dkist optical alignment plan · 2020. 1. 22. · dkist | proc-0018 dkist optical alignment plan...

DKIST Optical Alignment Plan

Procedure Number: PROC-0018

Authors: P. Sekulic, C. Liang, K. Gonzales, R. Hubbard, S. Craig, SEIC

Date: May 2017

Approved by:

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Revision Control

Version Date Revised by Items changed

A 8 May 2017 Original release

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Table of Contents

1 Introduction .............................................................................................. 1

1.1 Plan Overview ..........................................................................................................1

1.2 Exclusions ................................................................................................................1

1.3 Safety Issues.............................................................................................................1

2 Supporting Information ............................................................................ 3

2.1 Reference Documents ..............................................................................................3

2.2 Acronyms and Abbreviations ..................................................................................4

3 Alignment Plan Definition ........................................................................ 6

3.1 General Approach ....................................................................................................6 3.1.1 Prime and Gregorian Foci (M1 / M2) ........................................................... 6

3.1.2 OSS Transfer Optics ...................................................................................... 6

3.1.3 Coudé and FIDO............................................................................................ 7 3.1.4 Telescope Level .............................................................................................. 7

3.2 Alignment Simulation ..............................................................................................7 3.2.1 Sensitivity Analysis......................................................................................... 7 3.2.2 Tolerancing Analysis ..................................................................................... 8

3.2.3 Alignment Simulation ..................................................................................... 8

3.3 Alignment Preparation ...........................................................................................10

4 On-Sky Measurements ........................................................................... 11

4.1 Nighttime Star Observing ......................................................................................11

4.2 Daytime Star Observing .........................................................................................11

4.3 Lunar Observing ....................................................................................................11

4.4 Pointing Model and Beam Steering .......................................................................11

5 Optical Alignment Equipment ................................................................ 13

5.1 Required Equipment ..............................................................................................13

5.2 Metrology Requirements .......................................................................................14 5.2.1 Coordinate Measurement............................................................................. 14 5.2.2 Tilt Measurement ......................................................................................... 15 5.2.3 Wavefront Measurement .............................................................................. 15 5.2.4 Software ....................................................................................................... 16

6 M1 Alignment ......................................................................................... 18

6.1 Method Overview and Flowchart ..........................................................................18

6.2 Objectives and Preconditions .................................................................................19

6.3 Wavefront Measurement ........................................................................................20 6.3.1 Measure the Wavefront at the Initial EA ..................................................... 20 6.3.2 Measure the Wavefront at Other EAs .......................................................... 21 6.3.3 Analyze the Wavefront ................................................................................. 21

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6.4 Modal Correction ...................................................................................................21

7 M2 Alignment ......................................................................................... 23


7.2 Objectives and Preconditions .................................................................................24 7.2.2 Perform the Shack-Hartmann Alignment..................................................... 25 7.2.3 Perform the M2 Alignment ........................................................................... 25 7.2.4 Optimize the Wavefront................................................................................ 26

8 OSS Transfer Optics Alignment ............................................................. 27

8.1 Method Overview and Flow Chart ........................................................................27

8.2 Definition of the Telescope’s Altitude Axis ..........................................................29

8.3 M5 and M6 Alignment...........................................................................................30

8.3.1 M6 Alignment ............................................................................................... 31 8.3.2 Tower Stability Test ..................................................................................... 31

8.3.3 M5 Alignment ............................................................................................... 32

8.4 M3 Alignment ........................................................................................................32

8.5 Alt-Az Alignment ..................................................................................................33

8.5.1 Defining the Azimuth Axis ............................................................................ 33 8.5.2 Alt-Az Alignment .......................................................................................... 34

8.6 M7 Alignment and Coudé Characterization ..........................................................35 8.6.1 M7 Alignment ............................................................................................... 35 8.6.2 Coudé Rotation Characterization ................................................................ 36

8.6.3 Coudé Optics Alignment Preparation .......................................................... 36

8.7 M4 Alignment ........................................................................................................37

9 Coudé Transfer Optics Alignment .......................................................... 38


9.2 Alignment of M8, M9, and M10 Mirrors ..............................................................39

9.2.1 M8 ................................................................................................................ 39 9.2.2 M9 ................................................................................................................ 40 9.2.3 CMM Arm Setup .......................................................................................... 40 9.2.4 M10 .............................................................................................................. 41

10 Telescope Residual Error Alignment ..................................................... 42

10.1 Method ...................................................................................................................42

10.2 Cumulative Focus Error .........................................................................................42

10.3 Final Measurement and Correction ........................................................................42

11 FIDO Alignment ...................................................................................... 44


11.2 Alignment Procedures ............................................................................................46

11.2.1 WFC-BS1 ................................................................................................... 46 11.2.2 M9a ............................................................................................................ 47

11.2.3 CL2a ........................................................................................................... 48

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11.2.4 CL2 ............................................................................................................. 49 11.2.5 CL3a ........................................................................................................... 50 11.2.6 CL3 and CL4 .............................................................................................. 51

11.2.7 DL-FM1 ..................................................................................................... 52

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List of Figures

Figure 1: Schematic representation of the alignment tool used for simulation and during

telescope alignment ............................................................................................................. 9

Figure 2: HASO4-BB typical dynamic range of individual Zernike ............................................ 16

Figure 1: M1 mirror optical alignment flowchart ......................................................................... 19

Figure 2 : M2 mirror optical alignment flowchart ........................................................................ 24

Figure 3: OSS transfer optics alignment flowchart....................................................................... 28

Figure 4: Example of an AC target ............................................................................................... 29

Figure 5: Position of the theodolite, AC1 and AC2 mirrors ......................................................... 29

Figure 6: Theodolite turned 60º upward to align M6.................................................................... 31

Figure 7: Position of the theodolite to align M5 ........................................................................... 32

Figure 8: AC3 autocollimation mirror and theodolite in the Coudé room ................................... 34

Figure 9 : Position of theodolite for M7 alignment ...................................................................... 36

Figure 10: M9 alignment configuration ........................................................................................ 37

Figure 11 : Coudé transfer optics alignment flowchart ................................................................ 39

Figure 12: M9 alignment configuration ........................................................................................ 40

Figure 13 : M10 alignment configuration ..................................................................................... 41

Figure 14 : Coudé beamsplitters and mirrors alignment flowchart .............................................. 46

Figure 15 : WFC-BS1 beamsplitter alignment configuration ....................................................... 47

Figure 16 : M9a alignment configuration ..................................................................................... 48

Figure 17 : CL2a beamsplitter alignment configuration ............................................................... 49

Figure 18 : CL2 beamsplitter alignment configuration................................................................. 50

Figure 19 : CL3a beamsplitter alignment configuration ............................................................... 51

Figure 20 : CL3 and CL4 beamsplitter alignment configuration .................................................. 52

Figure 21 : DL-FM1 alignment configuration .............................................................................. 53

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1 INTRODUCTION

1.1 Plan Overview

This document presents a plan for aligning the following optical components of the

DKIST telescope:

The primary and secondary mirrors, M1 and M2

The transfer optics of the optical support structure (OSS), which include M3, M4,

M5, and M6

The Coudé transfer optics, which include M7, M8, M9, and M10

The facility instrument distribution optics (FIDO), which include beamsplitters

WFC-BS1, CL2, CL3, CL4, CL2a, and CL3a and mirrors M9a (which feeds the

Cryo-NIRSP instrument) and DL-FM1 (which feeds the DL-NIRSP instrument)

NOTE: The procedures included in this plan are intended to provide a general outline of steps to be completed for each phase of alignment. A list of detailed procedures is presented in Section 2.1.

1.2 Exclusions

Separate alignment plans and procedures will be developed for all DKIST

instruments, for the wavefront correction system (WFC), and for the polarimetry

analysis and calibration (PA&C) Gregorian optical station (GOS). The PA&C GOS

procedure will be developed after M1 and M2 have been aligned.

1.3 Safety Issues

Project safety guidelines are described in SPEC-0030, “Conditions for Working at

the Site / Safety & Health Specification for Contractors.”

Many activities performed during alignment will require working on elevated

structures, a moving telescope, and a moving enclosure. These activities pose fall,

falling object, and pinch hazards. Only adequately trained personnel shall operate the

telescope and enclosure. Those moving the telescope and enclosure must follow

proper safety procedures to maintain a balanced structure; they must also coordinate

with other personnel working within the enclosure. All safety precautions must be

followed to mitigate associated risks.

