Telescope Wavefront Errors

Transcription

1 Telescope Wavefront Errors Henri Bonnet 1

2 Tasks of WFC at E-ELT Help System Engineering develop and maintain the technical budgets Develop Control Strategy Define WFC I/F to instruments. 2

3 How we do it Define a WFC plan, describing: how we phase M1 how we maintain the telescope collima;on how we reject the dynamic perturba;ons Evaluate error propaga;ons -> Simulate FEM Dynamic simula;ons Ray tracing of segmented model AO simula;ons Wide range of spa;al and temporal ;me scales No end to end simula;ons Simula;on tools are customized and interfaced to one another for each ques;on addressed by the team. 3

4 Performance Analysis Purpose & Process Identify Errors Dynamic Perturbations Wind Allocate Errors Heat Vibrations Write Requirements Gravity Structure Integration Design Optical Propagation Define Control Strategy Model Input Wavefront Atmosphere Mirrors Delivered Wavefront Noise Evaluate Error Propagation Turbulence Actuators Metrology Internal Sky Manufacturing Control Algorithms Model Errors Adaptive Optics 35Hz ES Loop 1 Hz Main Axes Loop 1 Hz Low Order Optimization 0.1 Hz Calibrations << 1Hz

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6 WFC Products Sensi;vity analyses Provides the connec;on between sub-systems requirements and error budget. Calibra;on and wavefront control baseline WF interfaces Requirements to control equipments Wavefront Sensors Metrologies (e.g. Edge Sensors) Actuators (stroke, resolu;on, ) 6

7 Differences VLT vs. E-ELT The wavefront delivered by the VLT is seeing limited: Wavefront errors created by the telescope are con;nuous and slow Always a minor distor;on to the power spectrum of the free atmosphere. Outstanding excep;on = vibra;ons 10 to 15 mas rms of ;p-;lt at a few harmonic frequencies. VLT has few sensors and actuators Failure of an equipment = down ;me E-ELT is not like this. 7

8 Wind shake: Al;tude Structure From VLT to E-ELT: Larger structure Lower eigen frequencies Higher sensi;vity to dynamic perturba;ons K band diffrac;on limit from 56 to 12 mas. Large effort invested during all design phases Driver to Main Structure requirements 2 stage control strategy (M4 + M5) with enhanced rejec;on at low frequency Resulted in sa;sfactory performance 1.6 mas rms in standard condi;ons E-ELT is VLT-like in this respect. 8

9 Wind Shake: M1 9

10 Telescope collima;on M2 High op;cal sensi;vity Large iner;a Resolu;on of posi;oning system incompa;ble with AO performance Phase B design conducted under constrained that M2 would not be reposi;oned during observa;on (1 hour). Studies concluded that this was not doable L1 requirement relaxed to 1 reposi;oning every 5 minutes. Performance and stroke budgets now ok 20 mas (post AO) transient at low order op;miza;on. 10

11 Plate Scale / field rota;on The goal is that CCS holds the plate scale for 1 hour. Field rota;on may not be predictable at the diffrac;on limit. => Instruments may need to incorporate secondary guiding for plate scale / field rota;on. 11

12 M1 12

13 Segment Misfigure Polishing Polishing Requirements (E-SPE-ESO ) Warping Harness Model and FiDng Error 1 3 First polished prototype at SAGEM Higher orders: 1/f 2 PSD Prototype measurement 2 Simulated samples E-ELT Phase B Final Review, September 22 nd 2010 Slide

14 Phasing Phasing procedure demonstrated at GTC. Baseline: update of phasing solu;on every 2 weeks. Local metrologies (Edge Sensors) maintain the phasing between calibra;ons 14

15 þ þ þ þ þ GTC Data Measured OPD (nm) 3 Op;cal E-ELT Phasing Programme Ray Tracing Model Data Offline Machine Valida;on at GTC with ESO technical ;me Template Online processing Fringes Post-Processing FTP Guiding (manual) FTP Modus Operandi GTC performance es;ma;on (July 2010) Ømeasurement noise ~ 40nm ØOPD residual ~40nm 200 EELT Data Telescope Error Budget alloca;on Ø based on 100nm OPD measurement noise Ø WFE compensated by SCAO Ø Ø Sequencer Low order offloaded from M4 Residual WFE = 35nm Sensitivity Matrices Alignment Reference

16 Scalloping Scalloping is the result of a large focus error in M1 compensated elsewhere (e.g. with M2). Consequence: mismatch between radii of curvature of Segments Segments assembly High order wavefront error with first order discon;nui;es. Scalloping budget ~35nm but this is considered a technical risk. Risk mi;ga;on: Make Edge Sensors sensi;ve to M1 focus mode (PSG sensors). Guide Probe WFS capable of observing scalloping at preset. 16

17 Segment in plane displacements Correc;ons amplitude Piston Shear Gap Edge Sensors Op;cal calibra;on (Phasing) every 2 weeks PSG features Ø Ø 1mm / 1mrad Gravity Full observability of mirror state Ambient Performance against gravity and thermal perturba;ons Temperature limited by moun;ng errors. Installa;on Accuracy Ø Integra;on 4 nm 8 nm 2 nm 6K 2 weeks 4K 6 nm 1K/42m/axis 3 nm Temperature gradient Total Gravity: Zenith to 45deg Driven by capture range budget Shear Gap compensation OFF: 100 nm Shear Gap compensation ON: 10 nm Impact of differenqal gravity load from zenith to 45 deg 1 mrad clocking error on segment #429 SCAO seasonal Rota;on Error = 1 mrad Transla;on Error = 100 µm Warping Harness from FEA To be measured with an accuracy of 0.1 mrad Ø Error Op;cal Phasing Impact of 20K ambient temperature change 11 nm

18 Phasing Difficul;es: Large number of DOF In plane mo;ons (PSG sensors) Coupling between in-plane mo;on and ES signals. New segments every day (re-coa;ng) Shape of segments behind the spider poorly observable Local vibra;ons => Locally large discon;nui;es in the wavefront. Diffrac;on effects in WFS? 18

19 Technical Note Impact of spider obstructions on phasing Spider Doc. Number: ESO Doc. Version: 1.1 Modified on: Prepared by: Page: 7 of Results Shape of segments hidden by spider are poorly observable Surface discon;nui;es at the edges of these segments Propaga;on of phasing error Spider width = 530mm > r 0 Fragmenta;on of AO pupil For each case 10 random realization were made with 5 phasing iterations for each case. Figure 5 shows an exemplary phasing solution for each of the two cases. Figure 5: Exemplary phasing solution for Case A (left) and Case B (right). 19

20 Opera;onal incidents ES / PACT failures Missing segments 20