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Infrared Spectroscopic Observations in Astronomy

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Infrared Spectroscopic Observations in Astronomy Infrared Spectroscopic Observations in Astronomy Adwin Boogert NASA Herschel Science Center IPAC, Caltech Pasadena, CA, USA 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 1 Scope ●Lecture 1 (Monday): What you need to know when planning, reducing, or analyzing infrared spectroscopic observations of dust and ices. ●Lecture 2 (Tuesday): Basic physical and chemical information derived from interstellar ice observations. Not discussed: laboratory techniques (see Palumbo lectures) and surface chemistry (see Cuppen lectures). ●Lecture 3 (Tuesday): Infrared spectroscopic databases. What's in them and how (not) to use them. ●Drylabs (Tuesday): Using databases of interstellar infrared spectra and of laboratory ices. Deriving ice abundances and analyzing ice band profiles. NOTE: Please download all presentations and drylab tar file: spider.ipac.caltech.edu/~aboogert/Cuijk/ 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 2 Topics ●Infrared wavelength definitions ●Infrared facilities ●Vibration modes ●Observational challenges: ● Atmospheric absorption ● Atmospheric and telescope background emission ● Chopping and nodding ● Celestial background emission ●Infrared detectors ●Spectral resolution ●Spectrometer types ●Spatial resolution ●Sensitivity ●Data reduction ●Summary: Ground vs Space Based IR Astronomy 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 3 Reading Materials Lecture 1 Basic reading material on observing techniques: ● Chapters 2 (Infrared Sky) and 6 (Infrared Techniques) of Handbook of Infrared Astronomy by I. S. Glass ● Gemini Mid-IR pages: www.gemini.edu/sciops/instruments/michelle/mid-ir-resources More advanced reading materials on spectrometers and telescopes: ● Astrophysical Techniques by C.R. Kitchin 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 4 Infrared Astronomy Somewhat subjective definitions of infrared wavelength regions in astronomy (I.S. Glass, p. 27): ● Near-infrared: 0.75-5 um ● Mid-infrared: 5-25 um ● Far-infrared: 25-350 um ● Sub-millimeter: 350-1000 um Roughly based on key wavelengths: ● Human eye cutoff: 0.75 um ● Optical CCDs cutoff: 1.1 um ● Background emission dominates: >2.3 um ● Background emission peaks (T~300 K): ~10 um ● Longest wavelength mi-ir window: 25 um ● Heterodyne techniques feasible (<2008): >350 um 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 5 Facilities Selection of past, current, and future 3-200 um spectrometers suitable for ice and dust feature observations (not complete!). We will get back to aspects of this table during the lecture. Instrument Detector l R Remarks (>3 um) mm *1000 ISAAC/VLT InSb 1-5 1.4-10 - NIRSPEC/Keck InSb 1-5 2.0-25 optional ad. opt. SpecX/IRTF InSb 1-5 1.0-2.0 x-dispersed NIRSpec/JWST HgCdTe 1-5 0.1-3 IFU SWS/ISO InSb,SiGa, 2-45 0.1-1.5 l scanning SiAs,GeBe IRC/AKARI Insb,SiAs 2-26 0.1 prism+grating TreCS/GeminiS SiAs 8-26 0.1-1.0 - MIRI/JWST SiAs 5-28 0.1-3 IFU IRS/Spitzer SiAs,SiSb 5-35 0.06-0.6 - FORCAST/SOFIA SiAs,SiSb 5-50 0.1-1.0 5-15 um x-disp LWS/ISO GeBe,GeGa 45-200 0.2 - PACS/Herschel GeGa 57-210 1.5 IFU 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 6 Dust and Ice Transitions ●Typical wavelength ranges in which dust and ice vibrational modes occur. ●Whether a particular mode can be observed and with which instrument and technique depends on absorption and emission spectrum of the earth's Allamandola (1984) atmosphere (see next slides). 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 7 Challenge: Atmospheric Absorption ●Windows of Mauna Kea: 1 mm (black) and 3 mm H2O (red) good/fair transmission between 1-26 μm. ●Strong wavelength dependence, even within windows. ●Transmission depends strongly on water vapor column above site (use H2O water vapor monitor to assess!), and elevation on sky. 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 8 Challenge: Atmospheric Absorption Mauna Kea: 1 mm (black) and 3 mm H2O (red) ●Same plot as previous, but on log wavelength scale, highlighting NIR transmission. 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 9 More Challenges: Infrared Background Emission While atmospheric absorption reduces the stellar signal strength, strong infrared background emission increases the noise. Atmospheric and telescope background emission often much stronger than stellar emission: ●Fluctuations in background emission strength produce systematic noise effects. Can be minimized using sophisticated subtraction methods. ●Background emission important component statistical noise, because photon noise follows Poisson statistics. Noise in observed stellar signal (after full reduction): star∝N phot = N phot bgN phot star... 