REU Lecture
Spectroscopy and Instrument Design
Erik Richard
Erik.richard@lasp,colorado.edu
303.735.6629
Spectroscopy & Instruments – REU Lecture 2007
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Outline
•Brief Review: Nature of Light (Electromagnetic Radiation)
–Propagation of E&M waves
–Interaction with matter
– Irradiance definitions
–Wave-particle duality
• Brief Review: Optics Concepts
- Refraction Reflection
- Diffraction grating characteristics
–Imaging characteristics of lenses and mirrors
–Photomultiplier tube operation
•Instrument Design and Function
–Drawings
–Block Diagram
–Mechanisms
•Operational Modes
Spectroscopy & Instruments – REU Lecture 2007
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Nature of Light
(Electromagnetic Radiation)
Classical Definition: Energy Propagating in the form of waves
– Many physical processes give rise to E&M radiation including
accelerating charged particles and emission by atoms and
molecules.
Spectroscopy & Instruments – REU Lecture 2007
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Electromagnetic Spectrum
• Velocity, frequency and wavelength are related:
c=l*n where:
• c=3x108 m/sec is the velocity in vacuum
• l and n are the wavelength and frequency respectively
• Electromagnetic radiation is typically classified by
wavelength:
Spectroscopy & Instruments – REU Lecture 2007
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Nature of Light: Wave-Particle Duality
• Light behaves like a wave
– While propagating in free space (e.g. radio waves)
– On a macroscopic scale (e.g. while heating a thermometer)
– Demonstrates interference and diffraction effects
• Light behaves as a stream of particles (called photons)
– When it interacts with matter on a microscopic scale
– Is emitted or absorbed by atoms and molecules
• Photons:
– Travel at speed of light
– Possess energy: E=hn=hc/l
• Where h=Planck’s constant h=6.63e-34 Joule hz-1
• A visible light photon (l =400 nm) has n=7.5 x 1014 hz and E=4.97 x 10-19
J
Spectroscopy & Instruments – REU Lecture 2007
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Solar Spectral Irradiance
Irradiance: Power per unit area delivered by electromagnetic radiation (e.g.
Watts/square meter)
Spectral Irradiance: Power per unit area per wavelength interval delivered by
electromagnetic radiation (e.g. Watts per square meter per nanometer)
Ex: spectral irradiance is I=290
W/m2/mm at l=1500 nm. Then
the total irradiance in emitted in
the wavelength 1490 nm to 1510
nm is T=I*Dl where:
Dl=(1510-1490)nm*10-3mm/nm
Dl=20 *10-3mm = 2 *10-2mm
T=5.80 W/m2\
1mm=10-6 m
1nm=10-9 m
Spectroscopy & Instruments – REU Lecture 2007
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Atmospheric absorption
of solar radiation
N2, O, O2
~99% solar radiation
penetrates to the
troposphere
Altitude (km)
Solar FUV and MUV radiation is the primary
source of energy for earth’s upper atmosphere.
stratosphere
O3
troposphere
Altitude “contour” for attenuation by
a factor of 1/e
I(km) = 37% x Io
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Atmospheric Absorption in the Wavelength
Range from 1 to 15 mm
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Nature of Light: Photon Examples
Atoms and Molecules
Photoelectric Effect
The nature of the interaction depends
on photon wavelength (energy).
Electron kinetic energy: K.E.=hn-W.
W is the work function (depth of the
‘potential well’) for electrons in the
surface. 1ev=1.6x10-19J
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Mg ion transitions
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A closer look at the Sun’s spectrum
Note log-scale for irradiance
The hotter and higher layers produce complex EUV (10-120 nm) emissions
dominated by multiply ionized atoms with irradiances in excess of the
photospheric Planck distribution.
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“Radiation in equilibrium with matter”
Spectral Radiance (W/m2/mm/sr)
Black body radiation
Hot objects emit A LOT more
radiation than cool objects
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
I (W/m2) = x T4
The hotter the object, the
shorter the peak wavelength
T x lmax = constant
Wavelength (mm)
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Solar Spectral Irradiance
SORCE Instruments measure total solar irradiance and solar
spectral irradiance in the 1 -2000 nm wavelength range.
Spectroscopy & Instruments – REU Lecture 2007
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Solar Cycle Irradiance Variations
The FUV irradiance varies by ~ 10-100% but the MUV
irradiance varies by ~ 1-10% during an 11 year solar cycle.
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Solar variability across the spectrum
• Solar irradiance modulated by presence of magnetic structures on the
surface of the Sun……Solar Rotation (short) Solar Cycle (longer)
• The character of the variability is a strong function of wavelength.
Greatest absolute variability
occurs in mid visible
Greatest relative variability
occurs in the ultraviolet.
Spectroscopy & Instruments – REU Lecture 2007
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Functional Classes of Instruments
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Element of optical sensors characteristics
Sensor
Spectral Characteristics
Spectral bandwidth (l)
Resolution (Dl)
Out of band rejection
Polarization sensitivity
Scattered light
Spectroscopy & Instruments – REU Lecture 2007
Radiometric Characteristics
Detection accuracy
Signal to noise
Dynamic range
Quantization level
Flat fielding
Linearity of sensitivity
Noise equivalent power
Geometric Characteristics
Field of view
Instan. Field of view
Spectral band registration
Alignments
MTF’s
Optical distortion
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Reflection and refraction
refractive index 
speed of light in vacuum
speed of light in medium
Glass : n  1.52
Water : n  1.33
Air : n  1.000292
As measured with respect to the surface normal :
angle of incidence  angle of reflection
Snell ' s law :
n sin   n 'sin  '
Spectroscopy & Instruments – REU Lecture 2007
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Critical angle for refraction
An interesting thing happens when light is going from a material with higher index to
lower index, e.g. water-to-air or glass-to-air…there is an angle at which the light will
not pass into the other material and will be reflected at the surface.
Using Snell’s law:
n 'sin  '  n sin 
n
n
o
sin  c  sin 90 
n'
n'
Examples:
Water  to  air
 1 
 48.6
 1.33 
 c  sin 1 
Spectroscopy & Instruments – REU Lecture 2007
Glass  to  air
 1 
 41.1
 1.52 
 c  sin 1 
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Total internal reflection
At angles > critical angle, light
undergoes total internal reflection
It is common in laser experiments
to use “roof-top” prisms at 90°
reflectors.
(Note:surfaces are typically
antireflection coated)
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Prism refraction
sin 1 n sin  2
 
