Large Infrared Telescope in a Lunar South Polar Crater

- Motivation -

- Design Considerations -

- Commissioning -

Yuki Takahashi

2002 Fall

* Please refer to the accompanying report for references.

Outline1. Motivation

1.1. Astronomical Questions

1.2. Required Observations

1.3. Planned Telescopes

1.4. Next Step

1.4.1. Objectives

1.4.2. Spectral Range: mid-far infrared

1.4.3. Telescope Type/Size: large-aperture

1.4.4. Interferometry

1.4.5. Summary of requirements

2. Need for the Moon

2.1. Stable platform

2.2. Cooling

2.3. Thermal stability

2.4. Reliability

3. Telescope Requirements3.1. Design considerations3.2. Instruments3.3. Observatory location3.4. Interferometer configuration3.5. Other considerations

4. Commissioning the observatory4.1. Types of possible activities4.2. Telescope design approach to reduce commissioning burden4.3. Resource / Personnel requirement

1.1. Astronomical QuestionsWhere did we come from?

– How did the Universe begin and evolve to form structures like galaxies?

– How do stars and planetary systems form?– How do planets form and evolve to create habitable

environment?– How does life form?

Are we alone?– How common is life in the Universe?– Are there life-bearing planets around nearby stars?– If so, what is life like out there? – One of the most significant discoveries in history of

Earth. Everyone would be curious to know what kind of life might exist on other planets.

1.2. Required Observations• submillimeter, infrared, and visible

– Stars, and the interstellar medium that form stars, emit most of their radiation.

– Most of the photon energy density in the Galaxy is in this wavelength range.

– Molecular signatures of almost all chemistry important to life.

• Importance of IR observation– Light from the earliest galaxies is red-shifted to infrared.– Stars and planets form in regions surrounded by

obscuring dust.– Relative brightness between the planet and its host star is

typically more favorable in the infrared (~1:106) than in the visible (~1:109).

Spectrum of our Galaxy

Topic Observation Wavelengths Requirement

Universe’s origin

Cosmic microwave background radiation Microwave Sensitive temperature/polarization map

Galaxy formation

Resolve / image first galaxies thru dust SMM-FIR-MIR Sensitive imaging w/ high resolution

Measure redshift (z) SMM-FIR-MIR Spectroscopy

Galaxy evolution (early)

Find dusty star-bust galaxies at high z SMM-FIR-MIR Sensitive imaging

Chemical evolution (heavy elements - C) SMM-FIR Spectroscopy (C)

Galaxy evolution (late)

Trace galaxies/quasars from z~4 to present V-UV Sensitive imaging

Chemical evolution (heavy elements - metals) UV Spectroscopy (quasar absorption lines)

Star formation Image thru dust FIR, SMM, MIR

Sensitive imaging w/ high resolution

Cooling of H2 at z>10 (first star formation) FIR-SMM Spectroscopy (H2)

Planet formation

Image proto-planetary disks thru dust SMM-FIR-MIR Sensitive imaging w/ high resolution

Proto-planetary kinematics / chemistry SMM, MIR Spectroscopy

Planetary system evolution

Image planetary systems / Kuiper-Belt objects MIR Sensitive imaging

Organic molecules in proto-planet MIR Spectroscopy

Planet detection

Coronagraph / nulling interferometry MIR-NIR-V High resolution (coronagraph / nulling)

Planet imaging Interferometric imaging (aperture synthesis) V Very high resolution

Life detection Planetary atmosphere (O2, O3, H2O, CO2,

CH4, N2O)

