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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. Outline. 1. Motivation 1.1. Astronomical Questions 1.2. Required Observations

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Large Infrared Telescope in a Lunar South Polar Crater

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Large infrared telescope in a lunar south polar crater l.jpg

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.


Outline l.jpg

Outline

1. 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 Requirements

3.1.Design considerations

3.2.Instruments

3.3.Observatory location

3.4.Interferometer configuration

3.5.Other considerations

4. Commissioning the observatory

4.1.Types of possible activities

4.2.Telescope design approach to reduce commissioning burden

4.3.Resource / Personnel requirement


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1.1.Astronomical Questions

Where 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.


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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).


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Spectrum of our Galaxy


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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

z = redshift, SMM = submillimeter, FIR = far-infrared, MIR = mid-infrared, NIR = ear-infrared, V = visible, UV = ultraviolet


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1.3.  Planned Telescopes


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SAFIR.

SPECS.22

NGST.

TPF.

Darwin.

~ 2020

Wavelength rangeObservatory (year)Aperture (baseline)

SMMALMA (2010-)64x 24 m (12 km)

FIR single-apertureSAFIR (2015-)8 m

FIR interferometerSPECS (2015-)3x 4 m (1 km)

MIR single-apertureNGST (2010-)6.5 m

MIR interferometerTPF/Darwin (2015-)5x 3 m (40-1000 m)

NIR-VOWL (2020-)100 m

V-UVSUVO (2015-)8 m


Summary of topics covered at various wavelengths with various capabilities l.jpg

Summary of topics covered at various wavelengths with various capabilities


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1.4. Next Step

1.4.1. 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.


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    Wavelengths of key species and spectral resolving power required


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    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 (t):

      • 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!


    Point source sensitivity of a 28 m tpf telescope trw l.jpg

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


    Coronagraphy l.jpg

    Coronagraphy

    • With coronagraphy, minimum detectable planet-star separation is 3.6 l/D.

    • TRW (28m) can detect planets only within 3~5 pc.

    • Wave front must be controlled to l/3000 precision with l/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 l.jpg

    aperture size => angular resolution

    • To obtain an angular resolution q (in milli arc second) at wavelength l (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).


    1 4 4 interferometry l.jpg

    1.4.4. Interferometry


    Nulling interferometry l.jpg

    Nulling Interferometry


    Nulling interferometry18 l.jpg

    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 mm.

    • Control algorithm to sense phase errors.

    • Surface accuracy of order 1 nm.


    Slide19 l.jpg

    • 1.      Find extraterrestrial life.

      • a. Find extra-solar planets (TPF).

      • b.      Detect chemical dis-equilibrium.

        • i.      l: 5-20 mm (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.      l: ~20 mm [z~20: (z+1) mm]

        • 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.      l: ~20 mm [z~20: (z+1) mm]

        • ii.      Spectral resolving power


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    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 l.jpg

    sunlit rim with a relay station

    sky visibility

    Telescope in a permanently dark polar crater

    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.


    2 1 stable platform l.jpg

    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 < l/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.)


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    2.2.     Thermal Stability

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


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    2.3.     Cooling (sensitivity)

    • To stay sky-limited at ~20 mm, 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.


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    2.4.     Lower risk

    • Construction

    • Accessibility

    • Lifetime


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    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 l sqrt(N) / p.

    • Main phase fluctuations are due to imperfect mirror polishing.


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    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 q.


    3 2 instruments l.jpg

    3.2. Instruments

    • Si:As detector covers the wavelength range of 3-28 mm, 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 l.jpg

    Sky background compared to telescope temperatures


    Cooling l.jpg

    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 l.jpg

    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 q = angle from the south pole in degrees: so q=0 at the pole)

      • * To be permanently dark, the rim needs to be 1.5+q degrees high.

      • * To see the Galactic center, the rim should be lower than 7+q 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 l.jpg

    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.


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    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).


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    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 l.jpg

    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).


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    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 l.jpg

    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.

    • [1] Keck Interferometer: http://huey.jpl.nasa.gov/keck/.

    • [1] VLTI: http://www.eso.org/projects/vlti/.

    • [1] LSST: http://www.lssto.org.

    • [1] HST: http://hubble.nasa.gov/.

    • [1] FUSE: http://fuse.pha.jhu.edu/.

    • [1] SIRTF: http://sirtf.caltech.edu/.

    • [1] SOFIA: http://sofia.arc.nasa.gov/.

    • [1] SMART-2: http://sci.esa.int/home/smart-2/.

    • [1] Herschel: http://sci.esa.int/first/.

    • [1] Eddington: http://sci.esa.int/home/eddington/.

    • [1] SIM: http://sim.jpl.nasa.gov/.

    • [1] SPIRIT: http://gsfctechnology.gsfc.nasa.gov/spirit.htm.

    • [1] ALMA: http://www.alma.nrao.edu/, http://www.eso.org/projects/alma/.

    • [1] Gaia: http://sci.esa.int/gaia/.

    • [1] NGST: http://ngst.gsfc.nasa.gov/.

    • [1] TPF: http://planetquest.jpl.nasa.gov/TPF/tpf_index.html.

    • [1] Darwin: http://sci.esa.int/darwin, http://ast.star.rl.ac.uk/darwin.

    • [1] SPECS: http://space.gsfc.nasa.gov/astro/specs/.

    • [1] SAFIR: http://universe.gsfc.nasa.gov/roadmap/docs/SAFIR_Answers.htm.

    • [1] SUVO: http://origins.colorado.edu/uvconf/UVOWG.html.

    • [1] OWL: http://www.eso.org/projects/owl/.

    • [1] Life Finder: http://origins.jpl.nasa.gov/missions/lf.html, http://peaches.niac.usra.edu/files/library/fellows_mtg/jun01_mtg/pdf/374Woolf.pdf.

    • [1] Planet Imager: http://origins.jpl.nasa.gov/missions/pi.html.

    • [1] TRW. TPF Architecture Phase 2 Final Report. June 2002. http://planetquest.jpl.nasa.gov/TPF/TPFrevue/FinlReps/Trw/TRW12Fnl.pdf.

    • [1] http://www.estec.esa.nl/spdwww/future/darwin/images/interferometer1.jpg.

    • [1] Eugene Serabyn. Nulling Interferometry and Planet Detection. In Principles of

    • Long Baseline Stellar Interferometry. Course Notes from the 1999 Michelson Summer School,

    • August 15 –19, 1999.

    • [1] Olivier Absil. Nulling Interferometry with IRSI-Darwin: Detection and Characterization of Earth-like Exoplanets. PhD thesis, 2001.

    • [1] http://universe.gsfc.nasa.gov/roadmap/docs/SAFIR.pdf


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