AgendaGSMT Controls Workshop, 11 September, 2001

9:00 am      GSMT Overview – Brooke Gregory, Larry Stepp

9:30 am      Pointing Control for a Giant Segmented Mirror Telescope                 – Patrick Wallace

10:00 am      Implications of Wind Testing Results on the GSMT Control Systems -- David Smith

10:30 am      To be defined – Mark Whorton

11:00 am     MSFC's Heritage in Segmented Mirror Control Technology                 – John Rakoczy

11:30 am      Current concepts and status of GSMT control system                 – George Angeli

12:00 pm       Lunch

1:00 pm       Informal discussions

5:00 pm Adjourn

B. Gregory, L. Stepp 11 September 2001

GSMT Overview

AURA NIO: Mission

• In response to the AASC call for a Giant Segmented Mirror Telescope (GSMT) AURA formed a New Initiatives Office (NIO)

– collaborative effort between NOAO and Gemini to explore design concepts for a GSMT

• NIO mission

“ to ensure broad astronomy community access to a 30m telescope contemporary in time with ALMA and NGST, by playing a key role in scientific and technical studies leading to the creation of a GSMT.”

AURA New Initiatives Office

E n g in eerin g O vers ig h tJ im O sch m an n

P ro jec t S c ien tis tS teve S trom

C on trac ted s tu d iesTB D

P art t im e su p p ortN O A O & G em in i

A d m in is tra tive ass is tan tJen n ife r P u rce ll

O p ticsL arry S tep p

C on tro lsG eorg e A n g e li (N O A O )

A d ap tive O p ticsB ren t E lle rb roek (G em in i)

In s tru m en tsS am B ard en (N O A O )

M ech an ica l D es ig n erR ick R ob les

S tru c tu resP au l G ille tt

S ite Tes tin gA lis ta ir W a lker (N O A O )

P rog ram M an ag erL arry S tep p

S ys tem s S c ien tis tB rooke G reg ory

M an ag em en t B oardM att M ou n ta inJerem y M ou ldW illiam S m ith

Adaptive OpticsEllerbroek/Rigaut (Gemini)

ControlsGeorge Angeli

Opto-mechanicsMyung Cho

Contracted Studies

Objectives: Next 2 years

• Develop point design for GSMT & instruments

• Develop key technical solutions – Adaptive optics– Active compensation of wind buffeting– Mirror segment fabrication

• Investigate design-to-cost considerations

• Carry out conceptual design activities that support and complement other ELT efforts

• Develop a partnership to build GSMT

AURA New Initiatives OfficeApproach to GSMT Design

Parallel efforts:• Understand the scientific context for GSMT

• Develop the key science requirements

• Address challenges common to all ELTs• Wind-loading

• Adaptive optics

• Site

• Develop a Point Design • Based on initial science goals

• Key part is conceptual design of instruments

What is a “Point Design”?

A point design is a learning exercise that:– Explores a single, plausible design

– Helps identify key technical issues

– Helps define factors important to the science requirements

– Provides an opportunity to develop necessary analytical methods

A point design is not:– A trade study that evaluates all possible options

– A design that anyone is proposing to build

Point Design: Scientific Motivations

• Enable high-Strehl performance over several arc-minute fields– Stellar populations; galactic kinematics; chemistry

• Provide a practical basis for wide-field, native seeing-limited instruments– Origin of large-scale structure

• Enable high sensitivity mid-IR spectroscopy– Detection of forming planetary systems

Telescope design should be driven by needs of instruments

Point Design: Basic Design Concepts

• Explore a radio telescope approach– Possible structural advantages– Possible advantages in accommodating large instruments

• Use aspheric optics – good image after two reflections• Incorporate an adaptive M2

– Compensate for wind-buffeting– Reduce thermal background– Deliver enhanced-seeing images

• Explore prime focus option– Attractive enabler for wide-field science– Cost-saving in instrument design

Radio Telescope Structural Design Fast primary focal ratio Lightweight steel truss structure Small secondary mirror Secondary supported on tripod structure Elevation axis behind primary mirror Span between elevation bearings is less than

diameter of primary mirror – allows direct load path

Optical Design

Primary diameter: 30 meters

Primary focal ratio: f/1

Secondary diameter: 2 meters

Secondary focal ratio: f/18.75

Optical design: Classical Cassegrain

Point Design Structure

Concept developed by Joe Antebi of Simpson Gumpertz & Heger

• Based on radio telescope• Space frame truss• Single counterweight• Cross bracing of M2 support

Point Design Structure

Typical 'raft', 7 mirrors per raft

Special raft - 6 places, 4 mirrors per raft

1.152 m mirror across flats

Circle, 30m dia.