FIR-MIR-NIR Spectroscopy w/ coronagraph / nulling

1.3.  Planned TelescopesTelescopes Loc (m) S (nJy) (“) R () FoV D (m) 1KIA (2001-) G 2-10 (MIR) 0.003 2x 10 (85),

4x 1.8 (115) 2VLTI (1999-) G V 4x 8 (200) 3LSST G 0.3-1 (V) 24 mag 0.6 3-100 3 6.5 4HST (-2010) 600 0.1-2.5 (UV-NIR) 0.05 2.4 5FUSE (1999-2002) 0.09-0.12 (UV) 20000 6SIRTF (2003-07) 3-180 (MIR-FIR) 2900-4200 (5K) 5 0.85 7SOFIA (2005-20) Air 0.3-1600 (V-FIR) 1.5-30 2.5 8SMART-2 (2006) 0.6-1.0 (V) Formation 9Herschel (2007-12) L2 80-670 (FIR-SMM) 3K 20 3.5 10Eddington (2008-11) L2 V 1.2 11SIM (2009-14) (0.4-0.9) V 20 mag 0.01 0.3 (10) 12SPIRIT (2010) 40-500 (FIR-SMM) High 2.1 (l/300) 10000 3.4’ 2x 3 (30) 13ALMA (2010) 300-10000 (SMM) 64x 12 (12000) 14Gaia (2012) L2 V 2x 1.7, 0.75 15NGST (2010-20) L2 0.6-28 (NIR-MIR) 5-2600 (<50K) 0.05 (l/2) 5-5000 4x4 6.5 16TPF (2015-20) L2 3-30 (MIR) 350 (10K) 0.00075(l/3) 3-300 0.25-1” 4x 3.5 17Darwin (2015) L2 4-30 (MIR) 40K/8K 6x 1.5 18SPECS (2015) 40-500 (FIR-SMM) 0.06 (l/300) 10000 3.4’ 3x 4 (1000) 19SAFIR (2015) 30-300 (FIR) 300-100000 (4K) 0.8(l/30) 5-1000 6x6 8 20SUVO (2015) 0.2-1 (UV-V) 0.05-600 0.005-0.03 2000-200000 14’x14’ 8 21OWL (2020) G 0.3-5 V~38 0.001 3-100000 100 22LF MIR High 4x 25 23PI V High 5x 4x 8 (6000km)

1 http://huey.jpl.nasa.gov/keck/ 2 http://www.eso.org/projects/vlti/ 3 http://www.lssto.org 4 http://hubble.nasa.gov/ 5 http://fuse.pha.jhu.edu/ 6 http://sirtf.caltech.edu/ 7 http://sofia.arc.nasa.gov/ 8 http://sci.esa.int/home/smart-2/ 9 http://sci.esa.int/first/ 10 http://sci.esa.int/home/eddington/ 11 http://sim.jpl.nasa.gov/ 12 http://gsfctechnology.gsfc.nasa.gov/spirit.htm 13 http://www.alma.nrao.edu/, http://www.eso.org/projects/alma/ 14 http://sci.esa.int/gaia/ 15 http://ngst.gsfc.nasa.gov/ 16 http://planetquest.jpl.nasa.gov/TPF/tpf_index.html 17 http://sci.esa.int/darwin, http://ast.star.rl.ac.uk/darwin 18 http://space.gsfc.nasa.gov/astro/specs/ 19 http://universe.gsfc.nasa.gov/roadmap/docs/SAFIR_Answers.htm 20 http://origins.colorado.edu/uvconf/UVOWG.html 21 http://www.eso.org/projects/owl/ 22 http://origins.jpl.nasa.gov/missions/lf.html, http://peaches.niac.usra.edu/files/library/fellows_mtg/jun01_mtg/pdf/374Woolf.pdf 23 http://origins.jpl.nasa.gov/missions/pi.html

~ 2020Wavelength range Observatory (year) Aperture (baseline)SMM ALMA (2010-) 64x 24 m (12 km)FIR single-aperture SAFIR (2015-) 8 m FIR interferometer SPECS (2015-) 3x 4 m (1 km)MIR single-aperture NGST (2010-) 6.5 mMIR interferometer TPF/Darwin (2015-) 5x 3 m (40-1000 m)NIR-V OWL (2020-) 100 mV-UV SUVO (2015-) 8 m

SAFIR.

SPECS.22

NGST.

TPF.

Darwin.

Summary of topics covered at various wavelengths with various capabilities

(m) Capability Telescopes Topics S ALMA Galaxy evolution (early z~3, dusty star-burst galaxies)

Star/planet formation (dust, galactic centers), Kuiper-Belt objects ALMA Galaxy formation, dust distribution, Planet formation (proto-planetary disks)

SMM (300-1000)

R ALMA Galaxies chemical evolution (C-158um), Proto-star/proto-planetary kinematics/chemistry S SAFIR,

SPECS Galaxy evolution (early z~3, dusty star-burst galaxies) Star/planet formation (dust, galactic centers), Kuiper-Belt objects

SPECS Galaxies beyond HDF (resolve), Planet formation (proto-planetary disks)

FIR (30-300)

R SPECS 1st star formation @ z>10 (H2 cooling), Galaxies/stars chemical evolution (heavy elements) Life detection (CH4, N2O)

S NGST, TPF/Darwin

Galaxy/star formation/evolution, 1st luminous objects/galaxies at z~20, Supernovae at high z Planet formation/evolution, Kuiper-Belt objects

TPF/Darwin Star/planet formation (proto-star disk), Galactic centers, image AGN Planet detection (Earth-like)

MIR (2-30)

