Wavefront Correction System Specifications Document · Tom Berger Project Scientist T. Berger 16...

Project Documentation Document SPEC-0058

Rev C

Advanced Technology Solar Telescope 950 N. Cherry Avenue Tucson, AZ 85719 Phone 520-318-8102 [email protected] http://atst.nso.edu Fax 520-318-8500

Wavefront Correction System

Specifications Document

T. Rimmele1, K. Richards, L. Johnson, S. Barden, F.

Wöger WFC Group

14 May 2013

Name Signature Date

Approved By : Sam Barden

Optics Group Lead S. Barden 14 May 2013

Approved By: Simon Craig

Sr. Systems Engineer S. Craig 16 May 2013

Approved By: Tom Berger

Project Scientist T. Berger 16 May 2013

Approved By: Joseph McMullin

Project Manager J. McMullin 19 May 2013

Released By: Thomas Rimmele

Project Manager T. Rimmele 17 May 2013

1 Responsible Author.

Wavefront Correction Specification

SPEC-0058 Rev C Page i

REVISION SUMMARY:

1. Date: October 2006 – September 2011 Revision: Drafts 1-6 Changes: Initial document; minor edits; removed all references to Nasmyth; rewritten; reformatted WCCS section, added aO section; added to WCCS and aO sections.

2. Date: 8 September 2011 Revision: A Changes: Ready for release.

3. Date: April-June 2012 Revision: B Changes: Changes per CR-0164.

4. Date: May 2013 Revision: C Changes: Changes per CR-0367; remove boilerplate in Chapter 5.


SPEC-0058 Rev C Page ii

Table of Contents

1. OVERVIEW ........................................................................................................... 1 1.1 PURPOSE ............................................................................................................... 1 1.2 SCOPE ................................................................................................................... 1 1.3 DOCUMENT REVISIONS ............................................................................................ 1

1.4 RELATED SPECIFICATION DOCUMENTS..................................................................... 1 1.5 OTHER RELATED DOCUMENTS ................................................................................. 2 1.6 INTERFACE CONTROL DOCUMENTS AND DRAWINGS ................................................... 2 1.7 VERIFICATION METHODS .......................................................................................... 2 1.8 ABBREVIATIONS ...................................................................................................... 3

2. WORK BREAKDOWN STRUCTURE .................................................................... 4 2.1 WAVEFRONT CORRECTION ASSEMBLY DEFINITIONS................................................... 4

3. WFC REQUIREMENTS ........................................................................................ 5

3.1 WFC-OPERATIONAL MODES .................................................................................... 5

3.2 DELIVERED IMAGE QUALITY ..................................................................................... 5

4. WFC SUBSYSTEM REQUIREMENTS ................................................................. 6

4.1 HIGH ORDER ADAPTIVE OPTICS SYSTEM .................................................................. 6 4.1.1 Conventional HOAO ....................................................................................................... 6 4.1.2 HOAO Performance ....................................................................................................... 6 4.1.3 High Strehl Science Productivity ..................................................................................... 6 4.1.4 Functional Requirements ................................................................................................ 7 4.2 ACTIVE OPTICS (AO) ............................................................................................. 11 4.2.1 Overview .......................................................................................................................11 4.2.2 aO Performance Requirements .....................................................................................12 4.2.3 aO Functional Requirements .........................................................................................12 4.2.4 aO Status screen ...........................................................................................................13 4.2.5 Low Order WFS (LOWFS) .............................................................................................13 4.3 CONTEXT VIEWER ................................................................................................. 15 4.3.1 Update and storage rate ................................................................................................16 4.3.2 Wavelength ...................................................................................................................16 4.3.3 Field of view ..................................................................................................................16 4.3.4 Resolution .....................................................................................................................16 4.3.5 Pointing Reference ........................................................................................................16 4.3.6 Nighttime Pointing Maps ................................................................................................17 4.3.7 Laser Calibration Source ...............................................................................................17 4.4 NASMYTH UPGRADE .............................................................................................. 17 4.5 WAVEFRONT CORRECTION CONTROL SYSTEM ........................................................ 18 4.5.1 Overview .......................................................................................................................18 4.5.2 WCCS Requirements ....................................................................................................18 4.6 LIMB TRACKER ...................................................................................................... 19


SPEC-0058 Rev C Page 1 of 19

1. OVERVIEW

1.1 PURPOSE

High resolution observations of the solar atmosphere are a key science driver for the Advanced

Technology Solar Telescope (ATST) with diffraction limited observations of high Strehl being the

highest priority. The ATST wavefront correction system (WFC) is a centerpiece of the telescope facility.

The ATST will be operated in different WFC operational modes that flow directly from the Science

Requirements Document (SPEC-0001), and the Operational Concepts Document (SPEC-0036). Science

use cases for disk and coronal observations lead to a requirement for different WFC Operational Modes

(WFC-OPM), which flow from the SRD image quality specifications and the OCD and lead to the

Delivered Image Quality Error Budgets defined in the Systems Error Budgets specification (SPEC-0009).