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Likewise, many activities associated with equipment installation and calibration will

require working with heavy equipment and lifting devices such as cranes, personnel

lifts, and forklifts, which pose pinch hazards. Only adequately trained personnel shall

operate lifting equipment, and all personnel must apply proper rigging and

standardized visual communication practices. All safety precautions must be followed

to mitigate associated risks.

Important Guidelines for Handling Optics

At some point in the installation and alignment procedures, each optic will be

unprotected while work is undertaken to position it. During this time, the optical

surface is at risk for damage by inadvertent contact with lifting equipment, tools,

hands, clothing, and even airborne contaminants. Care must be taken to mitigate all of

these risks by following all steps and using all required protective equipment and

clothing when working near or above unprotected optics.

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2 SUPPORTING INFORMATION

2.1 Reference Documents

File Name Title

Safety Documents

SPEC-0030 Conditions for Working at the Site / Safety & Health Specification for Contractors

Integration, Test, and Commissioning Procedures

PROC-0082 M1 Mirror Installation

PROC-0098 PF Wavefront Measurements

PROC-0097 TEOA/PF SHWS LT Align

PROC-0103 M2 and HS on-TMA LT Alignment

PROC-0117 GF Wavefront Measurements

PROC-0243 Alignment of optical targets

PROC-0244 Characterization of TMA axis, Coudé rotation and tower stability

PROC-0136 M3 Assy Installation

PROC-0245 M3 alignment

PROC-0141 M4 Assy Installation PROC


PROC-0134 M5 surrogate installation

PROC-0247 M5 surrogate alignment


PROC-0129 M6 Assy Install


PROC-0250 M6 tilt adjustment (beam pointing)







PROC-0155 M10 Surrogate Installation

PROC-0254 M10 surrogate alignment


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File Name Title

PROC-0156 SHWS Installed and Aligned

PROC-0256 Coudé Wavefront Measurement

PROC-0164 M9a Assy Install/Align

PROC-0163 WFC-BS1 Assy Install/Align

PROC-0165 CL2a Assy Install/Align

PROC-0166 CL2 Assy Install/Align

PROC-0167 CL3a Assy Install/Align

PROC-0168 CL3 & CL4 Assy Install/Align

PROC-0169 DL-FM1 Assy Install/Align

PROC-0170 FIDO Beam Pointing verifications

Technical Notes

TN-0240 IT&C Focus Compensation Analysis

TN-0248 DKIST alignment simulation

2.2 Acronyms and Abbreviations

AOS: Adaptive optics system

BS: Beam splitter

CAD: Computer aided design

CMM: Coordinate measuring machine

FIDO: Facility instrument distribution optics

FOV: Field of view

GIS: Global interlock system

GOS: Gregorian optical station

IT&C: Integration, Testing, and Commissioning

LUT: Lookup table

NIRSP: Near-infrared spectro-polarimeter

OSS: Optical support structure

PA&C: Polarimetry analysis and calibration

SA: SpatialAnalyzer®

SHWS: Shack-Hartmann Wavefront Sensor

SMR: Spherically mounted retroreflector

TEOA: Top end optical assembly

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TMA: Telescope mount assembly

VBI: Visible-light broadband imager

WFC: Wavefront correction

WFCS: Wavefront correction system

WFE: Wavefront error

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3 ALIGNMENT PLAN DEFINITION

3.1 General Approach

Each optical component will be accurately positioned using a laser tracker, a

coordinate measuring machine (CMM), and/or a theodolite, followed by a Shack-

Hartmann wavefront sensor (SHWS) to achieve fine optical alignment of the

wavefront. This process will use three focal locations that precede the Coudé

laboratory: the prime focus (from the M1 mirror) and the Gregorian focus (between

M2 and M3).

The telescope coordinate system is defined by the rotation axis (altitude and azimuth)

of the telescope mount assembly (TMA). The mirrors will be initially positioned by

measuring their fiducials using a laser tracker with SpatialAnalyzer® (SA) software,

which integrates optical and mechanical computer aided design (CAD) models. The

TMA structure deflection at different elevation angles (EA) will be characterized by

measuring the position of mirrors with the laser tracker. These data will be analyzed

and processed to populate the look-up tables (LUT).

3.1.1 Prime and Gregorian Foci (M1 / M2)

At the prime focus, the SHWS will be used to measure the wavefront produced by

target stars. The objective of this effort is to characterize the focal plane by

identifying the best focus position and the center of the field of view (FOV). The

SHWS will also be used to characterize M1’s surface figure as function of EA. In

addition, the SHWS will be used to verify M1’s modal correction.

The M2 mirror will be aligned by placing the SHWS at the Gregorian focal plane.

The objectives are to align M2 to the M1 mirror to minimize WFE at Gregorian focal

plane and to measure the position the Gregorian focus for different elevation angles.

The WFE data and Gregorian focus position data will be used to refine M1 and M2

LUTs.

3.1.2 OSS Transfer Optics

The OSS transfer optics comprise three flat mirrors—M3, M5, and M6—and one off-

axis mirror, M4. The flat mirrors will be aligned using the laser tracker and a

theodolite to improve tilt accuracy. The off-axis mirror, M4, will be aligned using

only the laser tracker; however, the residual misalignment will be measured and

compensated for in the Coudé wavefront measurements.

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3.1.3 Coudé and FIDO

Similarly, the Coudé optics and FIDO will be aligned using a laser tracker, theodolite,

and CMM arm. The main challenge for this phase of alignment will be to connect the

telescope and Coudé coordinate systems with minimal errors. To achieve this goal, a

number of targets will be attached permanently to non-rotating walls on the Coudé;

the laser tracker measurements of these targets can then be transferred automatically

into the telescope coordinate system.

3.1.4 Telescope Level

The wavefront for the entire telescope will be aligned at the Coudé level using the

beam from the M10 mirror. Night-sky measurements will be used to determine the

telescope performances, and the M2/M4 mirrors will be used to compensate for

residual astigmatism, coma, and focus to achieve fine optimization. The tilt of the

M3/M6 mirrors will also be optimized to adjust the boresight. For each wavefront

measurement—at the primary, Gregorian, and telescope foci—dedicated alignment

software will be used to compute the mirror shift and tilt needed to compensate for

the measured residual aberrations and to position the focal plane at its nominal

location.

3.2 Alignment Simulation

Three analyses will be conducted to inform the alignment strategy:

An analysis of the alignment sensitivities for each optical element

The results of Monte-Carlo tolerancing

The results of a simulation to verify the alignment procedure

This approach is designed to account for the opto-mechanical design, the error

budget, and the assembly procedure.

3.2.1 Sensitivity Analysis

The sensitivity analysis will be conducted using Zemax© OpticStudio™ (Zemax).

This software will be used to develop a sensitivity matrix for all degrees of freedom

for each optical element and for different field points. Sensitivities will be computed

for several parameters:

Fringe Zernike coefficients (focus Z4, astigmatism Z5 and Z6, coma Z7 and Z8,

and spherical Z9) in nine predefined field points

Focus position

Boresight error

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The analysis will reveal the most and least sensitive elements, providing a basis for

designing a preliminary alignment strategy and identifying compensators. The

alignment sensitivities of each mirror will be computed for the prime, Gregorian, and

telescope focal planes.

By default, Zemax uses the vertex position as the center of the coordinate systems.

When tilting an off-axis mirror, it creates, at first order, a large focus term, with

additional Dx or Dy decenters at second order. To eliminate this effect, macros will be

used to transform the coordinate system for each off-axis mirror (M1, M2, M4, and

M8) before computing sensitivities. The goal of this effort is to use the local vertex

position (rather than the global vertex position) as the center of rotation of each

optics. For flat mirrors (M3, M5, M6, M7, M9, and M10), Zemax centers the

coordinate system on the optical surface by default.

TN-0240, “IT&C Focus Compensation Analysis,” gives the results of this analysis.

3.2.2 Tolerancing Analysis

Next, a Monte-Carlo tolerancing analysis will be performed. The input for this

analysis is the tolerance of initial mechanical positioning of each mirror. This value is

determined from the mirror’s error budget during the use of the laser tracker for the

initial positioning. The results of this analysis will establish DKIST’s expected initial

performance prior to wavefront alignment.

Zemax will be used to perform the tolerancing at different alignment steps. Each step

will use the results of the previous alignment step as input. Tolerancing for each will

be performed for 1,000 trials that can be analyzed to determine WFE (root mean

square (RMS) and peak-to-valley), Zernike Z4 to Z9, the focus position, and the

boresight error.

