Download Large Infrared Telescope in a Lunar South Polar Crater

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
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
3. Telescope Requirements
1.4.5. Summary of requirements
3.1. Design considerations
2. Need for the Moon
3.2. Instruments
2.1. Stable platform
3.3. Observatory location
2.2. Cooling
3.4. Interferometer configuration
2.3. Thermal stability
3.5. Other considerations
2.4. Reliability
4. Commissioning the observatory
4.1. Types of possible activities
4.2. Telescope design approach to
reduce commissioning burden
4.3. Resource / Personnel requirement
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.
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
Galaxy
formation
Cosmic microwave background radiation
Microwave
Resolve / image first galaxies thru dust
SMM-FIR-MIR
Measure redshift (z)
SMM-FIR-MIR
Sensitive temperature/polarization
map
Sensitive imaging w/ high
resolution
Spectroscopy
Find dusty star-bust galaxies at high z
SMM-FIR-MIR
Sensitive imaging
Chemical evolution (heavy elements - C)
SMM-FIR
Spectroscopy (C)
Trace galaxies/quasars from z~4 to present
V-UV
Sensitive imaging
Chemical evolution (heavy elements - metals)
UV
Spectroscopy (quasar absorption
lines)
Image thru dust
Cooling of H2 at z>10 (first star formation)
FIR, SMM,
MIR
FIR-SMM
Sensitive imaging w/ high
resolution
Spectroscopy (H2)
Image proto-planetary disks thru dust
SMM-FIR-MIR
Proto-planetary kinematics / chemistry
SMM, MIR
Sensitive imaging w/ high
resolution
Spectroscopy
Planetary
system
evolution
Image planetary systems / Kuiper-Belt objects
MIR
Sensitive imaging
Organic molecules in proto-planet
MIR
Spectroscopy
Planet
detection
Planet imaging
Coronagraph / nulling interferometry
MIR-NIR-V
Interferometric imaging (aperture synthesis)
V
High resolution (coronagraph /
nulling)
Very high resolution
Life detection
Planetary atmosphere (O2, O3, H2O, CO2,
CH , N O)
FIR-MIR-NIR
Galaxy
evolution (early)
Galaxy
evolution (late)
Star formation
Planet
formation
Spectroscopy w/ coronagraph /
nulling
1.3. Planned Telescopes
Telescopes
1
KIA (2001-)
Loc
G
 (m)
2-10 (MIR)
2
G
G
600
V
0.3-1 (V)
0.1-2.5 (UV-NIR)
0.09-0.12 (UV)
3-180 (MIR-FIR)
0.3-1600 (V-FIR)
0.6-1.0 (V)
80-670 (FIR-SMM)
V
(0.4-0.9) V
40-500 (FIR-SMM)
300-10000 (SMM)
V
0.6-28 (NIR-MIR)
3-30 (MIR)
4-30 (MIR)
40-500 (FIR-SMM)
30-300 (FIR)
0.2-1 (UV-V)
0.3-5
MIR
V
VLTI (1999-)
LSST
4
HST (-2010)
5
FUSE (1999-2002)
6
SIRTF (2003-07)
7
SOFIA (2005-20)
8
SMART-2 (2006)
9
Herschel (2007-12)
10
Eddington (2008-11)
11
SIM (2009-14)
12
SPIRIT (2010)
13
ALMA (2010)
14
Gaia (2012)
15
NGST (2010-20)
16
TPF (2015-20)
17
Darwin (2015)
18
SPECS (2015)
19
SAFIR (2015)
20
SUVO (2015)
21
OWL (2020)
22
LF
23
PI
3
1
2
Air
L2
L2
L2
L2
L2
L2
G
http://huey.jpl.nasa.gov/keck/
S (nJy)
 (“)
0.003
R ()
FoV
24 mag
0.6
0.05
3-100
3
D (m)
2x 10 (85),
4x 1.8 (115)
4x 8 (200)
6.5
2.4
20000
2900-4200 (5K)
3K
5
1.5-30
Formation
20
20 mag
High
0.01
2.1 (l/300)
10000
3.4’
5-2600 (<50K)
350 (10K)
40K/8K
0.05 (l/2)
0.00075(l/3)
5-5000
3-300
4x4
0.25-1”
0.06 (l/300)
0.8(l/30)
0.005-0.03
0.001
10000
5-1000
2000-200000
3-100000
High
3.4’
6x6
14’x14’
300-100000 (4K)
0.05-600
V~38
High
0.85
2.5
3.5
1.2
0.3 (10)
2x 3 (30)
64x 12 (12000)
2x 1.7, 0.75
6.5
4x 3.5
6x 1.5
3x 4 (1000)
8
8
100
4x 25
5x 4x 8 (6000km)
~ 2020
SAFIR.
SPECS.22
Wavelength range
SMM
FIR single-aperture
FIR interferometer
MIR single-aperture
MIR interferometer
NIR-V
V-UV
NGST.
Observatory (year)
ALMA (2010-)
SAFIR (2015-)
SPECS (2015-)
NGST (2010-)
TPF/Darwin (2015-)
OWL (2020-)
SUVO (2015-)
TPF.
Aperture (baseline)
64x 24 m (12 km)
8m
3x 4 m (1 km)
6.5 m
5x 3 m (40-1000 m)
100 m
8m
Darwin.
Summary of topics covered at various
wavelengths with various capabilities
 (m)
SMM
(300-1000)
FIR
(30-300)
Capability
S

