MAX-AT Workshop: Madison, WI August 28-29 1998
The purpose of the workshop was to develop the scientific basis and the critical performance specifications for a very large aperture telescope (30-50 meter). Both science and technical issues were examined in view of the existing and future role of space and ground base observatories (Next Generation Space Telescope, Keck, VLT, and Gemini).
- In view of the large number of science projects identified, there is sufficient scientific interest in building a 30-50m telescope observatory. Moreover, there was consensus already at the end of the first day of the meeting that MAX-AT should be maximized to do science based on high resolution imaging and spectroscopy.
- Regarding the type of telescope, it was agreed that the community should be educated about the limitations of the two types of designs considered (interferometers and single filled-aperture telescopes) before committing ourselves with either one. (Note that the literature contains already a large number of papers presenting astrophysical results based on optical interferometric observations.)
- Most of the participants, about 2/3 of them, favored science programs that required high signal-to-noise ratio high-resolution spectroscopy. The killer applications of MAX-AT can be summarized as follows:
- Planet formation
- Cepheids out to redshifts z~0.1 (measure Ho)
- Age determination of giants (measure to)
- Geometry of the Universe via Supernovae at z~3 (measure qo)
- MAX-AT's added value is complemented with existing and future facilities: NGST, MMA, Gemini, Keck, VLT, HST, VLA (plus upgrades), VLBI, Planck, MAP, and optical interferometers. In brief, science drivers outlined in (3) depend critically on NGST capabilities and similar timing while technological feasibilities depend on existing (8-10m telescopes) capabilities and on tests performed on 4-8m telescopes.
- Adaptive Optics will be essential in MAX-AT.
- The challenges faced by the community in building a MAX-AT are technological and financial in nature. First, spectroscopy is challenging for subarcsec slits, a technology that is not known yet. Second, the operations budget may be larger than NOAO and Gemini together so it is very likely that the (international) astronomical community will have to shut down working facilities to operate MAX-AT.
[III] SCIENCE DRIVERS:
One of the main purposes of the workshop was to identify science projects that will require very large apertures. In this Section, I tried to summarize the science projects that were identified by the participants. It is not meant to be a comprehensive text (as many of the topics are not my expertise) but they should provide you with a rough baseline of the fundamental science objectives that will drive MAX-AT.
There was an artificial division of different science topics. The division was as follows: Stellar Astrophysics, Galaxies/AGN, Cosmology and Large Scale Structure, and Technology. Each participant joined one or more panel(s) and presented what they felt were strong science drivers. Each session's chair gave a report at the end.
A) STELLAR ASTROPHYSICS
Chair: Stephen Strom
Similar to the main theme of the Origins Plan of the "HST and beyond Committee", the theme of this session turned out to be the formation of stellar and planetary systems. The three main tent-poles were identified as follows:
- Formation of stars and planetary systems (disks)
- Planet Formation
- Imaging of planets around nearby stars
i) Formation of stars and planetary systems
The picture of stellar formation is that of a stellar seed with low angular momentum from which a disk is formed. For a protostellar system in this phase MAX-AT could trace the infalling gas from the outer envelope to the disk down to the centrifugal radius (Rc ~ 10 AU). The mass infall rate could be quantified and measured as a function of position and angular momentum.
- Angular resolutions of 10-25 mas to study disks at Rc ~ 10 AU for stars at distances <~ 1 kpc.
- Spectral resolutions of R~105 to obtain velocity resolutions of at least 3 km/s to estimate mass accretion rate.
