Infrared Spectroscopic Observations in Astronomy Adwin Boogert NASA Herschel Science Center

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)
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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.
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NOTE: Please download all presentations and drylab tar file:
spider.ipac.caltech.edu/~aboogert/Cuijk/
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Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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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
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Interstellar Dust School (Cuijk): Interstellar Ices (Boogert)
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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
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Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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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
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Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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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.
Detector

(>3 um)
m
ISAAC/VLT
InSb
1-5
NIRSPEC/Keck
InSb
1-5
SpecX/IRTF
InSb
1-5
NIRSpec/JWST
HgCdTe
1-5
SWS/ISO
InSb,SiGa, 2-45
SiAs,GeBe
IRC/AKARI
Insb,SiAs
2-26
TreCS/GeminiS
SiAs
8-26
MIRI/JWST
SiAs
5-28
IRS/Spitzer
SiAs,SiSb
5-35
FORCAST/SOFIA SiAs,SiSb
5-50
LWS/ISO
GeBe,GeGa 45-200
PACS/Herschel
GeGa
57-210
Instrument
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R
*1000
1.4-10
2.0-25
1.0-2.0
0.1-3
0.1-1.5
Remarks
0.1
0.1-1.0
0.1-3
0.06-0.6
0.1-1.0
0.2
1.5
prism+grating
IFU
5-15 um x-disp
IFU
optional ad. opt.
x-dispersed
IFU
 scanning
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Dust and Ice Transitions
●Typical
wavelength
ranges in which dust
and ice vibrational
modes occur.
●Whether
Allamandola (1984)
04 June 2012
a particular
mode can be observed
and with which
instrument and
technique depends on
absorption and
emission spectrum of
the earth's
atmosphere (see next
slides).
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Challenge: Atmospheric Absorption
Mauna Kea: 1 mm (black) and 3 mm H2O (red)
●Windows
of
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)
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Challenge: Atmospheric Absorption
Mauna Kea: 1 mm (black) and 3 mm H2O (red)
04 June 2012
●Same
plot as
previous, but on
log wavelength
scale, highlighting
NIR transmission.
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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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 bg  N phot  star...
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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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 ~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)
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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 m!
●Sky
temperature
similar at most
wavelengths, but
O3 emission from
higher and colder
layers.
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Infrared Background Emission
Mauna Kea: 1 mm (black) and 3 mm H2O (red)
04 June 2012
●Same
plot as
previous, but on
log wavelength
scale, highlighting
NIR background
emission (OH
lines).
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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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
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Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Infrared Background Emission
04 June 2012
●
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)
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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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)
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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)
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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)
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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)
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Nodding and Chopping
10 m beamswitch observation
performed with T-ReCS
at Gemini-South.
●Actual
●This
is the variant with
off-array nodding.
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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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)
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Infrared Detectors
Infrared detection done using N2 or He cooled semiconductors:
bound electrons liberated by sky photons. Detectors can be
photovoltaic (measure current of electrons) or photoconductive
(measure change in resistance).
●
Nomenclature: Infrared detector arrays are not CCDs! They are
arrays of individual detectors, each of which are read out
individually. Advantage: one bad or saturated pixel does not
affect an entire column!
●
Infrared arrays hard to make and expensive. Still improving,
especially at longer wavelengths (motivated/funded by space
missions—HST, JWST).
●
Each detector material and array type has its own properties
(quantum efficiency, dark current,readout noise) and problems
(memory effects, standing waves, effect of cosmic ray hits).
●
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Spectral Resolution
Relatively broad dust and ice features (>0.1 m) need
spectral resolutions of ~100 to a few thousand.
But higher spectral resolution may matter (e.g., for narrow
CO, 13CO ice features):
●
●
improved sensitivity by looking 'between' sky lines
(instead of them being smeared out)
separation circumstellar gas phase emission and/or
absorption lines from ice band
Proper grating AND narrower slits needed for higher
spectral resolution. Narrower slits transmit less light.
Trade-off with seeing important.
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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High Spectral Resolution Needed?
Ro-vibrational transition rules lead to characteristic P and R
branch spectrum superimposed on ice absorption band. May be
problem at low spectral resolution.
Example: R=25,000 spectrum shows CO fundamental (J=1,
v=1) absorption lines. Other sources show broad emission
severely compromising the analysis.
Boogert et al. (2002)
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Spectrometer Types
Standard (but cooled!) grating spectrometers often used for ice
and dust observations (R~<few 1000).
●
Higher R sometimes desired: use echelle grating (optimized for
high diffraction orders), which in combination with cross disperser
(prism or grating) 'packs' orders efficiently on detector array
provided slit is short.
●
Longer slits (e.g. for larger field of view for extended emission)
may only give one order on array.
●
If desired wavelength range does not fit on array, move grating.
But ISO/SWS moves mirror to scans spectrum over (small) array.
●
Grism: prism+grating: all objects in field of view display spectra
at position of direct imaging (e.g., AKARI satellite). Spectra of
different objects may overlap.
●
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Spectrometer Types
Use of Echelle grating and normal grating modes for NIRSPEC/Keck
High resolution mode: Echelle
grating+cross disperse
grating
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Low resolution mode: move
Echelle grating out of the way
and only use cross disperse
grating
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Spectrometer Types
●1D
spectroscopy for spatial
distribution of solid state features
possible using long slits.
