity T Stress birrefringence T 3.4 Mechanical Performance Requirements Long term stability NV 3.5 Reliability Requirement Lifetime guideline NV 3.6 Environmental Requirements T 3.7 Packaging, Handling, Storage and Transportation Requirements Transport obligations I Handling and storage requirements I 4.1 Cleanliness D Table 6. Verification Matrix Information on material properties shall be provided by the glass supplier

19 VLT-SPE-ESP , Issue /20 Appendix A : Descriptive drawing NOTE: Wedge is not indicated in this drawing. It will be explicitly included in final version of the specifications.

20 20/20 ESPRESSO Project Appendix B : Opto-mechanical Concept The optics is hold onto a structural steel frame by 3 points in a kinematic and athermal way. At the same time each frame is fixed by 3 points to the OB in a quasi-kinematic way. Figure 6. Mount concept