IR Atmospheric Windows & Sky Ground based IR Observation

Blackbodies  Planck distribution  Wien displacement law T(K),λ(μm):  T ∗ λpeak=2898   10 micron   60 micron    1000 micron 

Infrared colors of BB  300K BB  peak at 10  m  Temp(K) J-H, H-K, K-L, L-N          

IR detecters Thermocouples, thermopiles (devices which convert heat to electric currents) PbS detector (change the resistance of the cell) : 1-4  m, cooled to 77 K Ge bolometer : 100x sensitivity, all IR, cooled to 4 K InSb, HgCdTe detectors : 1-5  m  Breakthrough : Array detector,1980’s

Earth's atmosphere  Most of the infrared light coming to us from the Universe is absorbed by water vapor and carbon dioxide in the Earth's atmosphere.  Only in a few narrow wavelength ranges, can infrared light make it through (at least partially) to a ground based infrared telescope.  The atmosphere itself radiates strongly in the infrared, often putting out more infrared light than the object in space being observed. This atmospheric infrared emission peaks at a wavelength of about 10 microns.

Q 1  Why do you think the ground based IR telescope itself is not cooled, while the space IR telescope is entirely cooled?  What do you think the important noises in space IR observations are?

Earth's atmosphere  Near-IR: mm  Mid-IR: 5-25 mm  Far-IR: mm  Submm/mm:  0.35 – 3 mm  Radio:  1 mm – 30 m Figure from Wikipedia, NASA, Spitzer project

Atmospheric windows define the IR pass bands  Z (0.89), Y (1.03),  J (1.25), H (1.64), K (2.2), K' (2.1), Ks(2.15) L' (3.8), M' (4.7), N (10.6), Q (21) L' (3.8), M' (4.7), N (10.6), Q (21) Wikipedia, public domain

Atmospheric extinction  extinction = absorption + scattering  d << λ : Rayleigh scattering ~  d ~ λ : Mie scattering (aerosols: sea salt, dust)  many atomic & molecular line transitions > absorption – gases in our atmosphere: N 2, O 2, A, CO 2, Ne, He, – gases in our atmosphere: N 2, O 2, A, CO 2, Ne, He, CH 4, Kr, H 2, N 2 O, CO, H 2 O, O 3, and other gases CH 4, Kr, H 2, N 2 O, CO, H 2 O, O 3, and other gases – absorption dominated by: H 2 O and CO 2 – absorption dominated by: H 2 O and CO 2 – Water vapor column density, expressed as – Water vapor column density, expressed as “precipitable water in mm”, varies from 1-15 “precipitable water in mm”, varies from 1-15

Atm Extinctions  Measured and extrapolated extinctions caused by the aerosol component, in magnitudes per unit air mass  Value   m, J, H, K  Max  Min  Mean  After the eruption of El Chicon  Max  Min

Molecular line spectrum of the Earth's atmosphere U.S. AirForce, MODTRAN v.3.7, Mid-latitude, summer, 1-10μm, airmass=1, 4 altitudes: 0, 1, 4, and 40 km

IR Bands Wavelength RangeBandSky Transparency Sky Brightnes s microns.Jhighlow at night micronsH highvery low micronsKhighvery low micronsL microns: fair microns: high low micronsMlowhigh micronsN microns and microns: fair others: low very high microns microns: Q microns: Z very lowvery high micronsvery lowlow

J, H, K, L, L’, M, M’

The ZY JHK bands  The UKIRT Infrared Deep Sky Survey ZY JHK photometric system:  Upper curve is for no atmospheric absorption. Middle curve is for 1 mm precipitable water and airmass=1.3. Lower curve for 5 mm water, airm=2. See Hewett et al. 2006, MNRAS 367, 454.