In most cases, a WFC-OPM can be mapped to a Delivered Image Quality Error Budget.

The WFC-OPMs that shall be supported are:

WFC-OPM1: Diffraction limited on-disk observations at visible and infrared wavelength.

WFC-OPM2: Seeing limited on-disk observations at near infrared wavelengths.

WFC-OPM3: Seeing limited coronal observations at near infrared wavelengths.

For WFC-OPM 1-3 requirements for the WFC are flown from the WFC-OPM definition and the

corresponding DIQ error budgets of SPEC-0009.

Additional WFC-OPM flow from the Instrument Science Requirements Documents (ISRD). For example,

near limb coronal observations require occulting of solar disk light and stabilization of the image of the

solar limb onto a fixed occulting edge at Gregorian Optical Station (GOS). The CRYO-NIRSP ISRD

(SPEC-0056) defines top level requirements for this WFC-OPM.

WFC-OPM4: Near limb occulting with image stabilization.

Nasmyth upgrade: The Nasmyth focal station of ATST is not developed as part of the ATST baseline.

However, a requirement exists to allow for future development of the Nasmyth. It is very likely that

during the first years of operations Nasmyth instrumentation will be developed. M2 is required to provide

atmospheric tip/tilt correction for future Nasmyth instruments.

1.2 SCOPE

This document lists requirements for all elements contained within the WFC WBS-2.0.

1.3 DOCUMENT REVISIONS

Changes to this document have to be coordinated with the responsible author. Formal change orders shall

be required to implement modifications to this document once it has been placed under Configuration

Control. While in Revision Control, any changes will need to be approved, but a change order will not be

required.

1.4 RELATED SPECIFICATION DOCUMENTS

SPEC-0001 Science Requirements Document

SPEC-0036 Operational Concepts Document

SPEC-0009 System Error Budget



SPEC-0056 Cryo-NIRSP ISRD

SPEC-0005 Software and Controls Requirements

SPEC-0012 Glossary and Acronym List

SPEC-0013 Software Operation Concepts Definitions

SPEC-0014 Software Design Description

SPEC-0022 ATST Common Services Users’ Manual

SPEC-0024 Feed Optics

SPEC-0091 Coudé Station

SPEC-0023 Telescope Control System Design

SPEC-0027 Coordinate System Definition

SPEC-0029 ATST Optical Prescription

SPEC-0063 ATST Interconnects & Services Specification

TN-0094 Environmental Conditions on Haleakala

1.5 OTHER RELATED DOCUMENTS

RPT-0021 Site Survey Working Group Final Report

RPT-0030 Haleakala Cn2 and wind profile data

TN-0073 High Order Adaptive Optics System Reference Design Performance Modeling

1.6 INTERFACE CONTROL DOCUMENTS AND DRAWINGS

ICD 1.1/2.1, TMA to WFC

ICD 1.2/2.3, M1 Assembly to WCCS

ICD 1.3/2.5, TEOA to WFC-LT

ICD 1.3/2.3, TEOA to WCCS

ICD 1.5/2.3, Feed Optics to WCCS

ICD 2.1/2.3, WFC-C to WCCS

ICD 2.1/3.1.3, WCCS to Coudé Station

ICD 2.1/3.4.2, WCCS to Cryo-NIRSP

ICD 2.1/4.3, WCCS to DHS

ICD 2.3/4.4, WCCS to TCS

1.7 VERIFICATION METHODS

Included in each major numbered specification listed herein this document is a requirement verification

method. These verification methods specify the minimum standards of verification required to ensure that

the individual requirements and specifications are met.

Examples of verification methods include:

Design Review. Verification by design review shall mean that it shall be demonstrated at the design

review that the equipment shall meet the specification by way of its intrinsic layout and configuration.

http://atst.nso.edu/library/icds/ICD%201.1-2.1%20TMA%20to%20Coude%20Wavefront.pdf

http://atst.nso.edu/library/icds/ICD%201.2-2.3%20M1%20Assy%20to%20WCCS.pdf

http://atst.nso.edu/library/icds/ICD%201.5-2.3%20Feed%20Optics%20Assy%20to%20WCCS.pdf

http://atst.nso.edu/library/icds/ICD%202.1-2.3%20WCCS%20to%20WFC%20Coude.pdf

http://atst.nso.edu/library/icds/ICD%202.3-4.4%20WCCS%20to%20TCS%20Rev%20A.pdf



Analysis. Verification by analysis shall mean that it shall be demonstrated by the analysis that the

design meets the specification. Such analyses may include finite element methods, computational

fluid analyses, closed form analyses, etc.

Inspection. Verification by inspection shall mean it shall be demonstrated by visual inspection that the

specification has been achieved on the as-built equipment during acceptance testing.

Test. Verification by test &/or measurement shall mean that it shall be empirically demonstrated that

the as-built equipment meets the specification.

