Motivation and Design Considerations for A Large Infrared Telescope at the Lunar South Pole

Motivation and Design Considerations for A Large Infrared Telescope at the Lunar South Pole Yuki D. Takahashi 2002 / 8 Summary 1. Astronomical Questi...
Author: Meagan Wiggins
3 downloads 1 Views 1MB Size
Motivation and Design Considerations for A Large Infrared Telescope at the Lunar South Pole Yuki D. Takahashi 2002 / 8 Summary 1.

Astronomical Questions 1.1. Where did we come from? 1.2. Are we alone? 2. Required Observations 3. Planned Telescopes 4. Next Step 4.1. Objectives 4.2. Spectral Range: mid-far infrared 4.3. Telescope Type/Size: large-aperture 4.4. Interferometry 4.5. Summary of requirements 5. Need for the Moon 5.1. Vibration control (coronagraph, nulling interferometry) 5.2. Cooling (sensitivity) 5.3. Thermal control (coronagraph) 5.4. Construction 5.5. Accessibility 5.6. Lifetime 6. Telescope Requirements 6.1. Design considerations 6.2. Instruments 6.3. Observatory location 6.4. Interferometer configuration 6.5. Other considerations 7. Commissioning the observatory 7.1. General description 7.2. Types of possible activities 7.3. Anecdotes from other telescopes 7.4. Telescope design approach to reduce commissioning burden 7.5. Resource requirement 7.6. Personnel requirement

1.

Astronomical Questions

Humans have long wondered about our origin and our place in the Universe. Today, we are gaining enough technical capabilities to find clues to these questions through astronomical observations with telescopes. One major such effort is the NASA’s Origins program1. 1.1. Where did we come from? Arguably one of the most important questions anyone can ask in life is the reason of our existence: Why do we exist? This is directly related to the question of our origin: Where did we come from? Through astronomy, we have learned that the Universe began with the Big Bang about 15 billion years ago, that it formed countless number of galaxies including our Milky Way Galaxy, that our Sun formed within it with planets, and that our planet Earth eventually evolved to bring about life. However, many mysteries remain: How exactly did the Universe begin? How did it then evolve to form structures like the galaxies? How exactly do stars and planetary systems form? How do planets form and evolve to create habitable environment? How does life form? 1.2. Are we alone? Looking at the countless stars, humans on Earth have also wondered whether there is life elsewhere. Considering the vastness of the Universe, there probably is, but we have no direct evidence of it so far. How common is life in the Universe? Are there life-bearing planets around nearby stars? If so, what is life like out there? Detecting signs of life elsewhere would make 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. 2.

Required Observations

We can best find answers to most of these important questions by observations in the wavelength range spanning submillimeter, infrared, and visible. This is because stars, and the interstellar medium that form stars, emit most of their radiation at these wavelengths. Most of the photon energy density in the Galaxy is in this wavelength range (Fig.1). In addition, this range includes molecular signatures of almost all chemistry important to life.2

Figure 1: Spectrum of our Galaxy, showing that most of the radiation is in the submillimeter, infrared, and visible wavelengths.3

Importance of infrared observation Observations at mid-infrared wavelengths and longer are especially crucial for mainly three reasons. First, even though stars emit most intensely in the visible, light from the earliest galaxies is red-shifted to infrared because of the expansion of the Universe. Second, stars and planets form in regions surrounded by obscuring dust; only long-wavelength infrared waves can penetrate through the dust to tell us about the formation. Moreover, the dust absorbs radiation at visible and ultraviolet wavelengths and re-radiates at the thermal infrared. Third, in detecting and studying planets, the relative brightness between the planet and its host star is typically more favorable in the infrared (~1:106) than in the visible (~1:109).4 Thermal emission of Earth-like planets peak at about 10 µm.5 Best contrast for Earth is at 12 µm. Table 1 summarizes the main astronomical observations required to answer the above questions of our origin and our place in the Universe. Table 1: Brief summary of required observations. Topic Universe’s origin Galaxy formation

Observation Wavelengths Requirement Cosmic microwave background radiation Microwave Sensitive temperature/polarization map Resolve / image first galaxies thru dust SMM-FIR-MIR Sensitive imaging w/ high resolution Measure redshift (z) SMM-FIR-MIR Spectroscopy Galaxy evolution Find dusty star-bust galaxies at high z SMM-FIR-MIR Sensitive imaging (early) Chemcial evolution (heavy elements - C) SMM-FIR Spectroscopy (C) Galaxy evolution Trace galaxies/quasars from z~4 to present V-UV Sensitive imaging (late) 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 Image planetary systems / Kuiper-Belt objects MIR Sensitive imaging evolution 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 = near-infrared, V = visible, UV = ultraviolet

3.