sin 1 n sin  2
  1   2  


1
1

 2

2

n
Spectroscopy & Instruments – REU Lecture 2007
n
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First issue: Optical transmission
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Second issue: Optical dispersion
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Spectral Irradiance Monitor SIM
•
•
•
•
•
•
Measure 2 absolute solar irradiance
spectra per day
Wide spectral coverage
– 200-2400 nm
High measurement accuracy
– Goal of 0.1% ( 1 )
High measurement precision
– SNR 500 @ 300 nm
– SNR 20000 @ 800 nm
High wavelength precision
– 1.3 m knowledge in the focal
plane
– (or
< 150 ppm)
In-flight re-calibration
– Prism transmission calibration
– Duty cycling 2 independent
spectrometers
Spectroscopy & Instruments – REU Lecture 2007
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SIM Prism in Littrow
Al coated
Back surface
n’
 sin 
2  sin 1 
 n'
Spectroscopy & Instruments – REU Lecture 2007

1  sin(   ) 

sin



n'
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SIM Measures the Full Solar Spectrum
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SIM Measurement Equation
Ideally,
El ( l s ) 
PD (ls )
A Dl
Spectroscopy & Instruments – REU Lecture 2007
(Wm 2 nm 1 )
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Instrument Block Diagram
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Solstice Instrument
The SOLar-STellar Irradiance Comparison Experiment consists of two identical
channels mounted to the SORCE Instrument Module on orthogonal axes. They each
measure solar and stellar spectral irradiances in the 115 - 320 nm wavelength range.
SOLSTICE Channels on the IM
SOLSTICE B
Single SOLSTICE Channel
SOLSTICE A
Spectroscopy & Instruments – REU Lecture 2007
- Dimensions: 88 x 40 x 19 cm
- Mass: 18 kg
- Electrical Interface: GCI Box
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SOLSTICE Grating Spectrometer
• SOLSTICE cleanly resolves
the Mg II h & k lines
Spectroscopy & Instruments – REU Lecture 2007
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Diffraction grating fundamentals
Beam 2 travels a greater
distance than beam 1 by
(CD - AB)
For constructive interference
ml= (CD-AB)
m is an integer called the
diffraction order
CD = dsin & AB = -dsin
ml= d(sin + sin)
Note: sign convention is “minus” when diffracted beam is on opposite side of grating
normal than incidence beam; “plus” when on same side
Spectroscopy & Instruments – REU Lecture 2007
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Diffraction grating fundamentals
Diffraction gratings use the interference pattern from a large number of equally spaced
parallel grooves to disperse light by wavelength.
Light with wavelength l that is incident on a grating with angle a is diffracted into a
discrete number of angles bm that obey the grating equation: m.l = d.(sin()+sin(m)).
In the special case that m=0, a grating acts like a plane mirror and =-
Blue (400 nm) and red (650 nm) light are
dispersed into orders m=0,±1, and ±2
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Grating example
Illuminate a grating with a blaze density of 1450 /mm With collimated
white light and a incidence angle of 48°, What are the l’s appearing at
diffraction angles of +20°, +10°, 0° and -10°?
1mm
6 nm
d
x 10
 689.7 nm
1450
mm
689.7nm
748.4
l
nm
sin 48  sin 20 
n
n
Wavelength (nm)
Spectroscopy & Instruments – REU Lecture 2007