1.8 ABBREVIATIONS

aO Active Optics

HOAO High Order Adaptive Optics

WFS Wavefront Sensor

QSA Quasi Static Alignment

PSF Point Spread Function

Cn2(h) Index of Refraction Fluctuations Structure constant

FOV Field of view

DM Deformable Mirror

FTT Fast Tip/Tilt Mirror

r0 Fried Parameter

TMA Telescope Mount Assembly

DIQ Delivered Image Quality



2. WORK BREAKDOWN STRUCTURE

2.1 WAVEFRONT CORRECTION ASSEMBLY DEFINITIONS

The WFC shall be comprised of the following major systems, subsystems, and components. Specific

details, requirements, and specifications of these items are specified in more detail later in this document.

WBS 2.0 WFC

The WFC contains the M5 Tip Tilt Mirror and Module; the M10 Deformable Mirror; the Beam Splitter

(BS1); the High Order Wavefront Sensor (HOWFS); the Low Order Wavefront Sensor (LOWFS); the

Context Viewer (CV); and the Wavefront Correction Control System (WCCS). It also contains the future

Nasmyth implementation which is unfunded at this time.

WBS 2.1 WFC Coudé (WFC-C)

This includes the hardware, software, and electronics for the WFC Coudé implementation.

WBS 2.1.1 Adaptive Optics (AO)

This work package contains the adaptive optics components inclusive of: modeling,

optical design, the HOWFS, the Fast Tip Tilt system (M5), and the Deformable Mirror

System (M10)

WBS 2.1.2 Active Optics (aO)

This work package contains the LOWFS and the Quasi-Static alignment.

WBS 2.1.3 Context Viewer (CV)

This work package contains the Context Viewer channel.

WBS 2.1.4 Common Items (CI)

This work package contains items common to the full WFC system, including the optical

tables and electronics.

WBS 2.1.5 WFC Tools and Equipment (WFCTE)

This work package contains miscellaneous tools and equipment for support of the WFC.

WBS 2.2 WFC Nasmyth (WFC-N)

The WFC Nasmyth implementation is not funded at this time.

WBS 2.3 WCCS

The Wavefront Correction Control System.

WBS 2.5 Limb Tracker Control (LTC)

This work package contains items necessary for control of the Limb Tracker.



3. WFC REQUIREMENTS

3.1 WFC-OPERATIONAL MODES

2.0-0005: The WFC shall operate in the following WFC-OPM as defined in SPEC-0009:

1. WFC-OPM1: Diffraction limited on-disk observations at visible and infrared wavelength.

2. WFC-OPM2: Seeing limited on-disk observations at near infrared wavelengths.

3. WFC-OPM3: Seeing limited coronal observations at near infrared wavelengths.

4. WFC-OPM4: Limb occulting with limb stabilization

2.0-0010: The system architecture shall be flexible in order to allow future implementation of

additional WFC-OPM.

Source: SPEC-0001, SPEC-0009, SPEC-0036, SPEC-0056, engineering,

Verification: Design, Acceptance Test

3.2 DELIVERED IMAGE QUALITY

2.0-0015: Given the assumptions made at the time of design in the relevant delivered image quality

error budgets about magnitude and characteristics of input wavefront errors each WFC subsystem shall

provide wavefront correction consistent with the relevant error budget allocations stated in the relevant

ATST Delivered Imaging Error Budgets.

Source: SPEC-0001, DIQ error budgets, SPEC-0056

Verification: Design, Analysis, Performance Modeling, Performance measurements using telemetry

data

NOTE: The delivered image quality specifications flow down to the performance requirements for the

subsystems. The subsystems are: high order adaptive optics (HOAO), active optics (aO), which includes

the quasi-static alignment (QSA), the context viewer (CV), the Wavefront Correction Control system

(WCCS) and the limb tracker (LT) for the occulting.



4. WFC SUBSYSTEM REQUIREMENTS

4.1 HIGH ORDER ADAPTIVE OPTICS SYSTEM

4.1.1 Conventional HOAO

2.1.1-0005: ATST shall have a conventional (single conjugate, to pupil plane) adaptive optics system

for visible and IR wavelengths.

2.1.1-0005-GOAL: The system shall be designed in a way that allows future implementation of MCAO.

Source: SPEC-0001

Verification: Design

2.1.1-0010: Wavefront sensing shall be performed at visible wavelengths using a correlating Shack

Hartmann wavefront sensor.

Note: At the time of design the correlating Shack-Hartmann WFS working at visible wavelengths is the

only demonstrated approach for solar AO. Hence it is considered a requirement to use this approach.

Source: SPEC-0001, Engineering


4.1.2 HOAO Performance

2.1.1-0015: While operating in WFC-OPM1 the HOAO shall correct within the isoplanatic patch the

HOAO relevant wavefront error sources listed in DIQ Case 1a and DIQ Case 1b. Given the assumptions

made at the time of design in DIQ Case 1a and DIQ Case 1b, about magnitude and characteristics of these

wavefront error sources the residuals after correction are not to exceed the relevant terms listed in DIQ

Case 1a and DIQ Case 1b.