Once the tolerancing is complete for each focus (prime, Gregorian, and telescope),

the best-, mean-, and worst-case results will be used as input to simulate wavefront

alignment and identify the corresponding system. This system will then be used as

input for the next tolerancing step.

The results of this analysis will be detailed in TN-0248, “DKIST Alignment

Simulation.”

3.2.3 Alignment Simulation

The goal of this final step is to identify the best achievable optical performances. It

entails simulating the optical alignment using a tool that computes the required

motions (decenters and tilts) to compensate for the measured residual WFE, focus

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position, and boresight error. The simulation will provide the data needed to define

the alignment specifications and to confirm the efficacy of the alignment strategy and

compensators.

The alignment tool will be used during both the mirror alignment simulation and the

on-site IT&C optical alignment. Developed specifically for DKIST, this tool

automatically identifies the required mirror compensations (Dx, Dy, Dz, Tx, Ty, and Tz)

and telescope pointing corrections to minimize the measured WFE (Zernike

decomposition), focus position, and boresight error. As inputs, it uses the sensitivities

computed for mirror alignment and telescope pointing, along with Zernike, focus

position, and boresight error values that are either measured or generated by the

Monte-Carlo simulation. Three matrices must be created to use this tool:

S, the sensitivity matrix, which contains the computed sensitivities (from Zemax)

of the parameters that will be measured (Zernike in nine field points, focus

position, and boresight error)

M, the measurement matrix, which contains the measured WFE (Zernike

decomposition) at the center of the field and at different field points, as well as the

focus position and boresight error

C, the compensation matrix, which contains the mirror motions and pointing

corrections

Figure 1: Schematic

representation of the alignment

tool used for simulation and

during telescope alignment

The objective is to find the

compensation matrix (C) using

the measured data (M) and

sensitivities (S). When these

motions are applied to the

mirrors, new measurements are

performed to generate a new matrix M that converges toward zero: S×C = –M and

then C = –S-1×M. Since matrices S and M are known, matrix C can be computed with

the pseudo-inverse function.

Individual tools have been created for each phase of alignment—at the prime focus

(M1), Gregorian focus (M1/M2), and telescope focal plane (M1 to M10).

The results of these individual alignments are detailed in TN-0248, “DKIST

Alignment Simulation.”

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3.3 Alignment Preparation

Preparation is critical to performing the alignment procedures effectively and

efficiently. The powerful SA software will be used for the following tasks:

Merging the optical Zemax design and mechanical CAD model.

Defining and creating the as-built alignment fiducials on the optical mounts to

establish their locations with respect to the optical surface.

Creating the coordinate systems that will be measured with the laser tracker or

CMM arm.

Extracting the values that will be used as success criteria during alignment.

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4 ON-SKY MEASUREMENTS

The on-sky procedures described in this section refer to nonsolar observing—the

equipment referenced here has not been designed for solar illumination.

Solar alignment and thermal testing are covered in other procedures.

4.1 Nighttime Star Observing

Almost all alignment activities will be performed at night on bright stars. This offers

several advantages. First, abundant data is available to provide full-sky coverage for

calibrating the LUTs for both WFE and beam steering. Second, a fully functional

thermal system is not required for nighttime measurements; consequently, they can

begin earlier in the project schedule. Another important consideration is the location

of the pole star. Not only is Polaris visible but it is also very close to the optimal EA

of 15° for best seeing. This will minimize the measurement error from other

sources—for example, the telescope bearing run out—as well as from gravitational

and trajectory errors. It may be possible to perform the alignment with the drives

powered off, which would remove telescope jitter. Finally, alignment activities can be

conducted in parallel with other IT&C activities, assuming the facility is in a state

that will allow observing.

4.2 Daytime Star Observing

Even when the sun cannot be observed, there are sufficient bright stars visible during

the day to allow some comparison with the nighttime results and to provide the data

required to populate the LUTs for thermal effects on the structure. The GIS will be

temporarily modified to prohibit anyone from entering the solar disc and solar

avoidance zones. This will prevent accidental solar illumination of the primary

mirror.

4.3 Lunar Observing

Although details are not yet available, there is a plan to observe the moon. Lunar

observation offers two main advantages. First, it allows tracking to be assessed on a

non-sidereal target. Second, because the full field of view is illuminated, aperture

“masks” can be used on the mirrors as a backup alignment check.

4.4 Pointing Model and Beam Steering

At each stage in the alignment process and throughout the IT&C phase, telescope

pointing runs will be conducted. The goal of these runs is to update the pointing

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model and, when light reaches the Coudé, the LUTs for the beam-steering optics (M3,

M6) to maintain alignment.

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5 OPTICAL ALIGNMENT EQUIPMENT

5.1 Required Equipment

The following equipment is needed to perform the IT&C optical alignment:

Qty Item

Laser Trackers and CMM Arms

1 API OmniTrac2

1 FARO Vantage E

1 API Axxis6-200™ (3.2m range)

1 FARO Edge (2.7m range)

Theodolite and Related Equipment

1 Leica T3000A plus X-Y stages

Theodolite supports for the Nasmyth optical table (+X), M5 assembly interface tower, M3 assembly interface, and Coudé room (floor and VBI/WFC optical tables)

1 Video eyepiece module for the theodolite

1 Optical table with four legs (at Nasmyth) to support the theodolite and targets

Software

1 SpatialAnalyzer Software

Wavefront Sensor and Related Equipment

1 Shack-Hartmann wavefront sensor — HASO4-BB (Imagine Optic)

1 Imaging system with support for the three measurement configurations — prime focus (f /2), Gregorian focus (f /13), and telescope focus in the Coudé

Mirrors and Targets

Spherically mounted retroreflectors (SMRs) for laser trackers, magnetic bases, and other associated toolings

1 Alignment mirror for M5 assembly re-alignment

1 Pentaprism for M7 alignment

3 Alignment autocollimation mirror targets (named AC1, AC2, and AC3) with tip-tilt / centering mounts

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5.2 Metrology Requirements

5.2.1 Coordinate Measurement

Laser Trackers

The laser tracker is an essential tool for making optical alignment measurements in

large volumes. This instrument measures the 3D coordinates of large objects by

tracking a laser beam to SMR targets that attached to the object. Based on these

coordinates, laser trackers can then measure the geometry and position of objects. The

DKIST optics contracts specify a number of line-of-sight requirements that will

permit the SMR targets to be seen from the planned mounting locations of the laser

tracker.

Laser trackers will be used in the interface metrology and TMA assembly; they will

also be used to position the mirror into the telescope structure, measure structure

deformation due to TMA rotation, align instrument tables in the Coudé laboratory,

and align instruments. Two laser trackers will be available during IT&C:

API Omnitrac 2 (OT2). The typical volumetric accuracy of this laser tracker is

±40 μm in the 5 m range, ±65 μm in the 10 m range, and ±115 μm in the 20 m

range. This laser tracker features a long measurement range—up to 50 m.

FARO Vantage E. The typical volumetric accuracy of this laser tracker is ±45

μm in the 5 m range, ±70 μm in the 10 m range, and ±120 μm in the 20 m range.

The Vantage E can measure up to 25 m. It features an integrated, high-resolution,

two-camera system that can help locate a specific target quickly and efficiently.

This system operates in all lighting conditions, from complete darkness to bright

sunlight.

CMM Arms

The CMM is a portable articulated arm that has six axes equipped with rotary

encoders. A probe attached to the arm of the instrument touches the object to measure

the coordinates of the contact point. The CMM arm would significantly reduce

integration time, particularly for bench-mounted optics, instruments, and the WFC. It

will be used to align local optics, Coudé instruments, and WFC optics in the Coudé

laboratory. The following data are for the full range of the arm; however, accuracy

and repeatability improve for smaller volumes.

API Axxis6-200. The accuracy of the Axxis6-200 arm is attributable to a

mechanical design that includes high-precision bearings; titanium joints; a

lightweight, aeronautical, aluminum structure; and carbon fiber tubes. Its

volumetric accuracy is ±64 μm over a full range of the 3.2 m. Repeatability over

the full range is ±45 μm.

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FARO Edge 2.7m. The Edge can be used remotely with WiFi connection and has

a built-in touchscreen and an on-board operating system. Its volumetric accuracy

is ±41 μm over a full 2.7 m range. Repeatability over the full range is ±29 μm.

