1. FELT 1 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

2. FELT 2 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

3. FELT 3 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

4. FELT 4 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

5. FELT 5 FELT features less “obstructive” tracker for improve IR performance

6. FELT 6 Why Build this ELT? Meets large fraction of “MAXAT” science goals Steppingstone to Even Larger Telescopes US history predicts “parsimonious” government funding The ELT is Cheaper and Has Less Technical Risk Than Any Other Currently Proposed 30-meter Approach

7. FELT 7 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”

8. FELT 8 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

9. FELT 9 Fixed Elevation: Look at how telescopes used C.R. Benn & R. Martin, 1987 QJRAS 28, 481 FELT effective limit

10. FELT 10 FELT has a sky coverage = Latitude +/- [Zenith Angle + 1/2 tracker FoV] 72-75 % conventional telescope

11. FELT 11 ~ 1 Hour Tracking time consistent with exposure time statistics available C.R. Benn & R. Martin, 1987 QJRAS 28, 481

12. FELT 12 Varying Pupil Will need varying cold baffle in IR instruments May present challenges for AO system Eliminate by making mirror diameter ~39 meters?

13. FELT 13 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!

14. FELT 14 Why Build this ELT? Could meet large fraction of science goals Steppingstone to Even Larger Telescopes US history predicts “parsimonious” government funding FELT is Cheaper and Has Less Technical Risk Than Any Other Currently Proposed 30-meter Approach

15. FELT 15 Why a low cost option? Expenditures on 8-meter class OIR telescope as fraction of GNP normalized to ESO countries With “private” telescopes US “national” funding; i.e remove philanthropy History has some lessons!

16. FELT 16 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

17. FELT 17 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

18. FELT 18 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

19. FELT 19 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

20. FELT 20 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

21. FELT 21 Deliverables Report detailing system concepts and performance Simulator for at least two baseline instruments