Source: SPEC-0001, DIQ error budgets

Verification: Design, Analysis, performance modeling, performance measurements

4.1.3 High Strehl Science Productivity

2.1.1-0020: Using state-of-the art technology at the time of design HOAO system shall maximize the

number of high Strehl observations by providing the highest order correction possible.

Source: SPEC-0001


Note: The HOAO system must have a sufficient number of degrees freedom and sufficiently high

bandwidth in order to meet its error budget allocation. In addition, the HOAO has to adhere to the SRD

and OCD requirements for maximizing science productivity. The highest priority science is achieved

while the ATST provides high Strehl to its instruments. The SRD states: "Making use of new technology

developments the ATST AO shall strive to provide the highest order correction possible."



4.1.4 Functional Requirements

4.1.4.1 Target structure

2.1.1-0025: The HOAO wavefront sensor shall be able to lock on granulation, with the exception of

near the very limb, and other solar structure, such as pores and sunspot structure in a sustained fashion

during seeing of r0(500nm) ≥ 7cm

2.1.1-0025-GOAL-A: The HOAO wavefront sensor shall be able to lock on granulation and other solar

structure, such as pores and sunspot structure during seeing of r0(500nm) ≥ 5cm.

Note: It is expected that performance and lock percentage of the system will gradually deteriorate as

seeing gets worse.

2.1.1-0025-GOAL-B: In addition to broad band visible on-disk structure the wavefront sensor shall be

able to lock on H limb and off-limb structure.

Source: SPEC-0001

Verification: Design, Analysis, modeling, measurement of lock status vs. r0, acceptance test

4.1.4.2 Target structure evolution

2.1.1-0030: The HOAOs wavefront sensor shall be able to stay locked within the seeing bounds

stated in 2.1.1-0025 on target structure that evolves in time.

Note: Solar granulation evolves on time scales of tens of seconds to minutes.

Source: SPEC-0001, SPEC-0036

Verification: Design, acceptance test

4.1.4.3 Robust operations

4.1.4.3.1 Intensity fluctuations

2.1.1-0035: The HOAO system shall function during transparency fluctuations typically encountered

in thin cirrus clouds.



4.1.4.3.2 Out-of-bound subaperture shift error checking and recovery

2.1.1-0040: The HOAO system shall automatically detect out-of-bound subaperture shifts and take

appropriate action.

Note: “Appropriate action” might include replace out-of-bound shifts with nearest neighbor and unlock

and re-lock the servo loop. The lock status of the servo loop must be reported to the event stream.





4.1.4.4 Reconstruction

2.1.1-0045: The HOAO system shall be able to measure interaction matrices in an efficient manner

and perform both modal and zonal reconstruction.



4.1.4.5 Telemetry data

2.1.1-0050: While operating in closed loop the HOAO system shall provide telemetry data at the

systems intrinsic update rate (nominal 2.0 kHz). At a minimum the system shall provide xy-shift data for

all subapertures and actuator displacement data for all actuators. The telemetry data shall be time stamped

and stored by the DHS.

2.1.1-0050-GOAL: Provide subaperture images at the highest feasible rate for engineering purposes.

Source: SPEC-0001, SPEC-0036, SPEC-0054


4.1.4.6 Fried parameter

2.1.1-0055: The HOAO system shall broadcast as an event the Fried parameter r0 to be displayed at

the WFC operator screen. The r0 data shall be logged in the engineering data base and be made available

to the header data base. The update rate shall be 1 second.

2.1.1-0055-GOAL: The HOAO system shall broadcast as an event the variance of the wavefront

sensor residuals to be displayed at the WFC operator screen. The update rate shall be 1 second. The WFS

residuals data shall be logged.

Source: Engineering


4.1.4.7 Automated Optimization

2.1.1-0060: The HOAO system shall perform automated real-time reconstruction matrix optimization

and servo-loop gain optimization based on seeing conditions.

Source: Engineering


Note: A number of implementations of optimization with different degree of complexity are possible. The

intention is to implement a simple scheme that uses the Fried parameter to load pre-computed

reconstruction matrices and gains optimized for various seeing conditions. It is expected that the

optimization process will occur during IT&C and continue during operations.

4.1.4.8 Automated Calibration

2.1.1-0065: The HOAO system shall be capable of performing automated calibration procedures in an

efficient manner. Routine calibration procedures shall be accomplished within 5 min or less.





2.1.1-0070: Flat field calibration of the HOAO WFS camera to 0.5% residual contrast for uniform

illumination shall be possible with and without random motion of the TMA.



Note: This flat field mode will ensure that flat field errors that build up on the WFS due to e.g. dust

particles that accumulate on optical surfaces (see DST experience) can be corrected without interrupting

entirely an observing sequence.