5.2.2 Tilt Measurement

For measuring the angle between two mirrors accurately, the Leica T3000A

theodolite offers a very good solution. The T3000A can be used as an alignment

telescope and as an autocollimation system; it measures tilt with a theoretical

accuracy of ±0.5 arcsec (less than ±2 arcsec in a working environment). An

illuminated green cross is collimated and projected at infinity; when reflected on a

mirror, the theodolite measures the tilt change (horizontal and vertical). It can also

measure the horizontal and vertical angles between two flat mirrors. This theodolite

will be upgraded by adding a CCD camera in front of the eyepiece, which allows

remote tilt measurement.

5.2.3 Wavefront Measurement

A WFS will be required during IT&C to align the telescope mirrors and help refine

the LUTs. The instrument will be used at multiple focal locations to:

Characterize the primary focal plane

Align M2 at Gregorian focus

Optimize alignment at Coudé focal plane

Measure the wavefront on the night-sky at Gregorian and Coudé

To properly size and condition the input beam into the WFS, a collimating optic will

be required for the primary (F/2), Gregorian (F/13), and Coudé foci.

The HASO4-Broadband (Imagine Optic™), an SHWS, has been selected for

wavefront measurement. It features 50x68 microlenses and a 5.2x7.0 mm2 sensor.

The absolute RMS wavefront accuracy is λ/100 over the full bandwidth (400–1100

nm), with a minimum of 6 nm RMS. The repeatability is λ/200. The HASO4-

Broadband has a relatively large Zernike dynamic range, which is required for

DKIST alignment (see Figure 2). Its tilt dynamic range of ±3° allows a large initial

wavefront measurement before starting alignment.

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Figure 2: HASO4-BB typical dynamic range of individual Zernike

The SHWS and associated optics will be mounted on a hexapod that will be attached

to following locations:

The top end optical assembly (TEAO), for the prime focus

The M3 interface, for the Gregorian focus

An optical table in the Coudé laboratory

Three SMRs will be calibrated to establish the location of the SH detector plane

within the telescope coordinate system when they are measured using the laser

trackers. The total position accuracy depends on the accuracy of the following:

The relation between the focus term (Z4) measurement and the Z position of the

SHWFS is determined by the SHWFS accuracy and the optical design. Since the

SH accuracy is 6 nm RMS, the position accuracy is <1 micron at prime focus and

0.05mm at Gregorian focus.

The environmental stability (thermal, vibration) value, as determined during

IT&C.

Calibration (< 0.05 mm, as determined using the CMM arm).

The laser tracker measurements (~0.1mm).

Assuming these conditions are met, the final position accuracy should be

approximately ±0.2 mm.

5.2.4 Software

SA software will be used for several purposes during IT&C. First, it will be used to

acquire measurements and process data from the laser trackers, CMM arms, and

theodolite. Because SA can integrate measurements from different instruments in the

same coordinate system, it facilitates the simultaneous use of the laser tracker and

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CMM arm. In addition, SA will keep acquiring measurements in the same coordinate

system even when an instrument is moved to a different position. Another important

role of this software will be to merge the mechanical CAD and optical Zemax

models. This provides several benefits for defining the metrology procedure, adding

the as-built mirrors, and extracting the data that will be used during alignment.

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6 M1 ALIGNMENT

6.1 Method Overview and Flowchart

During M1 polishing, four SMRs will be attached to the mirror to register the location

of its final optical surface. Initially, M1 will be installed in its cell and aligned onto

the TMA at its nominal position using the laser tracker and SMRs.

To facilitate the M1 alignment process, the SHWS will be mounted onto the TEOA

frame near the primary focus, and the wavefront from the pole star will be measured

at night. Similar wavefront measurements will also be performed at different EAs, for

several reasons: to determine the primary focus position for these angles, to verify

that they are within the expected range, and to verify that M1’s surface figure at

different EAs falls within the expected range after correction using the LUT. During

this step, the SHWS will also be used to verify the ability of the M1 cell to inject

specified Zernike, or mirror modes, onto the mirror’s surface.

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Position of the SH wrt primary focus at different Alt angles

Multiple measurement of a target star along the axial direction

Measurement at the best focus in the center and at different field positions

Analysis and determination of the center of the FOV

Wavefront measurement at optimum Alt angle

WF sensor

Measurement of M1 and SH position with the LT at different Alt angles

Programmation of the hexapod position for different Alt angles

Change of Alt angle

Primary focus position for different Alt angles (axially and laterally)

M1 Mirror surface figure at different Alt angles, after M1 LUT correction

Alignment of M1 mirror at the nominal position

M1 mirror installed into M1 cell

M1 cell installed onto TMA

Data collection and analysis

Injection of known M1 mode into M1 mirror using LUT

Retake of the wavefront measurement

Change of M1 mode

M1 response to modes injectionAnalysis of wavefront measurements

Laser tracker

Figure 1: M1 mirror optical alignment flowchart

6.2 Objectives and Preconditions

Objectives

To determine the axial position of the primary focus

To determine the lateral position of the primary focus (the center of the FOV)

To verify the LUTs—primarily the elevation LUT—for the M1 cell

To verify the M1 modal correction

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Preconditions

M1 is installed in its cell, the cell is installed on the TMA, and the nominal

position of the M1 mirror has been established using the laser tracker.

The TMA can point and track correctly.

The TEOA frame is mounted on the TMA and the SHWS is mounted on the

TEOA frame near the primary focus.

6.3 Wavefront Measurement

The position of the SHWS at different EAs will be programmed into the hexapod

based on the expected primary focus position, as determined by the M1 laser tracker

measurement and the TMA deflection analysis. Those positions will be measured

directly using the laser tracker and the SMRs that are attached to the SHWS

assembly.

6.3.1 Measure the Wavefront at the Initial EA

For this initial set of wavefront measurements, start with the telescope pointing at the

pole star at an EA of approximately 20° above the horizon.

1. Using the SHWS viewer, center the SHWS onto the pole star and take multiple

wavefront measurements along the axial direction (through focus measurement).

2. Analyze the measurement data and determine the position of best focus, where the

Z4 term is zero or at a minimum. Compare this with the expected position.

3. Using the best axial focus position, take a set of wavefront measurements on a

grid of lateral positions by first moving the SHWS to a predetermined position

and then pointing the telescope to center the star.

NOTE: The grid and step size will be based on the expected accuracy of the primary focus after it is aligned using the laser tracker.

4. Analyze the measured wavefront and compare it to the predicted one based on the

Zemax model at multiple lateral positions to determine the center of the FOV1.

5. Analyze the measured wavefront and compare it to the predicted shape of the

mirror surface.

6. Confirm and fine-tune the M1 cell LUT.

1 Off-axis aberration and LUT error can be distinguished by looking at the wavefront over multiple fields

and comparing it to the designed wavefront.

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6.3.2 Measure the Wavefront at Other EAs

1. Start at the first elevation angle (EA1) and lock onto a target star using telescope

pointing.

2. Use the SHWS viewer to center the SHWS onto the target star. Take multiple


3. Analyze the measurement data and determine the position of best focus. Compare

this with the expected position.


grid of lateral positions by first moving the SHWS to a predetermined position

and then using telescope pointing to center the star.

NOTE: The grid and step size will be based on the expected accuracy of the primary focus after it is aligned using the laser tracker.

5. Analyze the measurement data and determine the center of the FOV2.

6. Analyze the measured wavefront and compare it to the predicted mirror surface

shape; also, confirm and fine-tune the M1 cell LUT.

7. Move on to EA2 and repeat steps 1–6, and then repeat these steps for a number of

EAs.

8. After you have measured the wavefront at all of the EAs, confirm the primary

focus position for each (both axially and laterally). Also, confirm that…

The primary focus positions are within the expected range.

The mirror surface figure at each EA is within the expected range after

correction using the M1 LUT.

NOTE: Multiple iterations may be needed to fine-tune the surface figure LUT for M1.

6.3.3 Analyze the Wavefront

The detail analysis of the wavefront will be presented in the document PROC-0098.

6.4 Modal Correction

Once you complete the steps outlined in Section 6.3, use the SHWS to verify the M1

cell’s ability to inject specified Zernike or mirror modes onto the mirror.



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1. Establish a baseline by taking wavefront measurements using the pole star as the

target, approximately 20° above horizontal pointing.

NOTE: Baselines should be repeated periodically to monitor possible changes.

2. Inject a known mode into the M1 mirror (an input wavefront) using the LUT via

the M1 control system (CS).

