The Active Optics System

S. Thomas and the AO team

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The Active Optics System

Wavefront Sensor Estimation (WEP)

Active Optics Control System

• Maintain system alignment • Maintain surface figure on three mirrors

M1M3 Controller

M2 Controller

M2 hexapod Controller

Camera hexapod Controller

Camera DAQ for WFS images

Science images from DM?

For each detector:- Instrument Signature Removal- Source Selection- Masters computation- Compute the Zernike

Coefficients

Zernike Coefficients

for each detectors

Crosstalk corrected

images

Crosstalk corrected

images

- Optimal estimator- Control algorithm using a

cost function

Catalog Calibration Product

Focal plane map Sensitivity matrix Prior knowledge of the

system Bending modes

Forces

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WEP

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Curvature sensing enables significant flexibility in selecting sources due to the large field of view of the area sensorssplit sensors: because of the fast f-number (f/1.23) and crowded focal plane, using a beam splitter and delay line or physically moving the detector will not work.

Use multiple sources to increase S/N, to help average out atmosphere noise, and to alleviate problems due to vignetting.

LSST WFS challenges- 61% Central Obscuration- f/1.23- Off-axis Distortion & Vignetting (~1.7o)- Field Dependence (covering 1.51° to 1.84°)

The defocus for the WFS detectors is +/- 1mm (TBC)

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WEP Status- Publication of the update to the basic curvature

sensing algorithm by Bo Xin (large central obscuration, small f/number, distortion and vignetting)

- Requirement document close to completion- Understand interfaces- Understand calibrations

- WEP translated from Matlab to Python- Start to include the WEP in the DM stack

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On sky testsChuck Claver and Bo Xin had a one-night observing run on Magellan (3/30/2015)

• To test LSST curvature wavefront sensing software • IMACS f/2, SLOAN r filter, 26.5% central obscuration• Piston secondary mirror to get defocused donut images

• Nominal setup: 3.5mm image defocus.• Analysis of wavefront was done in real time.• Good agreement in general between system input and the curvature wavefront estimates

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AOCS Status

• An algorithm written in Matlab exists and Paul Lotz will be in charge of the translation in Labview.

• Include the telescope models and use PhoSim to built the look-up-table (temperature, elevation and azimuthal angle).

• “closed-loop” done using Zemax simulations and bending modes of the mirrors

• Need to close the loop using PhoSim

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Data Quality Control

• In the WEP:– Source Selection– Master

Consequence: The Zernike coefficient for the specific wavefront sensor is truncated.

• In the AOCS:– Check forces sent to the various controllers

Consequence: TBD?

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Future Work

• Write the ICDs for the different mirrors and hexapods, and also with Data Management

• Include the telescope models in the look-up-table (temperature, elevation and azimuthal angle).

• Include the AOS code in the DM software stack (one first step is to add the code on github)

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Issues

- Should we send the offsets to the mirrors or the actual forces

- Need to update the interface documents (to the mirrors and hexapods)

- How to make this system automatic and robust?

- Still a few unknown, some of which we won’t know before going on sky (such as how often do we want to update the forces)

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Wavefront sensor camera

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ICD provides specifications and tolerances:• Offset distance for intra/extra focal sensors

+/- 2mm??• Flatness

10 microns rms• Placement wrt science FPA

+/- 20 microns in along z-axis• Sensor noise and QE

Matches science sensors• Useable effective sensor area

80 arcmin2 x two halves• Data exchange and format

Image quality drives AOS related requirements

• Science Requirements Document (LPM-17)LSST System Requirement (LSE-29)– Mandates 0.4 arcsec FWHM from the system (Telescope + Camera)– Plate scale control not required

• Observatory System Specifications (LSE-30)– Optical prescription– Allocates 0.30 arcsec to the Camera– Allocates 0.25 arcsec to the Telescope– Additional AOS error is captured fully in the Telescope allocation– Mean slew and settle time of 5 sec. between visits

• Telescope and Site Requirements (LSE-60)– AOS functional requirements– Alignment and Compensation

• Detailed Image budgets– Telescope & Site image budget (LTS-123, LTS-124)– Camera System (LCA-17)

TitleTMA Review • Tucson, Arizona • May 9-10, 2013 13

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Covariance Analysis

(Algorithmic + atmospheric)Covariancematrix

Total covariance is almost entirely dominated by atmosphere

– Diagonal elements (lower left) similar

– Singular values (lower right) also similar; some increase for low singular values

Variance

Singular values