●2D
spectroscopy becoming more
popular as more instruments
(Herschel/PACS,
JWST/MIRI+NIRSPec) have
Integral Field Units.
●IFU
image slicer images sky
pixels onto entrance slit of grating
spectrometer
Example: Herschel/PACS IFU
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Spatial Resolution
●Telescopes
seeing limited in near-infrared (although
adaptive optics using (laser) guide stars can remove
much of the seeing effects), but often diffraction limited in
mid-infrared:
 m
arcsec =0.252∗
diameter m
●Some
infrared spectrometers are coupled with adaptive
optics for high spatial resolution (CRIRES, NIRSPEC). E.g.,
One could study ice/dust features in circumstellar disks
and envelopes as function of distance to star (see
example in lecture 2).
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Spatial Resolution
source: www.gtc.iac.es/en/media/canaricam/
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Sensitivity
Some ground-based and all space-based telescopes use online
integration time calculators to determine time needed for certain
S/N, source brightness, instrument settings (, and atmospheric
conditions).
●
For other facilities one needs to use sensitivity tables and tables
with other noise contributions and then take into account
instrument overheads.
●
Noise sources:
● Readout noise: property of readout electronics. Expressed as
standard deviation in unit of electrons per read-out.
● Photon or shot noise: intrinsic property of particle nature of
light. Scales with square root of number of photons per
readout, following Poisson statistics. Note: includes ALL
photons, incl. science object, thermal background, dark
current! Main reason why ground-based thermal infrared
observations are tough.
●
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Sensitivity
●
Sky background variations, even if the weather is good the sky
background varies. It follows behavior of “1/F noise” and
decreases if the chopper frequency F increases. Can be
corrected a bit by subtracting neighboring sky on 2D arrays.
In summary, signal-to-noise ratio achieved is
●
N el obj  N integr ÷  N el obj2 N el atmtel 2 N el dark  read
2
●
●
●
Nel(obj) is the number of electrons from the science target after
photon absorption in atmosphere, filters, mirrors, and taking
into account detector quantum efficiency.
At >3 m, Nel(atm+tel)>>Nel(obj), so main source of noise,
e.g., L=13.0 mag object yields 25 electrons/sec with ISAAC/VLT,
while background is 25,000 electrons/sec!
For satellites Nel(atm+tel) much smaller, but extraterrestrial
background becomes significant (cirrus, zodiacal)
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Data Reduction
Reduction of data taken with modern infrared detector arrays
fairly similar to optical CCD data reduction:
(1) [Bias level subtraction]. Optional, because taken care of
automatically in step (2) or (4).
(2) [Dark current subtraction]. Needs same integration time as
on science, calibration, flatfield targets. Optional, because
taken care of automatically in step (4).
(3) Division by flatfield:
● spectral response curve for 1D spectra
● Taken using lamp or on thermal background
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Data Reduction (Continued)
(4) Subtract Chop/Nod pairs to get rid of
● Telescope+earth background emission
● Sky background emission (Cirrus, interstellar)
● Hot pixels (Spitzer)
(5) Extract 1D spectrum from 2D image
● Second order background subtraction using neighboring
columns:
● Thermal background variations between nod/chop
● Local, spatial variable extended source emission
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Data Reduction (Continued)
(6) Divide by telluric line standard
● Can be used for photometric calibration as well
● Preferred spectral type, airmass difference, brightness
● Need stellar model spectra (e.g., from Kurucz)
● Correct for grating shifts before division. Telescope pipelines
generally don't do this.
● Gets rid of other multiplicative effects in case no flatfielding
done or that flatfield did not remove such as standing waves
(though not perfectly).
● Optionally use atm models to remove sky lines or residuals
thereof
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Data Reduction (Continued)
(7) Wavelength calibration:
● Use calibration lamp lines
● Lamp lines generally not available >4 um
● Use atmospheric emission lines for wavelength calibration or
fine-tune the lamp calibration (grating can shift between
science and lamp observations). Atmospheric lines come for
free with infrared observations and should give very accurate
calibration.
Software packages for infrared data reduction:
Some telescopes, especially satellites, provide packages
optimized for data reduction of specific instruments (VLT: MIDAS,
IRTF: an IDL-based package, ISO/SWS: IDL+OSIA, Herschel: HIPE).
●Spitzer/IRS provides tools for certain aspects of data reduction
(e.g., SPICE for source extraction).
●General purpose tools such as IRAF and IDL can be used, but
may require extensive script writing.
●
04 June 2012
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
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Summary :Ground vs Space Based IR
Astronomy
Given huge advantage of low background emission of satellites,
why bother doing ground based infrared spectroscopy?
●Large
ground based telescope apertures give high spatial
resolution (there will be JWST, but then there will be ELT, TMT..).
●Ground
based telescopes can be equiped with new, more
specialized instruments (e.g., high spectral resolution).
●Infrared
satellites have limited lifetime, making follow up
spectroscopy on new discoveries hard.
●Windows
of good/fair transmission between 3-4, 4.6-5, 8-13
and 16-25 μm. Benefit from cold high dry sites (Mauna
Kea/Chile) and low emissivity, high cleanliness.
●Last 3 points are in favor of SOFIA airplane too
Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert)
04 June 2012
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