3. Retake the wavefront measurements.

4. Analyze the measured wavefront and compare it to the input wavefront.

5. Repeat the measurement for as many modes as desired.

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7 M2 ALIGNMENT


Initially, the M2 mirror will be mounted into the TEOA and aligned, using a laser

tracker and SMRs, relative to the measured primary focus position. As described for

M1, three SMRs will be attached to the M2 mirror during polishing to register the

location of the optical surface. The SHWS will be mounted near the Gregorian focal

plane (near the M3 mirror mount).

During this phase of alignment, the LUT for the M1 mirror will be refined to optimize

the wavefront at Gregorian focus. As described for M1, the wavefront measurement

at Gregorian focus will be acquired at night on Polaris; the SHWS will be used to

determine the position of best focus, where the Zernike Z4 term (defocus) is at a

minimum. This measured wavefront will be compared to the wavefront by predicted

by the Zemax model at multiple lateral positions to determine the M1/M2 alignments

and the position (lateral and axial) of the Gregorian focus. The same measurement

will be repeated at different EAs, and an M2 LUT will be established to provide a

stable Gregorian focus position for these angles. Zernike measurements obtained in

the field and at different EAs will provide input data for the optical alignment tool;

they will also define the optimal position of M2 (6° of freedom) at each EA that

minimizes the WFE.

The M2 LUT may require further adjustments to balance between improving the

positioning of the Gregorian focus and improving the WFE. Three LUTs may be

created to effect this process:

An LUT that optimizes the WFE at Gregorian focus, but changes the focus with

EA.

AN LUT that keeps the Gregorian focus at the same position for all EAs, but

gives a degraded WFE.

An LUT that compromises between WFE and Gregorian focus positon.

Some of the residual WFEs can be corrected by updating the M1 LUT. After updating

the M1 LUT, additional iterations of wavefront measurement will be performed to

confirm the effectiveness of the updates.

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Laser tracker

Multiple measurement of a target star along the axial direction

Measurement at the best focus in the center and at different field positions

Analysis and determination of the center of the FOV

M2 alignment to optimize wavefront error

WF sensor

Mounting of the SH wavefront sensor onto M3 mirror mount

Change of Alt angle

Gregorian focus position for different Alt angles (axially and laterally)

Alignment of M2 mirror at the nominal position

M1 mirror aligned at primary focus

M2 mirror mounted onto TEOA

Alignment of the SH at the nominal Gregorian focus with the LT

Improvement of Gregorian focus position

Updating of M1 LUT

Telescope performances at Gregorian focal plane

Final wavefront measurement at Gregorian focus

Figure 2 : M2 mirror optical alignment flowchart

7.2 Objectives and Preconditions

Objectives

Establish a LUT for the M2 hexapod as a function of EA

Align the M2 mirror to M1 mirror

Position the Gregorian focus at its nominal position.

Refine the LUT for the M1 mirror to optimize the wavefront at Gregorian focus

Preconditions

The M1 mirror is aligned according to procedures defined in Section 6.

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The M2 mirror is mounted onto the TEOA and has been aligned with the laser

tracker. The initial mirror position should be near the hexapod’s center of travel.

7.2.2 Perform the Shack-Hartmann Alignment

1. Mount the SHWS onto the M3 mirror mount interface.

2. Align the SHWS to the expected Gregorian focus using the laser tracker.

7.2.3 Perform the M2 Alignment

The initial LUT for the M2 hexapod motion will be computed based on the Zemax

model and the movement of the primary focus position measured during M1

alignment (see Section 6).

1. As described for M1, start at the initial EA using the pole star as the target. Use

the SHWS viewer to center the instrument onto the target star. Take multiple


2. Analyze the measurement data and determine the position of best focus, where the

Zernike Z4 term (defocus) is zero or at a minimum. Compare this with the

expected position.


grid (possibly 5x5) of lateral positions by first moving the SHWS to a

predetermined position, and then using telescope pointing to center the star.

4. Analyze the measured wavefronts and compare to the predicted wavefront based

on the Zemax model at multiple lateral positions to determine the center of the

FOV3.

5. Move on to EA2 and repeat the wavefront measurements. Repeat them for a

number of EAs.

6. After wavefront measurements for all telescope EAs are complete, confirm that

the Gregorian focus positions (axial and lateral) have been determined and that

they are within the expected range. In addition, confirm the wavefront

performance of combined M1 and M2 at those positions.

7. Examine the WFEs for these EAs and compute their respective Gregorian focus

positions relative to other telescope reference points.

NOTE: You may need to further adjust the M2 LUT to balance between improving the positioning of the Gregorian focus and improving the WFE. For example, if you observe a larger-than-expected amount of astigmatism and/or coma, you can correct



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using the M2 position (and let the Gregorian focus position drift) or use M1 to correct the wavefront and fix the focus position.

7.2.4 Optimize the Wavefront

Once the M2 position LUT is established and provides a stable (or known) Gregorian

focus for different EAs, review the measured wavefront data to identify residual

errors. Some residual WFEs can be corrected by updating the M1 LUT. After

updating the LUT, be sure to perform additional wavefront measurement iterations to

confirm effectiveness.

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8 OSS TRANSFER OPTICS ALIGNMENT

8.1 Method Overview and Flow Chart

The OSS transfer optics include four mirrors—M3, M4, M5, and M6—that must be

aligned with respect to the telescope’s altitude and azimuth. Alignment will be

conducted in several stages that include defining the altitude and azimuth axes using a

theodolite and a set of mirrored targets; axis orientation will be determined based on

tilt of these targets while axis position is based on their centers. In addition to the four

OSS mirrors, the alignment procedure for M7 is included here because it can be used

to define the azimuth axis.

The altitude axis will be defined based on the Nasmyth platform and the azimuth axis

will be defined based on the Coudé laboratory. The theodolite will be positioned in

both of these locations, which will also facilitate characterization of the mechanical

stability of the telescope’s altitude-azimuth rotation.

Telescope pointing runs will be conducted at each stage of this process to update the

LUTs for the beam-steering optics (M3, M6) and to maintain the position of the

beam.

Mirrors M3, M4, M5, and M6 will be positioned into their mounts using a laser

tracker. The flat mirrors (M3, M5, and M6) will be accurately tilt-aligned with respect

to the altitude axis using the theodolite and mirrored targets. A (flat) mirrored target,

AC1, will be positioned at the M4 location during these alignment procedures;

consequently, the M4 mirror will be integrated last, after the procedures have been

completed.

Various alignment tests will be conducted using the theodolite located in the Coudé

laboratory. The rotation of the altitude-azimuth axis will be characterized when the

Coudé is static and the telescope is rotating. The theodolite will be used to confirm

the stability of folded (90°) beam produced by when M5 and M6 mirrors are aligned

on the altitude axis. If the azimuth and altitude axes are perfectly perpendicular

(within the ±2 arcsec accuracy of the theodolite), the beam’s tilt should remain

constant during alt-az rotation; if not, the theodolite will measure this deviation and

the mirrors will be adjusted to minimize it. When the Coudé is rotating, the tilt will be

optically measured with respect to the TMA and compensated as needed.

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Figure 3: OSS transfer optics alignment flowchart

Theodolite, AC1, AC2

Alignment of M6 angle with respect to Alt axis

Measurement of tower tilt stability when TMA Alt is rotated

Mounting and alignment of M5 assembly with laser tracker

Alignment of M5 angle with respect to Alt axis

Alignment of AC1 and AC2 on the Alt axis

AC1, AC2, laser tracker and theodolite install


Mounting of AC3 on the tower

Alignment of AC3 on the Az axis

M6 assembly

Laser tracker

Definition of the Alt axis

Tower stability with TMA Alt rotation

M5 assembly

AC3

Definition of the Az axis

Characterization of Alt-Az rotation and perpendicularity with respect to M5-M6

Adjustment of M6 tilt

M7 assembly mounting and alignment in Coudé coordinate system wrt Az axis

Coudé rotation characterizationTilt and decenter of the Coudé rotation

axis wrt Az axis

Alignment of theodolite on Alt axis and alignment of AC1 on M4 local tilt


M4 assembly

M7 assembly


Alignment of M3 angle with theodolite and GOS target

M3 assembly

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8.2 Definition of the Telescope’s Altitude Axis

Before any of the mirror assemblies (M3, M4, M5, and M6) are mounted, the

telescope’s altitude axis will be defined using mirror targets AC1 and AC2 and a

theodolite. These flat mirrors will be reflective and feature a target that defines their

centers. Their tilts will be used to define the tilt of the altitude axis, and their centers

will be used to define the axis position. The outside diameters of the ACs are

concentric to the reticle within 0.03 mm and cylindrical to within 0.008mm. They will

be mounted on a tool that allows X-Y tip-tilt and X-Y centering.