4.1.4.9 Low order offload

2.1.1-0075: The HOAO shall perform off-loading of seeing (time) up to 20 averaged low order modal

(e.g. Zernike, M1 natural modes) terms. The HOAO shall send the modal terms to the aO engine for

further aO processing at a user definable rate between 10 seconds and 10 minutes. The HOAO shall

perform sufficient error checking to ensure that valid wavefront information is provided to the aO engine.

Note: Error checks might include: don’t allow high noise, bad seeing wavefront information to contribute

to the average. Ensure time average has converged.

Source: SPEC-0001, DIQ case 2, Engineering


4.1.4.10 Operator status screen

2.1.1-0080: The HOAO system shall provide an operator status screen. At a minimum the dedicated

operator status screen shall display:

1. all subapertures at a minimum of 5Hz,

2. subaperture shifts,

3. actuator voltages,

4. modal (e.g. Zernike or KL) aberration spectrum, instantaneous and time-averaged.

5. Out-of-bound statistics

6. Lock status

7. Off-load parameters (modal coefficients)

8. Gain settings and reconstruction matrix identifier

9. Fried parameter



2.1.1-0085: The update rate of the operator display shall be ≥ 10 Hz.


Verification: Design, factory test



4.1.4.11 Location of corrector elements

2.1.1-0090: The DM shall be located at its designated position in the coudé lab (M10).

Source: SPEC-0029


2.1.1-0095: The Tip/tilt device for the HOAO shall be located at its designated position in the coudé

optical path (M5).

Source: SPEC-0029


2.1.1-0100: The WFC beamsplitter shall be located at its designated position in the coudé lab

(following M10) and remove no more than 4% of the light at any wavelength of interest to the AO

science instruments.



4.1.4.12 Thermal Control

2.1.1-0105: The WFC beam splitter, the DM and FTT mirror shall be thermally controlled to limit

local seeing and thermal distortion to values consistent with the relevant error budgets.

Source: DIQ error budgets

Verification: Design, modeling, laboratory tests, acceptance test

4.1.4.13 Alignment

2.1.1-0110: The HOAO DM and the HOAO wavefront sensor alignment shall be maintained to ≤5%

of a subaperture.

2.1.1-0115: The alignment of the pupil with respect to the WFS shall be actively controlled to ≤5% of

a subaperture.

Source: DIQ error budgets


4.1.4.14 Interface to the HOAO tip/tilt device (M5)

2.1.1-0125-Goal: WFC shall provide tip/tilt control for the Cryo-NIRSP instrument to the specified

precision (Cryo-NIRSP ISRD (SPEC-0056, 5.12).

Note 1: The requirement for image stabilization stated in Cryo-NIRSP ISRD (SPEC-0056, 5.12 ) is met

without T/T control for r0 > 2.7 cm, i.e., for all practical use cases (see TN-0168).

Note 2: It is envisioned that the Cryo-NIRSP context viewer will provide the T/T error signals. The

context viewer camera is expected to track on the faint coronal structure (e.g., coronal emission line). The

context viewer camera frame rate currently does not exceed 80 Hz).



Source: Cryo-NIRSP ISRD (SPEC-0056, 5.12)


4.1.4.15 HOAO WFS Off-Pointing

2.1.1-0125: The HOAO wavefront sensor shall be capable of locking on target structure located

within a field of view corresponding to a deviation in angle from the boresight of the telescope of up to 20

arcseconds.

Note: Potential lock structures can be found using the context viewer that has a FOV of 30 arcseconds

square (± 15 arcseconds deviation of the telescope boresight circular). This requirement allows selecting

target structure from the edge of the field of view of the context viewer.

Source: SPEC-0001


4.2 ACTIVE OPTICS (AO)

4.2.1 Overview

The Active Optics system (aO system) receives wavefront information obtained from either the HOAO or

the low order WFS systems as required by the particular WFC-OPM, and computes and delivers

corrections to M1CS, TEOACS and feed optics control systems (FOCS). The computed corrections are

needed in order to meet the seeing limited imaging error budget requirements. Given a measured

wavefront error the ATST uses an optimal combination of M2 rigid body displacements and M1 bending

modes to correct the aberrations. In addition, the ATST aO actively system maintains pupil alignment

using a mirror close to a focal plane (M3) on the DM/WFS systems as well as bore-sight using a mirror

close to a pupil (M6). Sources of aberrations include bending of M1, misalignment of, in particular, M1

and M2, but also feed optics elements due to gravitational and thermal effects and manufacturing

errors/tolerances of optical elements.