R NGST, TPF/Darwin

Galaxies formation/evolution (high z) Planet atmosphere (H2O, CO2, O3, CH4, N2O) - life, ISM / proto-planet organic molecules

S OWL, NGST Galaxy evolution (early), IGM to high z, SN cosmology, Star formation/evolution, Kuiper-Belt obj OWL Planet detection (Earth-like), Dark matter distribution (large scale), Star formation/evolution

NIR (0.7-2)

R OWL Planet atmosphere (O2, H2O, CO2, CH4) – life, Element creation S SUVO, OWL Galaxy evolution (late), Stellar surface (dynamic), SN @ z~10, HII regions @ z~3

Planet formation/evolution, Dark matter (weak lensing) SUVO, OWL Planet detection (Earth-like), Stellar interiors

V (0.3-0.7)

R SUVO, OWL Planet atmosphere (O3), radial velocities, temperature S SUVO Galaxy/quasar/cluster formation/evolution (z<3),

Dark matter/baryons detection/mapping, IGM (H, heavy elements) SUVO IGM density / structure, Galaxy chemical maps, Stellar activities

UV (0.1-0.3)

R SUVO Chemical origin/evolution (heavy elements), IGM (quasar spectroscopy), oxygen, chlorophyll

1.4. Next Step1.4.1. Objectives after ~2020

(1) Discover extrasolar life signatures

• Spectroscopic studies of extrasolar planets found by the TPF to detect any chemical dis-equilibrium.

(2) Discover and image the earliest galaxies in formation.

• Resolving objects far beyond any deep fields taken by the NGST and determining their redshifts.

1.4.2. Spectral Range: mid-far infrared– Imaging the earliest galaxies requires observations in the mid-infrared or

longer wavelengths because of the high redshifts.– Finding life requires spectroscopy in the near-infrared or the mid-infrared;– Molecular lines in the mid-infrared demands less resolving power (Fig.2).– Some lines for methane (CH4) and nitrous oxide (N2O) also exist in the

far-infrared.

Wavelengths of key species and spectral resolving power required

1.4.3. Telescope Type/Size: large-aperture • By ~2020, the NGST -> operational lifetime

– TPF/Darwin continuing its search for more extra-solar planets and utilizing its angular resolving power for astrophysical observations.

– The 6.5-meter NGST -> a different name (JWST:)– Much larger next generation telescope optimized for the mid-infrared will

be in the highest demand.

• How much larger should it be?– Larger than both the NGST (6.5 m) and the SAFIR (8 m)– Thermal emission of zodiacal clouds (~4.1K) around the Sun is too much

for extrasolar planet studies unless the telescope aperture is large. – Minimum detectable flux density (S) improves proportionally to the

collecting area (A) and the square root of integration time ():

– In general, the required integration time shortens with the collecting area squared.

– 25-meter telescope will be able to complete all the observation done during the 10-year lifetime of the NGST in only about 17 days!

AS

1

Point-source sensitivity ofa 28-m TPF telescope (TRW)

Coronagraphy• With coronagraphy, minimum detectable planet-

star separation is 3.6 /D.• TRW (28m) can detect planets only within 3~5 pc.• Wave front must be controlled to /3000 precision

with /10,000 stability (~1 nm rms).– Very difficult because of vibrations and thermal

variations, which produce large-scale deformation in the primary mirror.

– Hundreds of actuators for the primary mirror could take care of such large-scale imperfection.

– Also, the coatings need to be uniform to within a fraction of a percent, or about 10 nm surface accuracy.

– For small-scale corrections, a deformable mirror in the instrument will be required.

aperture size => angular resolution

• To obtain an angular resolution (in milli arc second) at wavelength (in micron), the aperture diameter (D) must be:

• Galaxies at the highest redshifts are likely to subtend angles on the order of 100 – 1000 mas (Hubble Deep Field).

)(

)(200

mas

mmD

1.4.4. Interferometry

Nulling Interferometry

Nulling Interferometry

• Two beam intensities, electric-field rotation angles, phase delays must all be matched to 2 sqrt (Null depth) simultaneously for both polarizations at every point in the aperture for all wavelengths.

• Optical delay lines need to be accurate to the order of 1 nm to allow 10-6 nulling at 10 m.

• Control algorithm to sense phase errors.• Surface accuracy of order 1 nm.

• 1.      Find extraterrestrial life.– a. Find extra-solar planets (TPF).– b.      Detect chemical dis-equilibrium.

• i.      : 5-20 m (Fig.2)

• ii.      Spectral resolving power: ~1000

• iii.      Method: coronagraph / nulling interferometer

• iv.      Target: nearby stars

• 2.      Image the earliest galaxies in formation.– a.       Resolve objects far beyond HDF.