Figure 4: Example of an AC target

1. Position the theodolite on an optical table on the Nasmyth platform at the +X side

of the mount. It should be fixed on the altitude axis, facing M4.

2. Place the AC1 mirror on the –X trunnion of the mount. It should be on the

rotating part of the TMA, at the optical surface position of M4 (located at

X = 3949.660 mm from the azimuth axis).

Figure 5: Position of the theodolite, AC1 and AC2 mirrors

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NOTE: The AC1 mount should enable tip-tilt and centering adjustments.

3. Position AC2 on the +X trunnion.

4. Rotate the TMA in the altitude direction over its full range (7° to 90°) in 10°

increments and use the theodolite to measure the AC1 tilt and center at each EA.

The AC1 mirror should rotate with the telescope while the theodolite stays fixed

on the Nasmyth floor.

5. Align AC1 iteratively on the altitude axis by adjusting its tip-tilt and centering so

it has no tilt or center variation in the theodolite during rotation.

6. Align the theodolite tilt on the altitude axis by adjusting it so the autocollimated

reticle is centered.

7. Follow the same procedure to center the theodolite on the altitude axis: In visual

mode, center the AC1 target into the theodolite by shifting the theodolite in the

two directions (without changing the tilt).

8. Place AC2 on the +X trunnion of the TMA and align it on the altitude axis using

the same method you used for AC1.

NOTE: You must be able to remove AC2 and replace it in the same location accurately.

The measurements for AC1 and AC2 should be identical.

This test will identify the difference between the rotation axes of the two

TMA trunnions and provide a second reference to reposition the theodolite on

the altitude axis.

8.3 M5 and M6 Alignment

The M5 and M6 mirrors will be aligned using a laser tracker and theodolite. This

procedure requires replacing the theodolite’s eyepiece with a camera to visualize and

acquire measurements during telescope rotation. Tilt will be measured with the

theodolite in autocollimation mode, with the focus on infinity, and centering will be

measured in visual mode, with the focus on the target surface. Because the focus can

only be adjusted manually, the tilt and centering must be aligned separately in two

steps (tilt first, then centering).

M6 will be aligned first, since the theodolite must be positioned at the M5 assembly,

followed by stability testing, and then alignment of M5.

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8.3.1 M6 Alignment

The M6 assembly will be integrated on the TMA tower and positioned in the

telescope coordinate system using a laser tracker. The tilt will be aligned using the

theodolite attached to the tower.

1. Position the theodolite on the tower on a dedicated support at the M5 assembly

interface.

Set the height of the theodolite on the altitude axis.

Position the theodolite to allow autocollimation on the M6 optical surface.

2. Allow the theodolite to autocollimate onto the AC1 mirror.

3. Set the theodolite at 60º upwards via a 90° pentaprism.

4. Tilt-align M6 on theodolite so it is 30º from the altitude axis. The horizontal tilt of

M6 should be the same as the altitude axis.

5. Check the M6 position using the laser tracker and realign if necessary by

monitoring the tilt with the theodolite.

Figure 6: Theodolite turned 60º upward to align M6

8.3.2 Tower Stability Test

This test entails rotating the TMA and measuring the tilt and decenter change on AC1

to characterize accurately the stability of the tower. In this test configuration, the

theodolite will be attached to the tower and aligned on AC1.

1. Autocollimate the theodolite onto the AC1 mirror.

2. Rotate the telescope mount around the altitude axis.

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3. Use the theodolite to acquire the tilt and position of AC1 during rotation at

different EAs.

If the tower is stable during rotation, the measurement should also be stable.

If not, the theodolite measurement will show the residual tilt and decentering

with respect to the altitude axis.

8.3.3 M5 Alignment

M5 will be aligned using the same method as M6, but with the theodolite placed at a

different position.

1. Dismount the theodolite and its support from the tower.

2. Position the theodolite on the altitude axis close to the M6 mount hole, facing

AC1.

3. Integrate the M5 assembly onto the tower and align it with the laser tracker in the

telescope coordinate system.

4. Allow the theodolite to autocollimate onto AC1 and set it horizontally at 180º and

downward at 15º.

5. Align the M5 tilt onto the theodolite; the horizontal tilt should be the same as the

altitude axis.

6. Check the M5 position with the laser tracker and realign if necessary by

monitoring the tilt with the theodolite.

Figure 7: Position of the theodolite to align M5

8.4 M3 Alignment

Once M5 is aligned, the M3 assembly interface can be freed from the TMA structure.

M3 alignment can be initiated after the AC1 target has been aligned. The M3

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assembly will be mounted and initially aligned in the telescope coordinate system

using the laser tracker and attached SMRs. The tilt will be aligned with respect to

AC1 using the theodolite in autocollimation mode with the focus on infinity. In order

to center the theodolite on the altitude axis close to the M3 mirror, a support tool will

need to be attached to the TMA structure between AC1 and M3.

The following steps describe the alignment sequence:

1. Align the theodolite tilt on AC1; you do not need to center the theodolite on the

elevation axis.

2. Rotate the theodolite horizontally and vertically to point toward the M3 optical

surface.

3. Align the tilt of M3 with respect to the theodolite.

8.5 Alt-Az Alignment

8.5.1 Defining the Azimuth Axis

Another mirrored target, AC3, will be used to define the azimuth axis. This target will

be aligned perpendicular to the azimuth axis and, like the AC1/AC2 targets, it must

enable tip-tilt and centering adjustments. The theodolite will be positioned in the

Coudé to align AC3 when the telescope rotates in azimuth. The following steps

describe the main stages of the procedure:

1. Position the AC3 mirror on the azimuth axis.

2. Position the theodolite on the Coudé floor and align its tilt using Coudé

references.

3. Most likely, the M7 mirror will be available.

If so, follow the alignment procedure described in Section 8.6 so it can be

used as a fold mirror during the alt-az alignment process (Section 0).

If not, use a small fold mirror (with a diameter of at least 10mm) mounted at

the M7 position and aligned 45º vertically using the theodolite. The horizontal

tilt can be defined using the Coudé references.

4. Rotate the TMA in the azimuth direction over ±180° in 10° increments. The

Coudé floor should not rotate.

5. Using the theodolite at different azimuth angles, measure the tilt and center of

AC3.

6. Align AC3 iteratively by adjusting its tip-tilt and centering to remove and tilt and

center variation during azimuth axis rotation.

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Figure 8: AC3 autocollimation mirror and theodolite in the Coudé room

8.5.2 Alt-Az Alignment

1. Align the theodolite on the azimuth axis by adjusting its tilt so that the AC3

autocollimated reticle is centered.

2. Align the theodolite position by centering it on the azimuth axis, in visual mode:

Center the AC3 target in the theodolite eyepiece by shifting the theodolite.

Do not change the theodolite’s tilt.

3. Remove AC3.

The theodolite stays fixed so the beam emanating from it passes through M7,

M6, and M5.

This beam should autocollimate on AC1 and return into the theodolite.

4. Rotate the TMA in the alt-az directions. The Coudé does not rotate at this stage.

5. Using the theodolite, confirm that the beam orientation does not change with

TMA rotation.

AC1 has been aligned on the altitude axis and AC3 on the azimuth axis. M5

and M6 have been aligned on the altitude axis to produce a 90° folded beam.

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If the azimuth and altitude axes are perfectly perpendicular (within the

theodolite accuracy ±2 arcsec), the alt-az measurement should remain constant

when the telescope rotates.

If the axes are not perfectly perpendicular, the theodolite will measure this

deviation during alt-az rotation.

6. Adjust the tilt of M6 to correct and reduce this deviation.

8.6 M7 Alignment and Coudé Characterization

8.6.1 M7 Alignment

M7 will be aligned with respect to the azimuth axis using a laser tracker and the three

SMRs that are attached to the mirror substrate. All of the measurements discussed

below reference the Coudé coordinate system.