In terms of error budgets the role of aO is to, starting with the coronal error budget, provide significantly

improved seeing limited imaging performance for on-disk observations. The difference between the

coronal error budget and the seeing limited error budget (Delivered Image Quality Case II) establishes the

requirement for the performance of the active optics system. The seeing-limited error budget contains the

residual errors not corrected when limited spatial and temporal frequency corrections are applied by both

the aO system and fast tip tilt correction. In addition, there are two residual aO error budget terms. The

first of these is errors associated with the effectiveness of M1 figure control due to effects such as

wavefront sensor errors, latency, figure stability etc. The second category of errors is due to the limited

effectiveness of the QSA alignment system in positioning the M2, M3 and M6 optics.

The most relevant error tree elements relevant to the aO performance requirements are as follows:

2.1.2 M1 – M1 residual figure error. The aO system is responsible for reducing this error term from the

level given in the Case III error budget down to the error level shown in the Case II error budget.

2.1.2 M2 – M2 residual figure error. The aO system is responsible for reducing this error term from the


2.1.7 Feed Optics residual figure error: The aO system is responsible for reducing this error term from the




2.1.6.1 Wavefront Sensor error – These errors include WFS measurement errors and errors in the

calibration of WFS in the aO optics. The WFS measurement error is likely dominated by residual

seeing “noise”.

These error budget terms flow down to performance requirements not only for the WFC but also on

various telescope subsystems. This document lists the top level performance requirements for aO. In

addition requirements for the aO components that are within the WFC WBS element 2.1.2 are listed here.

4.2.2 aO Performance Requirements

2.1.2-0005: The aO system shall be able to operate the WFC-OPMs specified in section 3.1 of this

document.



2.1.2-0010: The aO system shall be responsible for reducing wavefront errors from the level given in

the Case III error budget down to the error level shown in the Case II error budget.



Source: SRD, SPEC-0009, Engineering


2.1.2-0015: The compute time for the aO algorithm shall not be longer than 10% of the time required

by the WFS system(s) to accumulate.



4.2.3 aO Functional Requirements

2.1.2-0020: The aO system shall receive wavefront sensor measurements in form of time-averaged,

calibrated modal coefficients from either the low order WFS or the HOAO auxiliary computer.



2.1.2-0025: While in closed-loop the aO system shall provide corrections to LUTs of telescope

subsystem control systems, including, M1CS, TEOACS, and FOCS.



2.1.2-0030: The aO system shall allow implementation of different QSA algorithms (e.g. force

optimized, damped least-squares, neutral point). The aO system shall be able to switch between these aO

control algorithms within an aO update cycle.



2.1.2-0035: The modal basis set used and the number of modal coefficients sent to M1CS by the aO

system shall be user selectable.





2.1.2-0040: The aO system shall receive pupil x,y misalignment error signals from the HOAO WFS

and output corrections to FOCS in order to keep the pupil aligned on the WFS according to specifications.



4.2.4 aO Status screen

2.1.2-0045: The aO system shall provide an operator status screen. At a minimum the dedicated

operator status screen shall display:

1. all subapertures at a minimum update rate of 5Hz,

2. time averaged subaperture shifts,

3. Time averaged, modal aberration spectrum

4. Gain settings and reconstruction matrix identifier

5. Current M2 hexapod positions: The WCCS shall provide the current M2 hexapod six-axis

position in terms of percentage of full travel in x, y, z, , , as copied from an TEOA health

and status screen.

6. Current feed optics positions: The WCCS shall provide the current M3 and M6 values in

terms of percentage of full travel as copied from a Feed Optics health and status screen.



4.2.5 Low Order WFS (LOWFS)

While performing seeing limited on disk observations (WFC-OPM2) the WFC system via a QSA

algorithm provides, at a slow rate, correction signals to various control systems of telescope subsystems.

These include TCS, M1CS, TEOACS, and FOCS. These corrections are processed and applied by the

individual subsystem control systems as corrections to their intrinsically stored LUT values.

While operating in WFC-OPM2 the LOWFS wavefront sensor measures wavefront errors averaged over

many atmospheric realizations thus providing the needed information about slowly varying (quasi static)

aberrations due to optical misalignment, mainly between M1 and M2, and/or M1 figure errors. The

wavefront sensor output is provided to relevant systems for further processing.

4.2.5.1 LOWFS Wavefront Performance Requirements

2.1.2-0050: The LOWFS shall measure slowly varying wavefront aberrations with up to 20 correction

modes provided in a user selectable basis. At a minimum the following modal basis sets shall be

implemented:

1. Zernike

2. Natural Mirror Modes


Verification: Design, modeling, acceptance test



4.2.5.2 LOWFS Capture range

2.1.2-0055: The capture range of the LOWFS shall be sufficient to handle the sum of seeing motions

and quasi-static aberrations, which are expected to be as large as 3 micron P-V. A Fried parameter of

r0(500nm)=3cm shall be considered as worst case seeing during which the LOWFS has to be able to

capture.



4.2.5.3 LOWFS Wavefront sensor error

2.1.2-0060: The LOWFS error after averaging of seeing effects shall not exceed the error budget

allocation for this term in DIQ Case 2.

