Transcript Slide 1

CEE 540
Spring Term 2012
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go over syllabus and course requirements
project
spectra
lab demo:
• parts of a spectrograph
• image in focal plane
• hg spectral emission lines
• varying entrance slit width
• pixelation of the detector
• widths of spectral lines
• integration time
• spectral resolution
low pressure mercury
lab demo
we will spend a lot of time talking about spectra – the use of molecular spectral
absorbance is a common theme in instrumentation designed to measure
molecular concentrations – e.g. 515 EPA instrumentation.
we will start out with a very brief introduction to molecular spectra and then get
into the instrumentatioin
red shows radiation from a black body
at 5700K for the Sun and 288K for Earth
vertical scales for Sun and Earth are not
the same !
how important is the 7% UV from the Sun?
the solar and earth spectra
Solar spectrum at top of atmosphere and ground level
why are TOA, BB, and sea level
different?
What are the bumps and wiggles in the real solar spectrum??
solar + Earth spectrum : measurement made
from the ground through the atmosphere
sun
how do you distinguish spectral lines in the Sun from those in the atmosphere
when observing the sky or direct Sun with a spectrometer?
sun
sun
sun
• history of spectra
• physical basis of spectra – molecular and atomic
• 2-level atom
• spectral line profile – transitions should be monochromatic??
why does a spectral line have width?
• lifetime
• collisions
• pressure broadening
• temperature broadening - Doppler
Molecular transitions  emission/absorption
molecules
Electronic (A  B)
Vibrational (n’’  n’)
Rotational (J’’  J’)
Radiative
transitions to/from
various energy levels
HCl rotational spectrum
rotational structure of HBr
spectrum of the O3 molecule at T = 0°C
lab testing of the OMI space instrument prior to launch in 2004
0.02
OMI ze nith s k y
17 Aug 2002
90¡SZA (PM) r e s idual
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OMI residual
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700
-0.01
OMI NO2 xsec [cm^2]
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om i ze n s k y re s idual w /o NO2 r e m oval 750x10-21
om i no2 xs e c
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wavele ngth (nm)
sky spectrum compared to laboratory NO2 photoabsorption
cross section
the correlation of the observed spectrum to NO2 is clear
spectrum of the Sun + Earth as measured from ground at KPNO
think about spectral
resolution. The ability
to distinguish colors with
your instrument. What
is the spectral resolution
of your eye? Can you
see spectral features in
the Sun when you look at
it? What happens to
spectral resolution when
you put on sunglasses?
Newton experimented
with prisms to disperse
solar light into colors.
The spectral features
(lines) were first observations
in the early 1800’s and
were not understood.
What would the spectral
shown to the left here
look like with an instrument
that could barely separate
colors?
These data were taken
with a very high resolution
system at Kitt Peak. Blow up!
solar spectral lines are wide and Earth spectral lines are narrow – why?
difference between spectral SAMPLING and spectral RESOLUTION
absorbance
absorbance
absorbance
absorbance
absorbance
absorbance
another example – an atmospheric absorption spectrum
sampling affects the definition of the spectrum, but not
the spectral resolution
what is spectral resolution?
now take a bunch of spectral lines which are viewed with a spectrograph
of infinite resolving power – it can see the absorption spectrum in infinite detail
now look at this infinite resolution set of spectral features with a real
spectrograph of spectral resolution 1 nm
inf
1 nm
1 nm
8192 pixels
1 nm
2048 pixels
1 nm
8192 pixels
1 nm
2048 pixels
1 nm
8192 pixels
1 nm
512 pixels
not feasible
1 nm
8192 pix
NASA MFDOAS
1 nm
2048 pix
OMI
1 nm
512 pix
spectrum (cross section) of the NO2 molecule at T = +20°C
spectrum of the NO2 molecule at T = +20°C
spectrum of the NO2 molecule at T = +20°C
spectrometers and spectrographs
terms:
• spectral range [nm]
• spectral resolution [nm]
• spectral coverage [nm]
• spectral sampling [pixels]
• angle of grating
• focal length
• f/
• grating blaze angle
• s/n
• throughput
• expected signal level vs other parameters (e.g. spectral coverage)
• polarization
basic components:
• light source
• slit
• collimator mirror
• disperser
• camera mirror
• focal plane detector
diffraction gratings
exit slit or detector pixel
diffraction
grating
entrance slit
camera
mirror
collimator
mirror
Acton Research Corp. spectrograph, model 300i, s/n 300404, cost ~$10,000
• slit:
• why?