1. If a fold mirror was used in Section 8.5.1, remove it; otherwise, proceed to step 2.

2. Install the M7 assembly in the Coudé laboratory.

3. Position the laser tracker between M7 and M8.

4. Measure the SMRs and align the mirror to its nominal position.

5. Replace the laser tracker with the theodolite.

6. Align the theodolite tilt using the Coudé references.

7. Using either a pentaprism or dedicated theodolite support, point at 45º directly

toward the M7 optical surface.

If you use a 45º pentaprism, place it between the theodolite and the mirror.

A dedicated theodolite support requires a tool but is more accurate.

8. Align the tilt of M7 mirror with the theodolite.

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Figure 9 : Position of theodolite for M7 alignment

8.6.2 Coudé Rotation Characterization

The Coudé rotation axis should be characterized with the respect to the azimuth axis

as follows:

1. Rotate the Coudé floor over its full ±180° range in 10° increments.

2. Use the theodolite to measure the autocollimation tilt and center of AC1 through

M5, M6, and M7.

If the Coudé rotation and azimuth axes are colinear, the theodolite

measurement should be stable.

If there is a residual tilt or decenter between the two axes, this test will

measure the residual tilt and decenter of the Coudé axis.

8.6.3 Coudé Optics Alignment Preparation

A flat alignment mirror, AM, will be set up with a tilt related to the M7 mirror. This

mirror will be placed onto the M9a optical table and used to align the tilt of the M9

mirror.

1. While autocollimated on AC1, rotate the theodolite horizontally by 90º and place

AM on the M9a optical table in the theodolite’s FOV.

2. Align the tip-tilt of AM to autocollimate on the theodolite.

M7 M8

Theodolite

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Figure 10: M9 alignment configuration

8.7 M4 Alignment

Once M3, M5, M6, and M7 are aligned and the TMA and Coudé rotations have been

characterized, AC1 can be dismounted and replaced with the M4 assembly. M4 will

be aligned using only a laser tracker; a theodolite cannot be used because this mirror

is not flat.

The initial alignment step is basically mechanical because M4 has low alignment

sensitivities; the mirror can be aligned using its fiducials.

If M4 has a centered target, centering can be verified and corrected using the

theodolite, which is already aligned on the altitude axis.

Th

AM

M7

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9 COUDÉ TRANSFER OPTICS ALIGNMENT


Mirrors M8, M9, and M10 will be aligned in the Coudé coordinate system.

M8 will be aligned using only a laser tracker, M9 will be aligned using laser tracker

and theodolite, and M10 will be aligned with the CMM arm and theodolite. Note that

M8 and M9 both feature SMRs that can be used to position these mirrors accurately

in the Coudé laboratory.

Because the tolerance required for beams feeding the different instruments is tight

(typically ±10 arcsec), the most sensitive alignment parameter is the tilt of the flat

mirrors, M9 and M10. Consequently, the theodolite will be used to accurately align

the tilt of M9 with respect to M7, and the M9 mirror angle will then be used as

reference to tilt-align all other Coudé flat mirrors (and beam-splitters). Aligning all of

the flat optics with respect to the same reference eliminates the accumulation of

measurement error.

Once M9 is aligned in the Coudé coordinate system, the theodolite can be used to

align M10 with respect to M9’s optical surface. M10, a deformable mirror, will be

mounted on the optical table that supports the wavefront correction system (WFCS)

and the visible-light broadband imager (VBI). M10 can then be aligned using the

CMM arm instead of the laser tracker.

Measurements will initially be referenced in the Coudé coordinate system; however,

all Coudé measurements will be converted to the telescope’s coordinate system using

SA software. This conversion will require a set of transfer coordinates. To develop

these coordinates, a laser tracker will be used to measure SMRs (in the Coudé

system) that are located on a nonrotating Coudé wall at zero degree of the encoder.

Three SMRs will also be attached to the WFCS/VBI optical table (where the CMM

arm will also be set up) and measured using both the laser tracker and CMM arm.

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Laser tracker

Setup of the CMM arm and transfer of the coordinate system

Mounting and alignment of DM assembly in Coudé coordinate system

Alignment of M10 mirror angle wrt to M9 mirror with the theodolite

Mounting, alignment and calibration of the focusing optic and wavefront sensor

Mounting and alignment of M9 assembly in Coudé coordinate system

Mounting and alignment of M8 assembly in Coudé coordinate system

M8 assembly

Alignment of M9 mirror angle wrt to M7 mirror with the theodolite

Wavefront error measurement on the sky of selected stars in center, nominal and extended field at different Alt-Az angles

M2 optimization

Telescope performances at different Alt-AzFinal telescope characterization

M9 assembly

TheodoliteAM mirror

CMM arm

DM assembly

Focusing optic

Wavefront sensor

Figure 11 : Coudé transfer optics alignment flowchart

9.2 Alignment of M8, M9, and M10 Mirrors

9.2.1 M8

The M8 assembly will be mounted and then aligned using the laser tracker in the

Coudé coordinate system. Data extracted from the CAD model and as-built will be

entered into SA to calculate the real position of the vertex and optical axis with

respect to a local coordinate system that will be developed using the mirror SMRs.

During the alignment process, the mirror’s three SMR positions will be measured in

the Coudé coordinate system.

SA will be used to achieve two objectives during this process: to convert the

measured position of the vertex and optical axis to the telescope coordinate system

and to calculate the needed shift and tilt to position the M8 optical surface at its

nominal position.

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9.2.2 M9

The M9 assembly will be mounted and aligned using the laser tracker in the Coudé

coordinate system. The M9 mirror angle will be aligned with respect to M7 using the

theodolite (see Section 8.6.1).

1. Place the theodolite at position “Th” and allow M9 to autocollimate.

2. Set up the theodolite.

3. Perform an autocollimation on AM.

4. Rotate the theodolite toward M9 by 89.3425º.

5. Align M9’s tilt to autocollimate on the theodolite.

6. Check the position of M9 with the laser tracker, and correct it if needed by

keeping the theodolite autocollimated on the M9 optical surface.

Figure 12: M9 alignment configuration

9.2.3 CMM Arm Setup

The Coudé optics and CMM arm must be mounted on the same optical table. This

step will ensure that the SMRs are accessible to the CMM arm on the WFCS/VBI

bench.

1. Set up the CMM arm on the bench.

2. Place on the three SMRs on the optical table and measure their position with the

laser tracker in the Coudé coordinate system.

3. Measure the three SMRs with CMM arm.

4. In SA, cophase the CMM arm and laser tracker so they are measuring in the

Coudé coordinate system.

Th

AM

M7

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9.2.4 M10

The M10 assembly will be mounted and aligned with the CMM arm in the Coudé

coordinate system. The tilt of M10 will be set with respect to M9.

NOTE: The VBI and WFC optics should not yet be mounted on the benches.

1. Mount and set up the M10 assembly on its bench.

2. Align the position of M10 with the CMM arm.

3. Place the theodolite at the intersection between the normal of the two mirrors, M9

and M10.

This intersection is represented on the following drawing by Th.

M9 will serve as reference to align M10’s tilt.

5. Setup the theodolite on a stand.

6. Autocollimate the theodolite on M9.

7. Rotate the theodolite toward M10 by 155.0000º.

8. Align M10 in autocollimation on the theodolite.

Figure 13 : M10 alignment configuration

M7

M9

M8

M10

Th

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10 TELESCOPE RESIDUAL ERROR

ALIGNMENT

10.1 Method

The telescope’s final WFE will be measured by adding calibrated, focusing optics

that will re-image the collimated beam emanating from M10. The SHWS will be used

to measure the WFE on the sky in the center of the field and in nominal and extended

fields. The WFE will be acquired for the entire range of alt-az angles, and the M2/M4

mirrors will be used to compensate for any residual aberrations—typically

astigmatism, coma, and focus—that are related to the manufacturing tolerances of the

Coudé mirrors. Misalignment should not induce significant aberrations because the

alignment sensitivities of M7, M8, M9, and M10 are negligible.

10.2 Cumulative Focus Error

The procedures described in Sections 6–9 of this plan will result in a fully aligned

telescope; however, residual power errors within each optic will cause a non-ideal

configuration. To minimize the effect on M2 and the PA&C Gregorian pinhole,

adjustments will be made only to M4, in two iterations, based on the following:

An estimate of the initial shift offset from the as-built test results for each optic

The final measurement at the M10 location, where M4 will be offset to

compensate for the residual error

Initial estimates for the offset, described in TN-0240, “IT&C Focus Compensation

Analysis,” are for less than 5mm of movement in M4.

10.3 Final Measurement and Correction

This last task will indicate how the telescope will perform after optical alignment and

before setup of the WFCS. General steps are outlined below.