• i.      : ~20 m [z~20: (z+1) m]

• ii.      Sensitivity: ~(z+1)4 times better

• iii.      Angular resolution: ~10 mas (high-z galaxies 100~5000 mas)

• iv.      Exposure: long

• v.      Target: empty field

– b.      Determine redshift.

• i.      : ~20 m [z~20: (z+1) m]

• ii.      Spectral resolving power

2. Need for the Moon • Stable platform: The techniques for finding life on

extrasolar planets require extraordinary stability. This stability level may be feasible only on the Moon due to difficulty in formation flying and vibration control in free space.

• Thermal stability: These techniques also require not only low temperature but also very stable thermal condition. Permanently dark floors of polar craters are probably the most thermally stable locations. In free space, temperature on the mirror varies depending on its orientation with respect to the sun shield.

• Lower risk: Construction is much less risky on a solid platform with gravity than in free space where everything needs to be kept track of (e.g. by tethering). Accessibility from a nearby lunar base allows service / upgrades for never-ending contribution to astronomy.

Lunar Environment (South Polar Crater)

• Very stable platform (much simpler vibration control than in free space).

• Permanently dark and cold polar craters.

– Unmatched thermal stability with estimated temperature of 30~80 K.

• Nearby access to almost continuously sunlit areas for supporting facilities.

• Some gravity and an inertial platform to ease construction.

• Crater topography protects the telescope from outside disturbance.

• Meteoroid flux ~ ½ that of free space.

• Dust contamination is preventable.

– Nearby activities are possible without contact with the dusty lunar surface.

– Superconducting magnetic bearing can tolerate some dust.

sunlit rim with a relay station

sky visibility

Telescope in a permanently dark polar crater

2.1.     Stable Platform • Coronagraph requires pointing stability of ~ 1 mas.

This will demand that a space-based telescope wait about 2 hours every time it repositions (TRW).

• Nulling inteferometer requires a beam path-length error < /1000 (< few 10 nm). Almost impractical to build an interferometer in free space with a baseline longer than a few 100 m.

• Magnetic bearing isolates lunar seismicity.• (Mode: probably below 10 Hz.)

2.2.     Thermal Stability

• Everything should be very stable in time… including the telescope thermal emission, detector efficiency, and amplifier gains.

2.3.     Cooling (sensitivity)

• To stay sky-limited at ~20 m, telescope needs to be < ~30K.

• In space, passive cooling to 30K possible, but with huge multi-layer sun shields (which degrades over time with contamination by propellants and damages by meteoroids).

• Active cooling will be necessary anyways (for detectors), and the vibration must be controlled to a very high standard required for coronagraph / nulling interferometry.

2.4.     Lower risk

• Construction

• Accessibility

• Lifetime

3. Telescope Requirements• Jitter should be < 1 mas.• Rejection rate (1/N) above 105 for pointing errors

below 2 mas rms and optical phase delay fluctuations below 8 nm rms.

• Relative beam intensity error should be below 4 sqrt(N).

• Differential phase errors should be below 2 sqrt(N).

• This requires that the wavefront distortions are controlled to within sqrt(N) / .

• Main phase fluctuations are due to imperfect mirror polishing.

3.1. Design Considerations• In nulling interferometry (and in interferometric

imaging in general), having only 1 baseline (however long) is limited in capability: a single baseline nulling interferometry will be able to detect and study planets with a limited range of separations from the host star.

• To generate both a deep and wide null for star and to retain high angular resolution off the central null, multiple baselines are required.

• Outputs of each baseline nulls can be combined to produce a null of higher powers of the off-axis field angle .

3.2. Instruments• Si:As detector covers the wavelength range of 3-28

m, operating at 10 K (for 512x512). Coronagraph Beam combiner (nulling interferometer) Spectrometer (17K) Imager (6K)

• Rough mass/volume estimate: instrument module (1000 kg, 50 m3):– Instruments (800 kg)– Structure (200 kg)– Electronics (40 kg)– Cooler (30 kg) 50K->5K

Sky background compared to telescope temperatures

Cooling• Detecter dark current: < 10 e- / s / pixel per

spectral channel to remain negligible compared to the local zodiacal background.

• The vibration from the cooler can be isolated from the optics. This isolation almost impractical on a space-based telescope. An important reason for a telescope on the lunar surface.

3.3. Observatory Location• Galactic Center visibility

– Mid-far infrared observations essential in studying the dusty region around the Galactic Center (and the central black hole).