Note: Seeing is the major noise source for the LOWFS. This requirement determines how many

atmospheric realizations have to be averaged to obtain a single aO wavefront measurement. The number

of samples required per average is seeing dependent.



4.2.5.4 LOWFS Update rate

2.1.2-0065: The nominal update rate of the LOWFS shall be 60 seconds. This parameter shall be user

selectable for values between 30 – 120 seconds.



Note: This spec is driven by the timescale over which aberrations are evolving. Little information is

available at this point and will likely become available only during commissioning. It is, however,

unlikely that a faster than 30 sec cadence will be required. A slower cadence is likely required in bad

seeing conditions to achieve the allowable LOWFS error.

4.2.5.5 LOWFS FOV

2.1.2-0070: The correlating Shack-Hartmann LOWFS shall have a minimum FOV of 30x30 arcsec.

2.1.2-0070-GOAL: 40x40 arcsec.

Source: Engineering


Note: Simulations have demonstrated that a larger FOV (compared to the HOAO WFS FOV of typically

10x10 arcsec) provides improved tracking stability and accuracy and improves convergence while

averaging out seeing.



4.2.5.6 LOWFS Target structure

2.1.2-0075: The low order wavefront sensor shall be able to measure quasi static wavefront errors

using granulation, with the exception of near the very limb, and other solar structure, such as pores and

sunspot structure as target during seeing of r0(500nm) ≥ 4 cm.

2.1.2-0075-GOAL-A: The low order wavefront sensor shall be able to measure quasi static wavefront

errors using granulation, with the exception of near the very limb, and other solar structure, such as pores

and sunspot structure as target during seeing of r0(500nm) ≥ 2 cm.

2.1.2-0075-GOAL-B: In addition to broad band visible on-disk structure the low order wavefront

sensor shall be able to operate with H limb structure.

Source: SPEC-0001, SPEC-0036, Engineering

Verification: Design, experiment, acceptance test

4.2.5.7 LOWFS wavelength

2.1.2-0080: The low order WFS shall operate at visible wavelengths.

Source: Engineering


4.2.5.8 LOWFS Robust operations

2.1.2-0085: The low order wavefront sensor shall be capable to select a lock feature from within a

2x2 arcmin FOV. The lock FOV shall be in accordance with the WFS FOV requirement. The low order

WFS shall function during transparency fluctuations typically encountered in thin cirrus clouds.



4.2.5.9 LOWFS Automated Calibration

2.1.2-0090: The low order WFS shall be capable of performing automated calibration procedures in

an efficient manner. Routine calibration procedures shall be accomplished with 5 min or less. Flat field

calibration of the low order WFS shall be possible with and without random motion of the TMA.



4.3 CONTEXT VIEWER

Requirement: The WFC system shall have a context viewer that provides the operator and other actors

with a real-time image of the FOV that includes the HOAO lock point. During operations the context

imager shall be used for visual control of HOAO performance, target selection, alignment and calibration.

For engineering and HOAO performance evaluation context viewer images shall be stored to the DHS.



4.3.1 Update and storage rate

2.1.3-0005: The context imager display(s) shall be updated at ≥10Hz. On demand context viewer

images shall be stored to the DHS at this rate.

2.1.3-0005-GOAL: The context imager display(s) shall be updated/stored at ≥20Hz.

Note: The update rate is to be understood for the full field of view of the Context Viewer. The display(s)

should allow for brightness and contrast adjustment.

Source: Engineering, SPEC-0036

Verification: Design, Acceptance test

4.3.2 Wavelength

2.1.3-0010: The context viewer shall deliver broad-band visible images. Nominal wavelength shall be

530nm with a 10 nm band pass.

2.1.3-0010-GOAL: The context viewer shall provide a set of user selectable wavelengths.

Source: Engineering


4.3.3 Field of view

2.1.3-0015: The context viewer shall provide user selectable FOV of 30 arcsec and 60 arcsec.

Switching between the 30 arcsec FOV and 60 arcsec FOV shall be automated and accomplished within

≤5 sec.

2.1.3-0015-GOAL: The FOV shall be user selectable between 30 arcsec and 90 arcsec.



4.3.4 Resolution

2.1.3-0020: The 30 arcsec FOV shall be sampled using at a minimum 1.64 pixels per resolution

element, with a maximum of 2 pixels per resolution element.

2.1.3-0020-GOAL: The 30 arcsec FOV shall be sampled with 4 pixels per resolution element at the

primary wavelength of the context viewer.

Note: A resolution element is defined by RE=λ/D at the primary wavelength λ of the context viewer (D

corresponds to the telescope diameter).



4.3.5 Pointing Reference

2.1.3-0025: The context viewer shall serve as the pointing reference.





Note: The origin of the heliocentric coordinate system will be established by pointing at four solar limb

positions. During this procedure the limb will be aligned to the center of the context viewer. Instruments

will be co-aligned with the context viewer.

4.3.6 Nighttime Pointing Maps

2.1.3-0030: The Context Viewer shall be able to operate at night and observe in long exposure mode.