1. Mount and align the imaging optic system needed to measure the telescope

wavefront.

2. Mount the WFS.

3. Calibrate the WFS and imaging optic system.

4. Use the WFS to measure the wavefronts of selected stars—on axis, in the center

of the field, and in the nominal and extended field, at different alt-az positions of

the telescope. These measurements will determine the telescope performances.

5. Optimize the telescope performances initially using M4 as compensator. You can

also use the M2 mirror to compensate for residual astigmatism, coma, and focus.

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6. Measure the telescope’s focal length:

Select different known stars in the FOV.

Measure the relative position of the WFS to center these stars in the field.

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11 FIDO ALIGNMENT


The feed optics consists of the following:

The beamsplitter for the WFC (WFC-BS1).

Five additional beamsplitters (CL2, CL2a, CL3, CL3a, and CL4)

Two mirrors (M9a and DL-FM1)

The procedure for aligning these optics will be similar to the one used for the Coudé

mirrors: Their position will be determined using the laser tracker or CMM arm and

their tilt will be aligned using the theodolite with respect to M9. The initial alignment

is trivial. The main challenge stems from the need for interchangeability between

CL2, CL2a, CL3, CL3a, and CL4 during telescope operation. In addition, the lack of

space between optical elements imposes constraints, which are accounted for in this

alignment plan.

All alignments will reference the Coudé coordinate system. The beamsplitters and

mirrors will be aligned using a laser tracker and CMM arm. The beamsplitter mounts

will be used as metrology surfaces. For flat surfaces (M9a, DL-FM1, and all

beamsplitters), tilt—the most sensitive alignment parameter— will be aligned using a

theodolite. Once M9 is aligned in the Coudé coordinate system, it will serve as the

reference for all of the beamsplitter tilts, an approach that is designed to eliminate

theodolite measurement errors.

The M9a mirror will be aligned using the laser tracker and theodolite. The other

optics—WFC-BS1, CL2, CL3, CL4, CL2a, CL3a, and DL-FM1—will be aligned

using the CMM arm and theodolite. The theodolite will be mounted on a stand for

M9a, WFC-BS1, CL3, and CL4 and on the optical table for CL2, CL2a, CL3a and

DL-FM1. The alignment sequence described below considers the position and space

needed for the theodolite. Note that M9a can be aligned at any time in the sequence.

NOTE: Before the beamsplitter alignment procedure can be initiated, all of the benches must be integrated at their position in the Coudé room and only M10 must be installed on its bench.

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CMM arm

Alignment of M9a angle wrt to M9 mirror with the theodolite

Mounting and alignment of CL2a assembly with the CMM arm

Alignment of CL2a angle wrt to M9 mirror with the theodolite

Mounting and alignment of CL2 assembly with the CMM arm

TheodoliteAlignment of WFC-BS1 angle wrt to M9

mirror with the theodolite

Mounting and alignment of WFC-BS1 assembly with the CMM arm

WFC-BS1 assembly

Mounting and alignment of M9a assembly with the laser tracker

Alignment of CL2 angle wrt to M9 mirror with the theodolite

Laser tracker

M9a assembly

CL2a assembly

CL2 assembly

Mounting and alignment of CL3a assembly with the CMM arm

Alignment of CL3a angle wrt to M9 mirror with the theodolite

CL3a assembly

Setup the theodolite at the second position and transfer coordinate system

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Alignment of DL-FM1 angle wrt to CL2 with the theodolite

Mounting and alignment of CL3 and CL4 assemblies with the CMM arm

Alignment of CL3 and CL4 angles wrt to M9 mirror with the theodolite

CL3 and CL4 assemblies

Mounting and alignment of DL-FM1 assembly with the CMM arm

DL-FM1 assembly

Figure 14 : Coudé beamsplitters and mirrors alignment flowchart

11.2 Alignment Procedures

11.2.1 WFC-BS1

WFC-BS1 should be aligned after M10 because the theodolite will already be in close

proximity.

1. Mount and set up the WFC-BS1 assembly on the bench.

2. Align WFC-BS1 using the CMM arm.

3. Place the theodolite on a stand at the intersection between the normal of M9 and

WFC-BS1. M9 will serve as reference for aligning the tilt of WFC-BS1.



6. Rotate the theodolite toward WFC-BS1 by 154.5000º.

7. Align WFC-BS1 to be in autocollimation on the theodolite.

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Figure 15 : WFC-BS1 beamsplitter alignment configuration

11.2.2 M9a

M9a will be aligned using the laser tracker. Its tilt will be set with respect to M9 using

the theodolite:

1. Mount and set up M9a assembly.

2. Align M9a using the laser tracker.

3. Place the theodolite on a stand at the intersection between the normal of M9 and

M9a (coordinates TBD). M9 will serve as reference for aligning the tilt of M9a.



6. Rotate the theodolite toward M9a by 161.0000º.

7. Align M9a to be in autocollimation on the theodolite.

M9

WFC-BS1

Th

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Figure 16 : M9a alignment configuration

11.2.3 CL2a

CL2a will be aligned using the CMM arm. Its tilt will be set with respect to M9 using

the theodolite.

2. Mount and set up the CL2a assembly on its bench.

3. Align CL2a using the CMM arm.

4. Place the theodolite on the optical table at the intersection between the normal of

M9 and CL2a. M9 will serve as reference to align the tilt of CL2a.



7. Rotate the theodolite toward CL2a by 125.2434º.

8. Align CL2a to be in autocollimation on the theodolite.

M9

M9a

Th

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Figure 17 : CL2a beamsplitter alignment configuration

11.2.4 CL2

CL2 will be aligned using the CMM arm. Its tilt will be set with respect to M9 using

the theodolite:

1. Mount and set up the CL2 assembly on its bench.

2. Align CL2 using the CMM arm.


M9 and CL2. M9 will serve as reference to align the tilt of CL2.



6. Rotate the theodolite toward CL2 by 125.7434º.

7. Align CL2 in autocollimation on the theodolite.

M9

CL2a

Th

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Figure 18 : CL2 beamsplitter alignment configuration

11.2.5 CL3a

CL3a will be aligned using the CMM arm. The tilt of CL3a will be set with respect to

M9 using the theodolite:

1. Mount and set up the CL3a assembly on its bench.

2. Align CL3a with the CMM arm.


M9 and CL3a. M9 will serve as reference to align the tilt of CL3a.



6. Rotate the theodolite toward CL3a by 154.9991º.

7. Align CL3a in autocollimation on the theodolite.

M9 CL2

Th

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Figure 19 : CL3a beamsplitter alignment configuration

11.2.6 CL3 and CL4

CL3 and CL4 will be aligned using the CMM arm. Their tilt will be set with respect

to M9 using the theodolite. CL3 and CL4 can be tilt-aligned simultaneously because a

single theodolite position allows autocollimation on M9, CL3, and CL4.

1. Move the CMM arm to a new position on the bench that allows the measurement

of CL3, CL4, and the DL-FM1 mirror.

2. Measure the three SMRs using the CMM arm. These measurements will be

conducted in the Coudé coordinate system and used to create a new coordinate

system that is identical but shifted.

3. In SA, cophase the new coordinate system so the CMM arm is measuring in the

Coudé coordinate system.

4. Mount and set up the CL3 assembly on the bench.


6. Mount and set up the CL4 assembly.


8. Place the theodolite on a stand at the intersection between the normal of M9, CL3

and CL4 (coordinates TBD). M9 will serve as reference for aligning the tilt of

CL3 and CL4.




M9 CL3a

Th

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13. Repeat the autocollimation on M9.



Figure 20 : CL3 and CL4 beamsplitter alignment configuration

11.2.7 DL-FM1

DL-FM1 will be aligned using the CMM arm. Its tilt cannot be aligned directly with

respect to M9; instead, since its normal line intersects that of the CL2 beamsplitter,

DL-FM1 will be aligned with respect to CL2.

1. Mount the DL-FM1 assembly.

2. Align DL-FM1 using the CMM arm.


DL-FM1 and CL2. CL2 will serve as reference for aligning the tilt of DL-FM1.


5. Autocollimate the theodolite on CL2.

6. Rotate the theodolite toward DL-FM1 by 120.7440º.

7. Align DL-FM1 in autocollimation on the theodolite.

M9 CL3

Th

CL4

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Figure 21 : DL-FM1 alignment configuration

DL-FM1

CL2

Th