Note: These requirements are contrasted to the capabilities of the MMA and NGST. Both are restricted to R>15 AU with NGST having insufficient high spectral resolution capabilities. (The point being that the planetary systems found so far are very close to the central star so that in order to find these systems at larger volumes higher angular resolutions are needed.)
ii) Planet Formation (identify where, when, and how planets form)
In the current picture of disk formation, the question remains: Do planets form during or after the accretion phase? Absorption lines observed in accretion disks are believed to arise from gaps in the Keplerian disk. These gaps are postulated to be locations where planets are forming. From simulations of e.g. CO lines with resolutions of R~105 and S/N>>100 Gemini would require 10 hours to resolve the shapes of the spectroscopic lines, while NGST would require 1 hr. MAX-AT's gain in sensitivity would allow a survey of many of these systems (>>10) and enable a systematic study of the absorption patterns.
- High sensitivity to survey >>10 systems.
- Spectral resolution (R~105) to study line features.
iii) Image planets around nearby stars (observe maturing and mature planetary formation).
The goal is to image Jupiter mass planets out to ~ 100 pc.
MAX-AT requirements: Angular resolution of 60 mas to image planets out to R<5AU.
Note: NGST/MMA will do this at 1/5 of the angular resolution but will lack a large enough volume to survey.
- If NGST does image Jupiter size planets, the sensitivity gains of MAX-AT should enable a search for Earth size planets. Imaging could then encompass a range in planetary masses.
- One current mystery is that planets are found "too near" to the central star. This can be a result of accretion driven planetary formation.
Chair: Evan Skillman
Personal remarks of chair:
- No tent-pole projects were identified. The number of science projects increase with the number of scientists involved. (The sell to the astronomical community should be easy.)
- All AGN projects were tied to interferometry as their study involves the inner sub-parsec regions.
- No "Light Bucket" projects were identified as the gains were considered incremental over VLT.
Following are a number of science projects identified by the participants of this panel:
- Spectra of high redshifts, faint (NGST) protogalactic fragments. The redshift range is z~0.1-3. To study the dynamics of the galaxies a resolution of R~5,000 - 10,000 is needed.
- Spectra of star clusters to continue study of the age-metallicity degeneracy.
- Galaxy dynamics of nearby elliptical galaxies. Main goal is to break the mass-isotropy degeneracy. Spectral resolution: R~105.
- AGN: Interferometry better than 1 mas to see into the cores of AGNs. Orientation of link for unification pictures. Goals: to study BLR structure and test unification theory.
- Stellar population of nearby galaxies (< 25Mpc). Measure a variety of detailed SF histories using CMD analysis. Measure chemical evolution history.
- Individual stellar spectroscopy. Study single stars in different environments (e.g. HDF stellar content). Study intracluster stars metallicity out to Virgo and Fornax.
C) Cosmology and Large Scale Structure
Chair: Matthew Bershady
- Cosmological parameters - testing the standard model: Ho to >1 ??
- Flows 0.03<z<0.1 via Cepheids: Distances are measured on optical images at 20 mas resolution @ 0.8 microns on a fov of 20 - 30 arcsec. This will measure omega matter and Ho in far fields.
- to (age of stars) via radioactive decay of Thorium in old giants below RGB tip. It will require optical spectroscopy at R~3x105 and S/N=1000.
- Geometry of the Universe (qo, Lambda) via Supernovae at z~3. Main goal is to break degeneracy of omega matter and omega lambda. Optical-NIR spectroscopy at R ~ 6,000 - 10,000. As much angular resolution as possible but competing with MAP and NGST maybe a problem.
- Galaxy/Structure Formation
- SF History and protogalaxies: Pop III SN at maximum redshift (z~10?). Spectroscopic identification with redshifts of targets flagged by NGST. Optical and NIR spectroscopy at R ~ 6,000 - 10,000 and high angular resolution are required.
- Other projects identified are:
- QSO ALS: Deuterium problem; nature of the clouds; LSS probe. High R spectroscopy 3x105 - 106 in the optical down to 0.3 microns. ("Light Bucket" case.)
- High redshift LSS: CFA/Sloan @ z≤3 Spectroscopic redshifts will give clue of CDM and omega. It will require optical - NIR capabilities at R~6-10x103. Here FOV: bigger is better with diffraction from ambient (seeing limited) to diffraction limited case.