R
S

R
MIR
(2-30)
S

R
NIR
(0.7-2)
V
(0.3-0.7)
UV
(0.1-0.3)
Telescopes
ALMA
ALMA
ALMA
SAFIR,
SPECS
SPECS
SPECS
NGST,
TPF/Darwin
TPF/Darwin
S

R
S
NGST,
TPF/Darwin
OWL, NGST
OWL
OWL
SUVO, OWL

R
S
SUVO, OWL
SUVO, OWL
SUVO

R
SUVO
SUVO
Topics
Galaxy evolution (early z~3, dusty star-burst galaxies)
Star/planet formation (dust, galactic centers), Kuiper-Belt objects
Galaxy formation, dust distribution, Planet formation (proto-planetary disks)
Galaxies chemical evolution (C-158um), Proto-star/proto-planetary kinematics/chemistry
Galaxy evolution (early z~3, dusty star-burst galaxies)
Star/planet formation (dust, galactic centers), Kuiper-Belt objects
Galaxies beyond HDF (resolve), Planet formation (proto-planetary disks)
1st star formation @ z>10 (H2 cooling), Galaxies/stars chemical evolution (heavy elements)
Life detection (CH4, N2O)
Galaxy/star formation/evolution, 1st luminous objects/galaxies at z~20, Supernovae at high z
Planet formation/evolution, Kuiper-Belt objects
Star/planet formation (proto-star disk), Galactic centers, image AGN
Planet detection (Earth-like)
Galaxies formation/evolution (high z)
Planet atmosphere (H2O, CO2, O3, CH4, N2O) - life, ISM / proto-planet organic molecules
Galaxy evolution (early), IGM to high z, SN cosmology, Star formation/evolution, Kuiper-Belt obj
Planet detection (Earth-like), Dark matter distribution (large scale), Star formation/evolution
Planet atmosphere (O2, H2O, CO2, CH4) – life, Element creation
Galaxy evolution (late), Stellar surface (dynamic), SN @ z~10, HII regions @ z~3
Planet formation/evolution, Dark matter (weak lensing)
Planet detection (Earth-like), Stellar interiors
Planet atmosphere (O3), radial velocities, temperature
Galaxy/quasar/cluster formation/evolution (z<3),
Dark matter/baryons detection/mapping, IGM (H, heavy elements)
IGM density / structure, Galaxy chemical maps, Stellar activities
Chemical origin/evolution (heavy elements), IGM (quasar spectroscopy), oxygen, chlorophyll
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.
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 (t):
S
1
A 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)
Coronagraphy
• With coronagraphy, minimum detectable planetstar 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:
 ( m)
D  200m
 (mas)
• Galaxies at the highest redshifts are likely to
subtend angles on the order of 100 – 1000
mas (Hubble Deep Field).
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.
– b.
Target: empty field
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 neverending 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.
• Construction
• Accessibility
• Lifetime
Lower risk
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) / p.
• 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 328 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
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[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/.
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[1] SUVO: http://origins.colorado.edu/uvconf/UVOWG.html.