• Criteria for an ideal site:(let = angle from the south pole in degrees: so =0 at the pole)– * To be permanently dark, the rim needs to be 1.5+ degrees high.– * To see the Galactic center, the rim should be lower than 7+

degrees in at least one direction. (At the lunar south pole, the Galactic center is about 7 degrees above the horizon.)

– Thermal environment much more stable closer to the pole. Avoid infrared radiation from Earth.

– Big craters: rims further away from the telescope so that the scattered light/radiation weaker.

• Not enough topological / illumination data to decide on the site, but better for now to choose for our baseline a dark area that's potentially flat enough with better sky coverage.

3.4. Interferometer configuration• The Moon rotates (slowly), so does our baseline.• By the time our telescope begins operating, many

planets will already have been detected by TPF.• As each planet align with the interferometer baseline

(which rotates slowly with the Moon), allocate time to study that planet.

• Eventually study all the planets already detected by TPF (at least the ones visible from our location).

• Moon-based interferometer is not very useful for finding new planets, but can study already-detected planets with better sensitivity and spectral resolution for life signatures.

4. Commissioning• 'Commissioning' = the period between the "first

light" (when all the optical elements are aligned to produce a presentable image) and the beginning of actual science operations.– To bring telescope to the required level of system

performance and to verify the performance.– To fully assess & understand the telescope’s

characteristics (pointing, tracking, field stabilization / vibration, image quality).

– Fine-tune, adjust, debug, exercise, verify, quantify, qualify, optimize functionality & performance

• Typically takes ~ a year (for example, for each VLT).

4.1.   Types of possible activities• Ideally commissioning should be possible only through

software (no human required on site).• But humans often need to install little temporary instruments

(like a little scope, laser, lens, ...) to test/measure things when something goes wrong.– Keck (5 years * 15 staff). Mirror deformation during

construction.– VLT (1 year): A small 15 cm guidescope was temporarily

fitted for modeling pointing.– HET (3 years * 12~15 staff). Problem: couldn’t place target

stars in the field of view. Solution: Attached a 10 cm telephoto lens to increase the field of view temporarily. Also used audio/video systems, and laser for alignment.

– HST repair: 2 spacewalkers at a time. Many days of prior training (each spacewalker cross-trained).

4.2.   Telescope design approach to reduce commissioning burden

• The personnel requirement for commissioning depends heavily on how carefully & flexibly the telescope was constructed (how much adjustments are possible just through software).– HST was designed for on-orbit maintenance / refurbishment

(subsystems modular / standardized / accessible, grapple fixtures for mechanical arm, bolts and electrical connections designed for spacewalkers).

– Crew aids: handrails, handholds, footholds, translation devices, transfer equipment, protective covers, tethering devices, grapple fixtures, sockets, stowage, parking fixtures, …

– Each instrument replaceable like a drawer (HST).

• Test everything on Earth (Anticipate 1/6 g).• Limit to well-tested technology.

– VLT had a problem with a novel axis encoding system – replaced by more conventional one (1 month).

4.3.   Resource/Personnel requirement• Various tools for the unexpected.

– HST servicing: over 200 specifically designed tools (screwdrivers, programmable power wrench, temporarily guide rails / handholds / foot restraints, small tool bag, hardware).

• Commissioning could be possible with only a couple highly experienced (multi-disciplinary) technicians, IF we design and construct the telescope flexibly.

• At least 2 essential for one to monitor / assist the other’s activity (HST: When one was installing an instrument, the other watched to ensure alignment).

• Must gather the various skills to solve any unexpected problems (electronics, machining, optics,...). Probably want one technician/engineer in each area.

Reference• [1] NASA Origins: http://origins.jpl.nasa.gov/.• [1] National Research Council. Astronomy and Astrophysics in the New Millennium: Panel Reports. National Academy Press, 2001.• [1] http://space.gsfc.nasa.gov/astro/specs/.• [1] C.A. Beichman et al. Summary Report on Architecture Studies for the Terrestrial Planet Finder. June 2002. http://planetquest.jpl.nasa.gov/TPF/

TPFrevue/FinlReps/JPL/tpfrpt1a.pdf.• [1] BÉLY P.-Y., LAURANCE R.J., VOLONTE S., GREENAWAY A.H., HANIFF C.A., LATTANZI M.G.,  MARIOTTI J.-M., NOORDAM J.E., VAKILI

F., von  der LÜHE O., Kilometric Baseline Space Interferometry: Comparison of free-flyer and moon-based versions. Report by the Space Interferometry Study Team, ESA-SCI(96)7, 111 pages, June 1996.

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