Planned Telescopes

To carry out these observations, a variety of telescopes are in planning. The following table highlights the leading telescopes that will be available in the upcoming years. Table 2: Existing and Planned Leading Telescopes in the SMM-UV range. Telescopes 6 KIA (2001-)

L H

λ (µm) 2-10 (MIR)

S (nJy)

θ (“) 0.003

R (λ/Δλ)

FoV

7

D (m) 2x 10 (85), 4x 1.8 (115) 4x 8 (200) 6.5 2.4

VLTI (1999-) C V LSST G 0.3-1 (V) 24 mag 0.6 3-100 3 9 HST (-2010) 600 0.1-2.5 (UV-NIR) 0.05 10 FUSE (1999-2002) 0.09-0.12 (UV) 20000 11 SIRTF (2003-07) 3-180 (MIR-FIR) 2900-4200 (5K) 5 0.85 12 SOFIA (2005-20) A 0.3-1600 (V-FIR) 1.5-30 2.5 13 SMART-2 (2006) 0.6-1.0 (V) Formation 14 Herschel (2007-12) L2 80-670 (FIR-SMM) 3K 20 3.5 15 Eddington (2008-11) L2 V 1.2 16 SIM (2009-14) (0.4-0.9) V 20 mag 0.01 0.3 (10) 17 SPIRIT (2010) 40-500 (FIR-SMM) High 2.1 (l/300) 10000 3.4’ 2x 3 (30) 18 ALMA (2010) 300-10000 (SMM) 64x 12 (12000) 19 Gaia (2012) L2 V 2x 1.7, 0.75 20 NGST (2010-20) L2 0.6-28 (NIR-MIR) 5-2600 (10 (H2 cooling), Galaxies/stars chemical evolution (heavy elements) Planet atmosphere (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 (z5K The telescope temperature is what limits the maximum observable wavelength (in the infrared wavelengths we're interested in). We should aim to achieve telescope temperature of ~30K (you can see on the plot that 30K will permit sky-background limited observations instead of thermalbackground limited.

Figure 6: Sky background compared to telescope temperatures.33

Detecter dark current should be below 10 e- / s / pixel per spectral channel to remain negligible compared to the local zodiacal background.32 Everything should be very stable in time, including the telescope thermal emission, detector efficiency, and amplifier gains. Active cooling will be very desirable if not necessary (necessary for the detectors). The vibration from the cooler can be isolated from the optics. This isolation is almost impractical on a space-based telescope. This could be one of the most important reasons for a telescope on the lunar surface. 6.3. Observatory location We should make the Galactic Center visibility a *requirement*, rather than something extra, because it will expand the scientific potential of our telescope significantly. Mid-far infrared observations are essential in studying the dusty region around the Galactic Center (and the central black hole). We don't have enough topological / illumination data to decide on the site, but I think it's better for now to choose for our baseline a dark area that's potentially flat enough with better sky coverage, rather than going forward with the Shackleton which most likely has sky coverage of only ~1/4 of the sky (assuming it can see higher than 30 deg above horizon). cover half the sky from there. Here are some concerns: - scattered/radiated light from lit/heated regions within the crater

- infrared radiation from Earth always - temperature not as stable I think there are 2 major criteria for an ideal site: (let a = angle from the south pole in degrees: so a=0 at the pole) * To be permanently dark, the rim needs to be 1.5+a degrees high. * To see the Galactic center, the rim should be lower than 7+a degrees in at least one direction. (At the lunar south pole, the Galactic center is about 7 degrees above the horizon.) How about one of the 3 big permanently shadowed craters located 2~3 degrees from the pole (at longitudes of about 0, 45, 90 E)? These 30~50 km craters seem to be big enough (the rims low enough) to give us a good sky coverage. Also, the thermal environment would be much more stable closer to the pole. I think big craters have advantages that the rims can be further away from the telescope so that the scattered light would be weaker. 6.4. Interferometer configuration Telescopes arranged in a line will have baselines in only one direction (at a time) and this direction is fixed on the lunar surface (unable to rotate as in space). Such nulling interferometer will only be able to detect planets whose position with respect to the host stars happen to be in the direction of the interferometer baseline. In general this is very limiting, but the Moon rotates (slowly), so does our baseline. By the time our telescope begins operating, many planets will already have been detected by TPF. What I'm envisioning is that as each of these planets align with the direction of the interferometer baseline (that rotates slowly with the Moon), our interferometer will allocate its time to study that planet (for signs of life). This way, it should be able to eventually study all the planets already detected by TPF (at least the ones visible from our location). Moon-based interferometer won't be very useful for finding new planets, but it can study already-detected planets with better sensitivity and spectral resolution for life signatures. 6.5. Other considerations About dust, the mid-far infrared wavelength is pretty comparable to the typical dust particle size. Especially for coronagaph it's essential to avoid dust particles because they'll scatter the star light in to blind us from detecting planets that are millions of times dimmer. The telescope instrument will produce on the order of 100-300 kilobytes per second of raw data (plus other 'house-keeping' data). 7.

