FELT1

Study of the capability and configuration of a fixed mirror Extremely Large Telescope

(FELT)

• Low cost path to large telescope• Primary concern is compromise to science

missions of fixed mirror design – Address by honest and through assesment of

performance vis-à-vis a conventional design (CELT)

“Everybody” agrees that FELT can be built for ~$300million but “nobody” want’s this telescope

FELT2

•Arecibo Concept Using Spherical Primary• Spherical Aberration Corrected • Diffraction limited > 1.5 microns• Primary Utilizing 128 Hexagonal 2.3 m

Segments• 540 Azimuth Rotation to Access > 70% of

Sky• Tracking Via Hexapod…Provides All D.O.F.

•Space Frame Dome with Steel Frame for Shutter

• Design Compatible with AO; pupil in corrector

The FELT Design

FELT3

Basic FELT attributes

• Modeled on the Hobby-Eberly concept– 30 meter diameter– +/- 6 degree tracker range– 3 to 6 arc-minute science FoV

• FELT changes in HET concept– Tracking via a hexapod– Lower emissivity structure– Improved corrector design– Add AO capability

FELT4

The Hobby-Eberly Experience

• Breaking the Cost Curves– “If You Want Something Different to Happen

then You Have to Do Something Different”– Accept Some Performance Compromises

(tracking time, FoV) to Open Up Engineering Design Space

• Savings Best Realized by Focussing on the Major Cost Elements of Observatories

• Innovation Not Extrapolation Yields Significant Cost Savings

• Requires Enabling Boundary Conditions: e.g. Fixed Gravity Vector, Reduced Moving Mass, Identical Segments

FELT5

FELT features less “obstructive” tracker for improve IR

performance

FELT6

Why Build this ELT?

• Meets large fraction of “MAXAT” science goals

• Steppingstone to Even Larger Telescopes

• US history predicts “parsimonious” government fundingThe ELT is Cheaper and Has Less

Technical Risk Than Any Other Currently Proposed 30-meter Approach

FELT7

0

1000

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0 10 20 30 40 50Telescope Diameter

Tel

esco

pe

Co

st (

Mil

lio

n $

US

)

Keck

VLT

Gemini

HET

Cost ~ D 2.5

CELT must beak cost

curve 6.8 x to get to

$400 million

FELT at ~$300

million with no

“innovation factor”

FELT8

FELT Limitations

• Fixed Elevation– Fixed elevation limits sky coverage

– Sky coverage is zenith angle +/- tracking range

• Limited tracking time– Tracking time is limited by tracker angular FoV on

the sky

• Varying Pupil– The maximum exit pupil is only realized for a

limited time

– Varying pupil can present challenges in baffling

FELT9

Fixed Elevation:

• Look at how telescopes used

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0 10 20 30 40 50 60 70 80 90

Zenith Distance (degrees)

# O

bse

rvat

ion

s

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Cu

mu

lati

ve f

ract

ion

C.R. Benn & R. Martin,

1987 QJRAS 28, 481

FELT effective

limit

FELT10

FELT has a sky coverage

= Latitude +/- [Zenith Angle + 1/2 tracker FoV]

72-75 % conventional

telescope

FELT11

~ 1 Hour Tracking time consistent with exposure time statistics

available

t low t high #Obs cum # Cum fration

1 3 166 166 0.024 9 145 311 0.0310 31 578 889 0.1032 99 618 1507 0.17

100 316 2437 3944 0.44317 999 2231 6175 0.681000 3162 2827 9002 0.993163 9999 54 9056 1.00

10000 0 9056 1.00

Exposure times

C.R. Benn & R. Martin,

1987 QJRAS 28, 481

FELT12

Varying Pupil

Will need varying cold baffle in IR instruments

May present challenges for AO systemEliminate by making mirror

diameter ~39 meters?

FELT13

FELT as a Stepping Stone...

• Advance Telescope Structural Design

• Large Enclosure Dome Technology

• Optical Fabrication• AO technology • Instrumentation for ELTs

A 30-meter ELT is a Rational Technical Increment for Decade

2000-2010 and Leads for the Future

Also need instrument innovation factors of 5-10!

FELT14

Why Build this ELT?

• Could meet large fraction of science goals

• Steppingstone to Even Larger Telescopes

• US history predicts “parsimonious” government fundingFELT is Cheaper and Has Less

Technical Risk Than Any Other Currently Proposed 30-meter Approach

FELT15

Why a low cost option?

Expenditures on 8-meter class OIR telescope as fraction of GNP normalized to ESO countries

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0.6

0.8

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1.2

With “private” telescopes

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US “national” funding; i.e

remove philanthropy

History has some lessons!

FELT16

FELT Instruments?

• FELT instruments can be in tracker– New technology for

small IR instruments• R 10,000 Si Grism AO

spectrograph with 25 mm aperture!

• Very large instruments could be at fixed focus– Fixed focus optical

designs need to be explored

FELT17

Scope of Study

• Define a baseline observatory system (telescope and instruments) including accurate performance model and cost estimate.

• Develop a comprehensive science performance model that will permit the astronomical community to accurately judge the efficacy of the ELT concept

FELT18

Refinement of Requirements and Optical Configuration

• Optical configuration & Performance goals– Image scale– Field of view– Throughput– Pointing and tracking

• Science performance

• Technical challenges

Develop top level system error

budget

Adaptive Optics capability must be integral in the FELT concept

FELT19

Performance modeling 1

• Basic focal plane performance model– Natural seeing performance limited

– AO

– Use data for two “real sites”• MK, Atacama?

– Use standard packages (IDL, Matlab)

• Performance model both optical configurations– Prime focus

– Fixed focus

FELT20

Performance modeling 2

• Performance of science instruments– Survey CELT, OWL and NGST for comparisons

– Baseline defined in NBT Feb 2000 workshop

– Use small workshop to refine baseline for FELT

FoV (min) MOS Rlow Rhigh IQ low high Other

LSS, Galaxy Formation, star pops 30 10000 2000 10000 0.5 400 2400LSS 2 100 2000 10000 0.01 800 2400 background removalStar formation, star pops, Gal substructure 2 100 2000 10000 0.01359 800 10000 bkg removal, RV stabilityPlanet formation 1/60 1 100000 0.0302 4000 10000 high s/nPlanet Evolution 1/60 1 2000 0.01359 800 10000 mod strehlPlanet Evolution 1/10 Imager 5 5 0.01359 800 10000 high strehlStar pops, Galaxy substructure 3 Imager 5 5 0.01359 800 2400 mod strehl

FELT21

Deliverables

• Report detailing system concepts and performance

• Simulator for at least two baseline instruments