Terrestrial background radiation at night time  at > 2.3  m thermal radiation dominates (telescope and atmosphere) (telescope and atmosphere) terrestrial 300K ~ 10  m terrestrial 300K ~ 10  m  Scattered light from moon, dominates for Z and Y bands  Airglow= dominant source of noises at J, H, and much of K bands  : OH lines, H + O 3 -> OH + O 2 alt=85-100km, varies 10-50% over 5-15 min, alt=85-100km, varies 10-50% over 5-15 min, -dominates J and H band background -dominates J and H band background (The average integrated line flux in H band : (The average integrated line flux in H band : 3x10 4 photons s -1 m -2 arcsec -2  m -1 ) 3x10 4 photons s -1 m -2 arcsec -2  m -1 ) -lines blend together for R < 600 -lines blend together for R < 600  Sky (mag/sq “): J=15.5, H=13.8, Ks=12.9

OH airglow spectrum  Airglow emission is tiresome noise, but can be used as a wavelength calibration source  ESO Infrared spectrometer IRSPEC, Nasmyth foci of the ESO-NTT telescope. SBRC 62x58 InSb array A&A 254, 466, 1992, Oliva & Origlia

OH airglow spectrum   m   m   m

List of OH lines ; A&A 254, 466, 1992, Oliva & Origlia  OH

List of OH lines  OH

List of OH lines  OH

Non-thermal emission in the Atm above Mauna Kea  MN 259, 751, Ramsey aet al  R=250

Non-thermal emission in the Atm above Mauna Kea  MN 259, 751, 1992, Ramsey aet al  R=250

Non-thermal emission in the Atm above Mauna Kea  MN 259, 751, 1992, Ramsey et al  R=800

Good IR observing sites – dry, cold and “high up”  Cooled telescopes in space, balloons  Dome C, Antartica (T=-60C,alt=2800m, water vapor from 0.1 to 0.3 mm !)  Atacama desert (ALMA): < 1.6 mm for 75% of time and < 1 mm for 50% < 1 mm for 50%  Hawaii (UKIRT, JCMT, Keck): < 1.7 mm for 25% of time, < 2.9 mm for 50% < 2.9 mm for 50%  La Palma, GTC site testing: < 3 mm for 39% of nights, < 1 mm for 10% < 1 mm for 10%

Special requirements for IR instruments  The whole instrument in a vacuum cryostat puts heavy constraints on all materials used: – tolerate temperatures -200°C < T < 120°C – tolerate temperatures -200°C < T < 120°C – no out-gassing allowed – no out-gassing allowed – different expansion coefficients (use of springs) – different expansion coefficients (use of springs)  IR sensitive detectors with rapid readout  Chopping(obs alternatively the field of the target and a nearby empty one) /nodding/dithering facilities

Ground based infrared observatories  Ground based infrared observatories, using advanced techniques such as Adaptive Optics are providing fascinating views of the infrared Universe viewed through our atmosphere's infrared windows. Mauna Kea Observatories

airborne observatory SOFIA, an airborne observatory, is schedule to start operations in SOFIA, an airborne observatory, is schedule to start operations in 2004.

Space Telescope  The Spitzer Space Telescope, launched in August 2003, is NASA's next great observatory in space.

Extraterrestrial Bg sources  1. Zodiacal light BG :  at nearer IR out to about 3.5  m, scattered light from dust in the solar ecliptic and at longer, light due to direct emission from the dust particles  COBE satellite :  DIRBE(Diffuse IR Bg Experiment) : maped the sky in 1.25, 2.2, 3.5, 4.9, 12, 25, 60, 100, 140, and 240  m with spatial resolution of 0.7 X 0.7 deg  FIRAS(Far IR Absolute Spectrometer) : Michelson interferometer providing cover from 100  m to 10mm with a FOV 7deg FWHM  2. ISM : 100  m < < 300  m : emission by dust within ISM, IR cirrus  3. Cosmic Microwave BG (0.03% of its peak between 500  m and 5mm, dominant Bg at about 300  m

Astron. Astrophys. Suppl. Ser. 127, 1-99 (1998)

South pole  Altitude of the scientific base at the South pole is 2800m and the low T, typically -60 C in winter  make it very promising site for IR  Precipitable water vapor ; 0.1 ~ 0.3 mm  Ashely et al (1996) : low resolution spectrophotometry :   m window (air glow minimal) ; two order of mag less airglow emission  L band ;sky brightness times less