n=1
n=2
n=3
20
748
374
249
10
632
316
211
0
513
256
171
-10
393
196
131
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Reflection Grating Geometry
Gratings work best in collimated light and auxiliary optical elements are
required to make a complete instrument
Plane waves, incident on the grating,
are diffracted into zero and first order
Rotating the grating causes the
diffraction angle to change
650 nm
l  d (sin( )  sin(  ))
400 nm
Zero order


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Auxiliary Optical Elements for Gratings
Lenses are often used as elements to collimate and reimage light
in a diffraction grating spectrometer.
Imaging geometry for a concave mirror.
Spectroscopy & Instruments – REU Lecture 2007
Tilted mirrors:
1. Produce collimated light when p=f
(q=infinity).
2. Focus collimated light to a spot with
q=f (p=infinity).
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Typical Plane Grating Monochromator Design
Grating spectrometer using two concave mirrors to
collimate and focus the spectrum
Only light that leaves the grating at the
correct angle will pass through the exit
slit. Tuning the grating through a small
angle counter clockwise will block the
red light and allow the blue light to reach
the detector.
Entrance Slit
Exit Slit
Detector
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Resolving Power
The resolving power R of a grating is a measure of its ability to
separate adjacent spectral lines of average wavelength λ. It is usually
expressed as the dimensionless quantity
l
R
 mN
Dl
Here ∆λ is the limit of resolution, the difference in wavelength between
two lines of equal intensity that can be distinguished (that is, the
peaks of two wavelengths λ1 and λ2 for which the separation |λ1 - λ2|
< ∆λ will be ambiguous).
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Resolving Power
• Rayleigh’s resolving limit
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Free spectral range
For a given set of incidence and diffraction angles, the grating equation is
satisfied for a different wavelength for each integral diffraction order m. Thus
light of several wavelengths (each in a different order) will be diffracted along
the same direction: light of wavelength λ in order m is diffracted along the same
direction as light of wavelength λ/2 in order 2m, etc.
The range of wavelengths in a given spectral order for which superposition
of light from adjacent orders does not occur is called the free spectral range
Fλ.
m 1
l1  Dl 
l1
m
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Photomultiplier Tube Detectors
Single photon detection (pulse counting) with an PMT
Output
pulse
Ground
-1200 V
•A photon enters the window and ejects an electron from the photocathode (photoelectric effect)
•The single photoelectron is accelerated through a 1200 volt potential down series of 10
dynodes (120 volts/dynode) producing a 106 electron pulse.
•The electron pulse is amplified and detected in a pulse-amplifier-discriminator circuit.
•Solstice uses two PMT’s in each channel that are optimized for a specified wavelength range
–CsTe (‘F’) Detector Photocathode) 170-320 nm
–CsI (‘G’) Detector Photocathode) 115-180 nm
Spectroscopy & Instruments – REU Lecture 2007
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TIM Bolometer - “Very sensitive heat balance”
Fancy name: Electrical Substitution Radiometer
Dr. George Lawrence
Adjustable
“weight”
Reference
“weight”
NiP “Black”
Accurately known
Aperture area (m2)
Spectroscopy & Instruments – REU Lecture 2007
Result: W/m2
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SOLSTICE: Science Objectives and Measurements
Science Objectives:
•
•
•
Measure solar irradiance from 115 to 320 nm daily with a
spectral resolution of 0.5 nm and an accuracy better than 5%
Monitor solar irradiance variation with an accuracy of 0.5%
during the 5 year SORCE mission
Establish the ratio of solar irradiance to the average flux of
an ensemble of bright, early-type stars with an accuracy of
0.5% for future studies of the long-term solar variability
Measurements:
Wavelength Coverage: 115-320 nm
Solar Spectral Resolution: 0.1 nm
Stellar Spectral Resolution: 1.1 nm
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SOLSTICE: Experiment Concept
Solar Observation: Modified Monk-Gillieson Spectrometer
Solar Exit Slit
Photomultiplier Detector
Diffraction
Grating
Camera Mirror
Entrance Slit
Stellar Observation: Objective Grating Spectrometer
Stellar Exit Slit
Camera Mirror
Photomultiplier Detector
Diffraction
Grating
Entrance Aperture
•
•
•
•
The optical configuration matches illumination areas on the detector.
Interchanging entrance slits and exit slits provides ~ 2x105 dynamic range.
Different stellar/solar integration times provide ~ 103 dynamic range.
A optical attenuator (a pair of neutral density filters), which can be measured in
flight, provides additional ~ 102 dynamic range in the F Mode for l>220 nm.
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SOLSTICE: Channel Assembly
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SOLSTICE: Channel Block Diagram
Grating
Encoder &
Control
Diffraction
Grating
Solar/Stellar
Radiation
Solar
Radiation
Sunshade
Vacuum
Door
Solar/Stellar
Entrance Slits
Fold Mirror I
Solar
Position
Monitor
Door
Control
Slit
Control
Fold Mirror II
SPM
Electronics
Elliptical
Mirror
Optical Path
Mirror
Control
Filter 1
Mechanism
Control
Filter 2
Mechanism
Control
Filter 1 &
Mechanism
Filter 2 &
Mechanism
MUV
PMT
MUV
Detector
Electronics
FUV
PMT
FUV
Detector
Electronics
Solar/Stellar
Exit Slits
Control
GCI
Signal Path
Spectroscopy & Instruments – REU Lecture 2007
Slit
Control
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SOLSTICE: Mechanism and Component Summary
Mechanism and Component Summary
Component
Type
Prototype
Gear Motor
Cycles
(5yr)
1
Entrance Slit
90ϋ Bi-Stable
27,500
Yes
Grating Drive
250,000
Yes
P()
Off Axis Ellipse
40ϋ Precision
Gimbal
8ϋ Bi-Stable
<<27,500
Yes
P()
Exit Slit Filter (2)
30ϋ Bi-Stable
110,000
Yes
Exit Slit
90ϋ Bi-Stable
27,500
Yes
Door
No
Detector Head
Yes
Optical Bench
and Cover
R: Required Test
P: Planned Test
Yes
Spectroscopy & Instruments – REU Lecture 2007
Prototype Te st Status
Vibration Thermal/Vac Life Te st
P()
P()
R()
P()
R()
P()
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SOLSTICE: Channel Assembly
‘A’ Channel During Preliminary Alignment Test
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SOLSTICE Channel Overview
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Instrument Type: Diffraction Grating Spectrometer
Wavelength Range: 115-320 nm
Wavelength Resolution: 0.1 nm, 1.1 nm
Detector: Photon Counting Photomultiplier Tubes
Absolute Accuracy: 5%
Relative Accuracy : 0.5%
Long-term Accuracy: 0.5%
Field of View: 0.75˚ calibrated, 1.5° total
Pointing Accuracy/Knowledge: 0.016˚/0.008˚
Mass: 18 kg
Dimensions: 88 x 40 x 19 cm
Orbit Average Power (w/GCI & heaters): 20.0 W
Orbit Average Data Rate: 0.902 kbits/s
Redundancy: 2 Redundant Channels
Heritage: UARS SOLSTICE
Pre-flight Cal. Std: NIST SURF-III
In-flight Cal.: Stars, Redundant Channels
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So, just how “bright” is the Sun?
If T = 5780 K @ Sun’s surface
Then the Sun’s emission from the photosphere is
I Sun   x T
4
ISun ~ 63,000,000 W/m2
(6.3 kW / cm2)
What does this mean for Earth?
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2
Surface area  4 R1AU
I Sun  R1AU 