In this commissioning mode the Context Viewer shall be used by the Telescope Control System to

determine the pointing error of the telescope. The Context Viewer shall provide long exposure images to

the DHS for further processing.

Source: Engineering (TCS pointing)

Verification: Design Review, Inspection

4.3.7 Laser Calibration Source

2.1.3-0035: It shall be possible to image with the context viewer a laser point source injected at the

GOS center of FOV.

Source: Engineering


Note: The laser source will be used for calibration and PSF (transfer optics) evaluation purposes.

4.4 NASMYTH UPGRADE

2.2-0005: The secondary mirror of ATST (M2) shall have the capability to correct atmospheric and

instrumental tip/tilt correction for future Nasmyth instrumentation.

Source: SPEC-0001


2.2-0010: The secondary mirror of ATST (M2) shall have a minimum -3dB bandwidth of 60 Hz.

2.2-0010-GOAL: The secondary mirror of ATST (M2) shall have a minimum -3dB bandwidth of

100 Hz.

Source: SPEC-0001


Note: The Nasmyth focal station of ATST is not developed as part of the ATST baseline. However, a

requirement exists to allow for future development of the Nasmyth. It is very likely that during the first

years of operations Nasmyth instrumentation will be developed. M2 is required to provide tip/tilt

correction for future Nasmyth instruments.



4.5 WAVEFRONT CORRECTION CONTROL SYSTEM

4.5.1 Overview

The WCCS is the principle control system for the Wavefront Correction System. The WCCS shall follow

the commands delivered from the TCS to control the WFC subsystems and to communicate status back to

the TCS. The WCCS shall configure all WFC subsystems and coordinate the activities within the

Wavefront Control System. It shall control directly the interaction of the WFC subsystems and

components, with the exception of the components and subsystems controlled directly by the HOAO real-

time computer.

The major task of the WCCS is to implement the operator selected wavefront correction strategy for the

telescope under direction from the TCS. The WCCS responds to one of the possible wavefront correction

modes by configuring the high-order and low-order wavefront correction systems and ensuring that the

determined correction values are sent to the relevant telescope subsystems. It monitors the behavior of the

relevant telescope subsystems to assure that the appropriate wavefront correction mode is operational.

The WCCS receives wavefront information obtained from the aO system and drive signals are sent from

the WCCS to the pertinent telescope subsystems. Each of these recipient subsystems can either use

internal lookup tables to perform their corrections (open-loop wavefront correction) or use the wavefront

information sent by the WCCS (closed-loop wavefront control). It is the responsibility of the operator or

observer to select the appropriate mode. It is the responsibility of the WCCS to ensure that the appropriate

corrections from the aO system based upon the selected mode are generated and subsequently sent to the

relevant telescope subsystems.

4.5.2 WCCS Requirements

4.5.2.1 WCCS Operation

2.3-0005: The WCCS shall be able to configure the WFC-C subsystems for operation in all WFC-

OPMs defined in this document The WCCS shall respond to possible wavefront correction modes by

configuring the high-order ,low-order wavefront correction systems, limb tracker and context viewer and

sending the determined correction values to the pertinent subsystem control systems.

2.3-0010: The WCCS shall also have the capability to maintain LUTs which duplicate the

functionality of the LUTs contained with optical subsystems.

Source: Engineering


2.3-0015: The WCCS shall accept or reject a command given on its public interface within 0.1

seconds.


Verification: Design Review, Test

2.3-0020: The WCCS shall be operational and ready to receive and act upon commands within 5

minutes of a cold, power-off start of its hardware.



2.3-0025: The WCCS shall have the capability to directly command the PA & C while it is in

calibration mode.





2.3-0030: Where possible the WCCS shall use the ATST Common Services (SPEC-0022) software

library, which provides communication interfaces, event services, logging and a suite of other useful

services.



2.3-0035: The WCCS shall provide an operator command screen for the OCS.



4.5.2.2 WCCS Engineering screens

2.3-0040: The WCCS shall provide a GUI interfaces called the WFC Engineering screens that

allow control and operation of all the WFC subsystems, including operation of individual mechanisms.

Source: SPEC-0005, SPEC-0036, Engineering


4.6 LIMB TRACKER

2.5-0005: The WFC system shall accept x,y error signals from GOS limb sensor assembly.

2.5-0010: The WFC system shall implement servo control with appropriate gain settings that

account for difference in sensitivity between perpendicular and parallel (to limb) direction.

2.5-0015: The limb shall be stabilized to 0.”5 P-V, which corresponds to 0.”15 rms in the direction

perpendicular to the limb. Parallel to the limb stabilization shall be on a best effort basis.

2.5-0020: The bandwidth of the limb tracker shall be limited only by the bandwidth of the M2

subsystem.

Source: SPEC-0056


Wavefront Correction System Specifications Document · Tom Berger Project Scientist T. Berger 16...

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