felt 1 study of the capability and configuration of a fixed mirror extremely large telescope (felt)...
TRANSCRIPT
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
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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
0
0.2
0.4
0.6
0.8
1
1.2
With “private” telescopes
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1.2
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