- Galaxy Formation: mass assembly (mergers?) Spatially resolved kinematics at R~104 and angular resolutions of 0.02 - 0.05 arcsec using multi-IFUs: "MIF." This project is photon starved but recognized to be fundamental.
Note: No thermal IR projects were identified.
Chair: Roger Angel
As a working example, the SOR 3.5m telescope achieves 0.06 arcsec at 0.85 microns with a correction cycle of 700 microsec with ambient seeing of <1.5 arcsec.
In brief, closed loop natural guide star AO works. This is a big milestone as AO will be essential to MAX-AT.
For a thin mirror (5 kg/m2 compared to the HST of 250 kg/m2) the support structure will be much smaller than existing cases. Angel has already built a 2m mirror 2mm thick and the next goal is to build an 8m thin mirror. A thin mirror 30m in diameter is estimated to cost $100M. On the other hand, using 0.3m Dobsonian telescopes at $500 each will cost $5M to assemble a 30m aperture. This design will need mass production of actuators with a final cost of $6M.
A MAX-AT design will involve wind loading, dynamic flexure, perhaps including mechanics from the radio experts.
Deformable mirror (dm): How much can wind deformation be corrected at dm? (~0.01")
Enclosure: Need to simplify and reduce costs.
- Glass fabrication and methods, quality, cost $/m2.
- Segment control: mass producing actuators 10k for 30 meter.
- Coating: how much weather protection will be needed? (see "Enclosures" item)
- Wavefront sensor and reconstructor for MAX-AT will require much bigger arrays with very low noise. Since the reconstuctor speed goes as D4, this requirement maybe still a problem.
- dm: number of elements goes as D2, stroke as D.
- focal plane instrumentation: spectrograph cost/size if telescope is not diffraction limited.
Laser: (Jim Oschmann)
- To increase information/data of off-axis atmospheric layers, laser launching telescopes (diameters 0.5-1m) will have to be installed on/near primary. As a result the PSF will have more scattered light and there will be obscuration.
- An estimated 10-50 lasers will be needed.
- Need test on 4-8m telescopes of a number of Rayleigh scattering lasers (3-10km atmospheric layer).
[IV] FINALE: Jay Gallagher
- There is sufficient scientific interest in building a 50+ m telescope. (caveat: can't build it now, don't know how, etc.) At a minimum the requirements are high angular resolution and low background levels.
- MAX-AT is a complementary observatory. An added value is complementarity with existing/future facilities; there is ample room of complementarity with optical/IR/submm/mm/radio community.
- Critical issue is technology:
- Mirror: should it be fully active/adaptive? 1m/8m segments?
- Wind loading-mechanical control
To build a cost effective telescope, both (a) and (b) have to be in place.
- Natural Guide Star (NGS) system is at a reasonable state today. However, science is specific to 15mag optical star.
- Laser Beacon Guide Star (LGS) will have to incorporate stitching which is specific to large aperture telescopes (D~>10m). Tomography may offer wider fov. High throughput Na lasers needed (other lasers should be/have been developed? Potassium? - 03 (ozone) and H lasers have been experimented with.)
- Science cases are not hurt by small fov.
- Seems like spectroscopy technology is challenging for subarcsec slits. Technology is not known yet.
- Operations budget: Is community willing to shut down other facilities to operate MAX-AT? Operation budget > NOAO + Gemini? Do different operating modes reduce cost? (e.g. 5 yrs dedicated to NIR spectroscopy @ R~105)
- VLA with upgrades (almost quantum limited devices)
- Optical interferometry
- Road map:
Basic needs are:
- Figures of merit for science drivers,
- All straw man concepts from engineers.
These two ingredients will show the cost-benefit ratio and provide a baseline design. Cross talk between science and engineering to provide an observatory (note not a telescope).
- Time scale for MAX-AT should be similar to that of NGST.