Commissioning the observatory

'Commissioning' is usually defined as 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. This typically takes on the order of a year (for example, for each VLT). 7.1. General description

Commissioning (fine-tune, adjust, debug, exercise, verify, quantify, qualify, optimize functionality & performance). • •

To bring telescope to the required level of system performance and verify. To fully assess & understand the telescope’s characteristics (pointing, tracking, field stabilization / vibration, image quality).

7.2. Types of possible activities List: Ideally commissioning should be possible only through software (no human required on site), but people often need to install little temporary instruments (like a little scope, laser, lens, ...) to test/measure things when something goes wrong. VLT: A small 15 cm guidescope was temporarily fitted for modeling pointing. 7.3. Anecdotes from other telescopes VLT (1 year): Problem with a novel axis encoding system – replaced by more conventional one (1 month). => Limit to well-tested technology. 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

7.4. Telescope design approach to reduce commissioning burden The personnel requirement for the commissioning phase depends heavily on how carefully the telescope was constructed and how much flexibly it was constructed (how much adjustments are possible just through software). 7.5. Resource requirement For the commissioning crew to perform its function 7.6. Personnel requirement How many people for how much time might be needed

Commissioning could be possible with only a couple highly experienced (multi-disciplinary) technicians, IF we design and construct the telescope flexibly. It depends on how many people it would take to gather the various skills to solve any unexpected problems (electronics, machining, optics,...). Realistically, we may want to have one technician/engineer in each area.

Reference: 1

NASA Origins: http://origins.jpl.nasa.gov/. National Research Council. Astronomy and Astrophysics in the New Millennium: Panel Reports. National Academy Press, 2001. 3 http://space.gsfc.nasa.gov/astro/specs/. 4 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. 5 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, ESASCI(96)7, 111 pages, June 1996. 6 Keck Interferometer: http://huey.jpl.nasa.gov/keck/. 7 VLTI: http://www.eso.org/projects/vlti/. 8 LSST: http://www.lssto.org. 9 HST: http://hubble.nasa.gov/. 10 FUSE: http://fuse.pha.jhu.edu/. 11 SIRTF: http://sirtf.caltech.edu/. 12 SOFIA: http://sofia.arc.nasa.gov/. 13 SMART-2: http://sci.esa.int/home/smart-2/. 14 Herschel: http://sci.esa.int/first/. 15 Eddington: http://sci.esa.int/home/eddington/. 16 SIM: http://sim.jpl.nasa.gov/. 17 SPIRIT: http://gsfctechnology.gsfc.nasa.gov/spirit.htm. 18 ALMA: http://www.alma.nrao.edu/, http://www.eso.org/projects/alma/. 19 Gaia: http://sci.esa.int/gaia/. 20 NGST: http://ngst.gsfc.nasa.gov/. 21 TPF: http://planetquest.jpl.nasa.gov/TPF/tpf_index.html. 22 Darwin: http://sci.esa.int/darwin, http://ast.star.rl.ac.uk/darwin. 23 SPECS: http://space.gsfc.nasa.gov/astro/specs/. 24 SAFIR: http://universe.gsfc.nasa.gov/roadmap/docs/SAFIR_Answers.htm. 25 SUVO: http://origins.colorado.edu/uvconf/UVOWG.html. 26 OWL: http://www.eso.org/projects/owl/. 27 Life Finder: http://origins.jpl.nasa.gov/missions/lf.html, http://peaches.niac.usra.edu/files/library/fellows_mtg/jun01_mtg/pdf/374Woolf.pdf. 28 Planet Imager: http://origins.jpl.nasa.gov/missions/pi.html. 29 TRW. TPF Architecture Phase 2 Final Report. June 2002. http://planetquest.jpl.nasa.gov/TPF/TPFrevue/FinlReps/Trw/TRW12Fnl.pdf. 30 http://www.estec.esa.nl/spdwww/future/darwin/images/interferometer1.jpg. 31 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. 32 Olivier Absil. Nulling Interferometry with IRSI-Darwin: Detection and Characterization of Earth-like Exoplanets. PhD thesis, 2001. 33 http://universe.gsfc.nasa.gov/roadmap/docs/SAFIR.pdf 2

Suggest Documents