I1AU  rSun 
2
Surface area  4 rSun
63 MW/m2 here
rSun  696, 000 km
How much
here?
R1AU  149, 600, 000 km
I@Earth  1360 W / m
2
Historically know as “Earth’s Solar Constant”
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2
TIM Design
Four
Radiometers
TIM Instrument
Detector
Head
Board
Heat Sink
Vacuum Door
Shutter
Precision Aperture
Spectroscopy & Instruments – REU Lecture 2007
Light Baffles
Radiometer
(Cone)
Vacuum Shell
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QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Spectroscopy & Instruments – REU Lecture 2007
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SOLSTICE Instrument Experiment Summary
• Two identical instrument channels meet the SORCE Mission lifetime requirement.
– Channel A primary wavelength range: 170-320 nm ( CsTe (‘F’) Detector Photocathode)
– Channel B primary wavelength range: 115-180 nm (CsI (‘G’) Detector Photocathode)
• Each channel covers both wavelength ranges for redundancy and cross calibration.
• Solar and stellar irradiance are measured with the same optical-detector chain.
• Accurate pre-flight calibration using the NASA beam line at the NIST Synchrotron
Ultraviolet Radiation Facility (SURF III)
• Precise measurements of solar and stellar irradiance of bright, early-type stars that,
according to stellar theory, vary by <1% in 104 years
• Stellar measurements provide
– Accurate in-flight instrument calibration tracking
– The basis for comparing SOLSTICE solar irradiance measurements with future work
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