1. The Moon as the ultimate infrared site ?
Jean-Pierre Maillard
Institut d’Astrophysique de Paris

2. Introduction: overview on the development of IR astronomy
The progress of IR astronomy has been fully dependent on:
advances in infrared detectors technology (from single cells to 2D arrays, extension from near-IR to sub-mm wavelengths) development of appropriate sites on ground
launch of specialized space missions.
The main challenge on ground and in space:
reduction at most as possible of the thermal background emission from the sky, the telescope and the instrument, received by the detectors to improve the final sensitivity.

3. On Earth: looking for the driest sites
1000
500
300 μm
Transmission in the sub-mm windows of a ground-based site and PWV values

4. Mauna Kea as a major step
The development from the 80’s of Mauna Kea, a dormant volcano of 4205 m altitude in the middle of the Pacific Ocean (Hawaii), as an international site, marked a major step for the development of optical and IR astronomy: mean pwv ~1 mm, mean temp. ~0°C, visible seeing ~0.6’’ (better in the near-IR)
First generation: UH88’’, UKIRT, CFHT, IRTF – 4 m
Second generation: Keck I and II, Gemini North, Subaru 8 – 10 m
Third generation: TMT ?? m
A site for sub-mm astronomy:
CSO m
JCMT m
SMA (Sub MM Array) x6 m

6. Looking for better infrared sites
Sites drier than MK have been explored, particularly for the extension to the sub-mm windows. The atmospheric absorptions also increase the sky brightness  two sites: Chajnantor (ALMA) and Antarctica (Dome C).
PWV values measured at Cerro Chajnantor (CCAT site) and at Dome C between 2008 and Important difference of scale range for PWV between the two sites. From Tremblin et al. (2012)

7. Dome C as ground-based infrared site
Dome C with the Concordia station – elevation 3200 m - appeared as the most promising ground-based site (FP contract 2006 – 2009 ARENA: Antarctic Research, European Network for Astrophysics).
Driest conditions on Earth: pwv ≈ 0.25 mm  opening of the THz window
Cold conditions in the long winter: - 70°C  reduction of the thermal emission
Low sky brightness in the infrared windows  ‘’ ‘’ ‘’
Several months long observing time using the polar night and the twilight time on circumpolar sources  programs of source monitoring, of asteroseismology
85% of clear nights in winter
However, it remains just an European roadmap:
Discovery of a ground layer of ~ 30 m thickness = poor seeing conditions, strong thermal gradient ~1°/m  an IR telescope on a 30 m tower, in a closed dome
Problem of access to Concordia for heavy equipments
Power requirements – no solar energy in the polar night = large quantity of fuel
Better site: Ridge A (4050 m, lower pwv, no wind) but even more difficult to access

8. Topographic map of Antarctica. Main research stations

9. Airborne observatories
By carrying the telescope to a high altitude by a balloon or an airplane, it makes possible to avoid most of the water vapor absorptions for IR observations. Ex.: H2O detection in Halley’s comet (1986) from the KAO
Two successful airplane observatories
KAO (Kuiper Airborne Obs.) [ ]
91 cm telescope on C-141. IR astronomy from 1 to 600 μm
SOFIA (Stratospheric Observatory for IR Astron.) [ ? ] 2.5 m telescope on a B-747; highest altitude 13.7 km
IR astronomy from 1 to 655 μm
Drawbacks
observing time limited to flight rate
moderate telescope size
optics temperature: ≈ 240 K
instability of the platform
Cometary emission lines of H2O compared with the Moon spectrum showing the residual atmospheric lines

10. The solution: IR astronomy in space
For an IR space telescope
The main parameters are :
the telescope diameter
the type of orbit
the telescope temperature (passive cooling/cryo-cooling)
IRAS
ISO
SPITZER
HERSCHEL
1983
1995
2003
2009
57 cm
60 cm
85 cm
350 cm
Geocentric
Elliptical geocentric
Earth-trailing L2
Helium (2K)
12 to 100 μm
10 months
Helium (1.7K)
2.4 to 240 μm
24 months
p. cooling (35K) + He
3.6 to 160 μm
6 years + warm mission
p. cooling (80K) + He
55 to 672 μm
4 years

11. Next space telescope: JWST
Launch oct at L2
6.5 m telescope (8m initially as NGST) made of 18 hexa mirrors – operating at 50 K
N-IR camera (NIRCam) 0.6 – 5 μm twice 2.2’x2.2’ FOV
N-IR Spectro (NIRSpec) 0.6 – 5 μm, 3’x3’ FOV Multi-object and Integral Field modes
Mid-IR Spectro (MIRI) – 28.5 μm 3.9’’x3.9’’ to 7.7’’x7.7’’, low & mid-res ≈2500
N-IR Guidance Sensor (FGS/NIRISS) 0.8 – 5.0 μm grism R ≈ 700

12. Limitations of the current solutions
Next generation ground-based telescopes: ELT
30 to 40 m diameter (E-ELT, TMT, GMT)
Warm telescopes with complex adaptive optics systems (MCAO …)
In the atmospheric windows (no access to the ~18 – 350 μm region)
Next generation space telescopes:
WFIRST (Wide field IR Survey Telescope): 2.4 m primary, geosynchronous orbit, warm telescope, near-IR range from 0.6 to 2 μm
top-priority NASA project; parallel to EUCLID for ESA
SPICA (SPace Infrared telescope for Cosmology and Astrophysics): 3 m primary, at L2, cryo-cooling ≈ 6 K, IR range from 12 to 210 μm
ESA/JAXA proposal submitted as M5 mission in 2016 – not selected
OST (Origins Space Telescope): 9.3 m primary, at L2, cryo-cooling ≈ 5 K, IR range from 5 to ~700 μm NASA decadal survey
Only way to combine large telescope diameter (>40m), low thermal background, diffraction-limited images, access to all the IR range: a lunar-based ELT

13. The Diviner Lunar Radiometer
North Pole
Thanks to the Diviner Lunar radiometer aboard NASA’s Lunar Reconnaissance Orbiter mapping of the day-night surface temperature of the Moon since 2009 to 2014 (Williams et al. Icarus, 2017).
At the lunar poles, detection of permanently shaded crater floors  very cold
Lowest temperature 26 K measured in the Hermite impact crater ( ~4° from the North pole). Flat bottom, Diameter : 108 km
Hermite
crater
Other potential site at the South Pole: Shackleton. Diameter: 21 km
Zuber et al., Nature 2012
a site for an IR telescope ?

14. Advantages of such a site for IR
Access to the IR domain up to the millimeter range
Operating temperature lower than at L2. With a low temperature of 26K limitation only by the zodiacal emission up to ≈450 μm
Possibility of building a telescope bigger than any ground-based telescope, with the advantages of a space telescope.
Could make a 100-m steerable telescope a reality. Has been a dream at ESO (before E-ELT) with the OWL Telescope
Major limitation due to wind-buffeting problem!

15. The 100 –m OWL Telescope project (primary made of 3048 elements)
The 100 –m OWL Telescope project (primary made of 3048 elements). The mirror covers stacked on the left and a sliding enclosure during day time no longer needed.

16. Scientific justification made for OWL on the spectacular angular resolution, to the condition to be diffraction-limited = a very complex adaptive optics system with 6 mirrors. Much simpler optical system for a lunar OWL. Only an active optics system to keep the primary in shape.

17. Thank you Owerwhelming Science !
Lunar OWL
3D survey and characterization of the most distant galaxies to unprecedented depth
integral field spectroscopy of giant molecular clouds over the full IR range
imaging and high-resolution spectroscopy of Jupiter-size planets …