Plan View of Structure Pattern of segments

Gemini

Lower Elevation Structure

Primary Mirror Segments

• Size chosen for point design: • 1.15-m across flats -- 1.33-m corner to corner• 50 mm thickness

• Number of segments: 618 • Maximum asphericity ~ 110 microns (equal to Keck)

Segment Support

• Point design axial support is 18-point whiffletree• FEA Gravity deflection 15 nm RMS

Wind Loading

• Primary challenge may be wind buffeting– More critical than for existing telescopes

• Structural resonances closer to peak wind power• Wind may limit performance more than local seeing

• Solutions include:– Site selection for low wind speed– Optimizing enclosure design– Dynamic compensation

• Adaptive Optics• Active structural damping

Initital Structural Analysis

X Y

Z

Output Set: Mode 1, 2.156537 Hz, Deformed(0.0673): Total Translation

Horizon Pointing - Mode 1 = 2.16 Hz

Structural Analysis

• Total weight of elevation structure – 700 tonnes• Total moving weight – 1400 tonnes• Gravity deflections ~ 5-25 mm• Wind buffeting response ~ 10-100 microns

Deflections are primarily rigid-body motions• Lowest resonant frequencies ~ 2 Hz

Instruments

• NIO team developing design concepts– Multi-Object, Multi-Fiber, Optical Spectrograph – MOMFOS– Near IR Deployable Integral Field Spectrograph – NIRDIF– MCAO-fed near-IR imager– Mid-IR, High Dispersion, AO Spectrograph – MIHDAS

• Build on extant concepts where possible• Define major design challenges• Identify needed technologies

Instrument Locations on Telescope

• Fixed Gravity Cass• Direct-fed Nasmyth• Fiber-fed Nasmyth• Prime Focus• Co-moving Cass

View showing Fixed Gravity Cassinstrument

MOMFOS with Prime Focus

Corrector

Conceptual design fits in a 3m dia by 5m long cylinder

Instrument Locations on Telescope

View showing Co-moving Cassinstrument

MCAO/AO foci and instruments

MCAO opticsmoves with telescope

Narrow field AO ornarrow field seeing limited port

MCAO Imagerat vertical Nasmyth

elevation axis

4m

Oschmann et al (2001)

MCAO System: Current Layout

Instrument Locations on Telescope

MCAO-fedNIRDIForMCAO Imager

Cass-fedMIHDAS

Fiber-fedMOMFOS

Mayall, Gemini and GSMT Enclosuresat same scale

Mayall Gemini GSMT

McKale Center – Univ of Arizona

GSMT – at same scale

Key Point-Design Features

• Radio telescope structure– Advantages:

• Direct load path to elevation bearings • Cass focus can be just behind M1• Allows small secondary mirror – can

be adaptive• Allows MCAO system ahead of

Nasmyth focus• Allows many gravity-invariant

instrument locations – Disadvantage:

• Requires counterweight• Sweeps out larger volume in enclosure

Key Point-Design Features

• F/1 primary mirror– Advantages:

• Reduces size of enclosure

• Reduces flexure of optical support structure

• Reduces counterweights required

– Disadvantages:• Increased sensitivity to

misalignment

• Increased asphericity of segments

Key Point-Design Features

• Paraboloidal primary– Advantages:

• Good image quality over 10-15 arcmin field with only two reflections

• Lower emissivity for mid-IR

• Compatible with laser guide stars

– Disadvantages:

• Higher segment fabrication cost

• Increased sensitivity to segment alignment

Key Point-Design Features

• 2m diameter adaptive secondary mirror– Advantages:

• Correction of low-order M1 modes

• Enhanced native seeing

• Good performance in mid-IR

• First stage in high-order AO system

– Disadvantages:• Increased difficulty (i.e. cost)

Goal: 8000 actuators30cm spacing on M1

Key Point-Design Features

• Prime focus location for MOMFOS– Advantages:

• Fast focal ratio leads to instrument of reasonable size

• Adaptive prime focus corrector allows enhanced seeing performance

– Disadvantages:• Issues of interchange with M2

Key Enabling Techniques: Active and Adaptive Optics

Active Systems:• M1 segment rigid body position

– ~ 1 Hz– Piston, tip & tilt

• M1 segment figure control – – Based on look-up table ~ 0.1 Hz– Low order -- Astigmatism, focus, trefoil, coma

• M2 rigid body motion– ~ 5-10 Hz– Five axes – active focus & alignment, image stabilization

• Active structural elements (?)– Active alignment– Active damping

Key Enabling Techniques: Active and Adaptive Optics

Adaptive Systems:• Adaptive secondary mirror

• ~ 20-50 Hz

• ~ 1000-10,000 actuators

• Adaptive mirror in prime focus corrector

• Multi-conjugate wide-field AO

• ~ 3 DMs

• Laser Guide Stars

• High-order narrow-field conventional AO

• ~ 10,000 – 50,000 actuators

Active and Adaptive Optics will be integrated into GSMT Telescope and Instrument concepts from the start