REU Lecture

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REU Lecture
Spectroscopy and Instrument Design Erik
Richard Erik.richard_at_lasp,colorado.edu 303.735.662
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Outline

• Brief Review Nature of Light (Electromagnetic
  Radiation)
• Propagation of EM 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

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Nature of Light (Electromagnetic Radiation)

• Classical Definition Energy Propagating in the
  form of waves
• Many physical processes give rise to EM
  radiation including accelerating charged
  particles and emission by atoms and molecules.

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Electromagnetic Spectrum

• Velocity, frequency and wavelength are related
  cln?where
• c3x108 m/sec is the velocity in vacuum
• l and n are the wavelength and frequency
  respectively
• Electromagnetic radiation is typically classified
  by wavelength

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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 Ehnhc/l?
• Where hPlancks constant h6.63e-34 Joule hz-1
• A visible light photon (l 400 nm) has n7.5 x
  1014 hz and E4.97 x 10-19 J

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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 I290 W/m2/mm at
l1500 nm. Then the total irradiance in emitted
in the wavelength 1490 nm to 1510 nm is
TIDl?whereDl(1510-1490)nm10-3mm/nmDl20
10-3mm 2 10-2mmT5.80 W/m2\ 1mm10-6
m1nm10-9 m
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Atmospheric absorption of solar radiation
N2, O, O2
Solar FUV and MUV radiation is the primary
source of energy for earths upper atmosphere.
99 solar radiation penetrates to
the troposphere
Altitude (km)
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 WavelengthRange
from 1 to 15 ?m
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Nature of Light Photon Examples
Atoms and Molecules
Photoelectric Effect
Electron kinetic energy K.E.hn-W. W is the
work function (depth of the potential well) for
electrons in the surface. 1ev1.6x10-19J
The nature of the interaction depends on photon
wavelength (energy).
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Mg ion transitions
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A closer look at the Suns 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
Black body radiation
2 key points
Hot objects emit A LOT more radiation than cool
objects
I (W/m2) ????x T4
Spectral Radiance (W/m2/?m/sr)
The hotter the object, the shorter the peak
wavelength
T x ?max constant
Wavelength (?m)
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Solar Spectral Irradiance
SORCE Instruments measure total solar irradiance
and solar spectral irradiance in the 1 -2000 nm
wavelength range.
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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
  SunSolar 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.
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Functional Classes of Instruments
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Element of optical sensors characteristics
Sensor
Spectral bandwidth (?) Resolution (??) Out of
band rejection Polarization sensitivity Scattered
light
Detection accuracy Signal to noise Dynamic
range Quantization level Flat fielding Linearity
of sensitivity Noise equivalent power
Field of view Instan. Field of view Spectral band
registration Alignments MTFs Optical distortion
Spectral Characteristics
Radiometric Characteristics
Geometric Characteristics
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Reflection and refraction
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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-airthere is an
angle at which the light will not pass into the
other material and will be reflected at the
surface.
Using Snells law
Examples
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Total internal reflection
At angles gt critical angle, light undergoes total
internal reflection
It is common in laser experiments to use
roof-top prisms at 90 reflectors. (Notesurface
s are typically antireflection coated)
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Prism refraction
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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 _at_ 300 nm
• SNR ? 20000 _at_ 800 nm
• High wavelength precision
• 1.3 ?m knowledge in the focal plane
• (or ???? lt 150 ppm)
• In-flight re-calibration
• Prism transmission calibration
• Duty cycling 2 independent spectrometers

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SIM Prism in Littrow
Al coated Back surface
n
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SIM Measures the Full Solar Spectrum
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SIM Measurement Equation
Ideally,
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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
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SOLSTICE Grating Spectrometer

• SOLSTICE cleanly resolves the Mg II h k lines

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Diffraction grating fundamentals
Beam 2 travels a greater distance than beam 1
by (CD - AB) For constructive interference m??
(CD-AB) m is an integer called the diffraction
order CD dsin? AB -dsin? m?? 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
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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 ? that is
incident on a grating with angle a is diffracted
into a discrete number of angles bm that obey the
grating equation m.? d.(sin(?)sin(?m)). In
the special case that m0, a grating acts like a
plane mirror and ?-?
Blue (400 nm) and red (650 nm) light are
dispersed into orders m0,1, 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 ?s appearing at
diffraction angles of 20, 10, 0 and -10?
Wavelength (nm)
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Reflection Grating Geometry
Gratings work best in collimated light and
auxiliary optical elements are required to make a
complete instrument
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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.
Tilted mirrors1. Produce collimated light when
pf (qinfinity).2. Focus collimated light to a
spot with qf (pinfinity).
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Typical Plane Grating Monochromator Design
Grating spectrometer using two concave mirrors to
collimate and focus the spectrum
Entrance Slit
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.
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 ?. It is usually
expressed as the dimensionless quantity
Here ?? 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 ?1 and ?2 for
which the separation ?1 - ?2 lt ?? will be
ambiguous).
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Resolving Power

• Rayleighs 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 ? in order m is diffracted along the
same direction as light of wavelength ?/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 F?.
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Photomultiplier Tube Detectors
Single photon detection (pulse counting) with an
PMT
Output pulse
-1200 V
Ground

• 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 PMTs 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

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TIM Bolometer - Very sensitive heat balance
Fancy name Electrical Substitution Radiometer
Reference weight
Adjustable weight
Accurately known Aperture area (m2)
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

• 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 lgt220 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/Stellar Entrance Slits
Vacuum Door
Fold Mirror I
Sunshade
Solar Radiation
Solar Position Monitor
Slit Control
Door Control
Fold Mirror II
SPM Electronics
Filter 1 Mechanism Control
Filter 2 Mechanism Control
Elliptical Mirror
Filter 1 Mechanism
Filter 2 Mechanism
Optical Path
MUV Detector Electronics
Mirror Control
Solar/Stellar Exit Slits
MUV PMT
FUV PMT
FUV Detector Electronics
Control
Slit Control
GCI
Signal Path
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SOLSTICE Mechanism and Component Summary
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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 _at_ Suns surface
Then the Suns emission from the photosphere is
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TIM Design
Detector Head Board
Heat Sink
Vacuum Door
Shutter
Vacuum Shell
Light Baffles
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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 lt1 in 104
  years
• Stellar measurements provide
• Accurate in-flight instrument calibration
  tracking
• The basis for comparing SOLSTICE solar irradiance
  measurements with future work