• width variable from 10 µm to 2 mm
• height – 5 mm
• collimator mirror/lens
• size – 60 mm diameter mirror
• focal length F = 300 mm
• f/number = F/a = f/5
• disperser
• grating/prism – diffraction grating
• lines/mm = 1800
• size 68 mm x 68 mm
• blaze – 320 nm = 32.7°
• how are these made
• included angle f = 13.7°
• detector
• film
• eye
• photodiode
• photodiode array
• channel plate
• CCD – xx pixel x yy pixel
basic parameters for a spectrometer/spectrograph
• dispersion [nm/mm] – will compute below
• spectral sampling [number of detector resolution elements/slit width or for an array
detector, number of detector pixels/FWHM of the spectral line profile
• resolution [nm]
• what is the importance of spectral resolution
• what is the effect on your instrument of increasing or decreasing spectral resolution
• throughput [fraction] = efficiency of the unit [photons detected/photon in]
• polarization [percent] – comes mainly from the diffraction grating
• can be a major problem when looking at polarized light
• what are examples of light sources that are polarized?
• free spectral range [nm]
• bandwidth
• overlapping orders – will talk about more when we discuss grating details
• grating blaze angle
other types of spectrographs
Ebert-Fastie Mounting
Rowland circle mounting
Dutch OMI instrument – launched 2004 and still operational on NASA/AURA
NASA JPL Orbiting Carbon Observatory
launch April 2013
diffraction gratings
diffraction gratings and spectrographs
grating equation:
nl = a [sina + sinb]
where,
a = angle of incidence relative to grating normal [°]
b = angle of diffraction relative to the grating normal [°]
n = diffraction order = think of the multiple slit problem, a grating is just a
multiple slit used in reflectance, not transmission
a = line spacing on the grating [mm]
For a Czerny-Turner spectrograph like the one we will use in the lab:
a=qf
b=q+f
where q is the angle the grating is rotated [°] from the mirror angle
and
f is the half angle of the angle between the center of the grating and the 2 mirrors
substituting into the grating equation and doing the arithmetic
nl = 2a sinq cosf
[for Czerny-Turner/Ebert types only – never ever use this eq. on another type of
spectrograph – go back to the basic grating equation]
dispersion [number of wavelength units per physical dimension at the focal plane]
nl = a [sina + sinb]
simple differentiation 
Dl/Db = [a cosb]/n 
Db = Dx / F  Dl/Dx = [a cosb] / (Fn)
which is now the linear dispersion in the focal plane [nm/mm]
Dx
Db
F
for the Acton spectrograph:
a = [1/1800] mm = 556 nm actual separation of the grating rulings
l = 400 nm  b = 35.4° from the grating equation nl = 2a sinq cosf
F = 300 mm
n=1
 Dl/Dx = 1.51 nm/mm
e.g. compute spectrograph entrance slit size for 0.1 nm spectral resolution at 400 nm
wavelength in the Acton spectrograph focal plane:
1.51 nm/mm dispersion  0.15 mm slit size = 150 µm
what spectrograph parameters determine the resolution of the instrument
Dl/Dx = [a cosb] / (Fn)
• size of entrance slit
• slit on our NASA instrument is about 100µm = 0.82 nm spectral
• can go to perhaps 10µm as smallest easily achievable size
• remember the amount of light going to the detector changes linearly
with size. Small slit  low light  low s/n
• big slits  poor spectral resolution
• big time tradeoff between these two items
• focal length of camera mirror
• longer focal length  higher dispersion, higher resolution, lower s/n
• Acton is 300mm, a nice size
• the OH spectrograph is 2m focal length – huge – with required spectral
resolution 0.0025 nm
• grating spectral order
• increasing “n” gives more resolution, but overlapping orders are problem
•  (e.g.) 1 x 400 nm is the same grating angle as 2 x 200 nm so both
200 nm radiation and 400nm radiation is falling simultaneously onto detector
• n = 1 gives no overlapping orders, but also the lowest resolution
• story about Elmo Brunner and OSO
• difraction angle – can’t make too big and fit in box
• grating groove density
• finest gratings are about 5000 g/mm
• fine gratings are extremely expensive
• normal gratings are inexpensively available with groove densities
of ~ 150 -2400 g/mm
The SCIENCE determines spectral resolution needed. Figure it out and see if
you can do the science with a real instrument.
Higher spectral resolution  fewer nm onto your detector  lose information
lower spectral resolution  more nm onto your detector, but poorer ability
to distinguish spectral features
types of detectors
silicon photodiod