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 10 Infrared Background Emission Main sources of background emission: (1) Earth's atmosphere ● Solutions: go cold (I∝T4), go high. Space, balloon (airship?), airplane, Antarctica, high mountains. ● Monitor weather conditions: good conditions at Mauna Kea have ~1 mm Precipitable Water Vapor (CSO 225 GHz t~0.05), but can be much higher. ● Causes 'sky noise': unstable weather, thin cirrus and other structured cloud, wind-borne dust (e.g., Saharan dust storms affecting Canary Islands). 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 11 Infrared Background Emission Mauna Kea: 1 mm (black) and 3 mm H2O (red) ●Atmospheric emission spectrum (model) ●Rise by 3-5 orders of magnitude above 3 mm! ●Sky temperature similar at most wavelengths, but O3 emission from higher and colder layers. 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 12 Infrared Background Emission Mauna Kea: 1 mm (black) and 3 mm H2O (red) ●Same plot as previous, but on log wavelength scale, highlighting NIR background emission (OH lines). 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 13 Infrared Background Emission Main sources of background emission (continued): (2) Telescope mirrors + support structures. Solutions: ● Low emissivity coatings (~5% for aluminum [most telscopes], ~2% for silver [Gemini]), needs re-coating every ~5 years. ● Thermally stable telescope ● Keep mirrors uniformly clean. M1 segment gaps M2 support structure 10 um entrance pupil image of Canaricam at Grantecan 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 14 Infrared Background Emission ● Telescope emission peaks at ~15μm, corresponding to temperatures of ~270 - 290 K ● Mauna Kea sky emission compared to emission from a telescope with 2% emissivity (Gemini) 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 15 Infrared Background Emission ● Keep them cold and avoid temperature fluctuations: ● Go to space ● Go far from earth's radiation, avoid going in and out of Earth's shadow (Herschel, JWST: L2, Spitzer: earth trailing) ● Sun still heats telescope. Use Helium to make telescope mirror+structures very cold (Spitzer ~5.5 K, ISO ~4 K, AKARI ~6 K) or cold (Herschel ~80 K). L2 orbit for Herschel and JWST 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 16 Infrared Background Emission Main sources of background emission (continued): (3) Instrument window. ● Window separates cooled spectrometer from outside world. Prone to condensation and ice formation. Solved by using a fan, esp. if humid conditions. If it happens anyway, do careful flatfielding. Background cancellation via nodding or nodding+chopping secondary, want small stable residual offset signals 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 17 Nodding and Chopping Background emission subtracted by ●Telescope nodding only, if the background is stable on time scales of 10s of seconds (e.g., NIRSPEC/Keck at Mauna Kea). ●Beamswitching: move (chop) secondary mirror between 2 sky positions at fast rate. Background emission not perfectly canceled as beams have slightly different optical paths, which have different defects, dust, etc., leading to radiative offset between the two chop positions. Compensate by nodding the telescope so that the object and reference positions are switched. Most used beam switch variants: ● Nod the telescope by a distance equal to the chop throw along the chop axis ● On-array and off-array nodding 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 18 Nodding and Chopping ●A-B gives net signal corrected for radiative offset. ●BUT flexure and temperature changes mean that offset changes with time, so take data in sequence A,B,B,A to remove linear gradient in offset (instead of A,B,A,B). 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 19 Nodding and Chopping ●Actual 10 mm beam- switch observation performed with T-ReCS at Gemini-South. ●This is the variant with off-array nodding. 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 20 Sky Background ●Galactic Cirrus and Solar System Zodiacal light largest contributors to extraterrestrial sky background in 3-200 um wavelength region. ●Intensity strongly direction dependent. ●Affects sensitivity of infrared satellite observations Diffuse background emission, away from ecliptic. Leinert et al. 1988 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 21 Infrared Detectors ●Infrared detection
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