Byron Smiley*, Jerry Harder, Juan Fontenla, George Lawrence, and Gary Rottman
*smileyb@lasp.colorado.edu
The measurement equation for spectral irradiance, ελ ,
contains the prism transmission:
IR photodiode (InGaAs)
Féry
prism
Visible2 photodiode (silicon p-n)
Visible1 photodiode (silicon n-p)
A Féry prism made of Suprasil 300
disperses sunlight onto detectors in the
focal plane of the SIM instrument.
Laboratory for Atmospheric and Space Physics (LASP)
Boulder, Colorado
80303-7814
USA
2
ελ
Electrical Substitution Radiometer (ESR)
1
ó
f ⋅ L ⋅ ô τ prism( λ ) ⋅τ HRT( λ ) ⋅Φ Diff ( λ ) ⋅ W y ( λ ) − y set ⋅ α ( λ ) dλ
ô
õ
(
⋅
)
V 7 ⋅R H
(
)2
M⋅ R s + R H
æ 1 + G Z H ö p ⋅D
⋅
÷⋅
è G Z R ø Λp ⋅ Q
⋅ç
The Prism Transmission Is Monitored On-Orbit
One SIM uses a periscope, mirror, and detector to monitor the prism transmission of the other SIM.
sunlight
Thus, the prism transmission vs. wavelength must be
known pre-launch and monitored on-orbit.
UV photodiode (silicon n-p)
(curvature exaggerated)
The first SIM acts as a
monochromator, directing
one wavelength into the
periscope.
The Prism Transmission Is Measured Pre-Launch
Experimental Setup
UV configuration was used from 200 to 1000 nm
p polarization
1 meter focal
length
monochromator
beam target
ensures that
angle of
incidence is
symmetric
reference intensity
transmitted intensity
The transmission coefficient is transmitted/reference,
measured repeatedly to check for stability.
raw transmission data
bandpass
filter
Xe arc
lamp
CaF2
lens
transmission coefficient (fraction)
0.85
0.8
0.75
0.7
Focal Plane
0.6
0.5
0.4
400 groove/mm
IR grating
200
250
300
350
400
450
500
550
600
wavelength (nm)
650
700
750
800
850
900
950
FEL
lamp
detector
0.05
0.04
Silicon n-on-p
photodiode
Sample of
raw data:
50
100
150
all data points
best reference set
best transmitted set
200
250
300
350
400
450
500
8 Hz data sample rate, 5 sec intervals
The data set with the lowest noise is used.
entrance
slit
flight witness
Féry prism
slit image on
focal plane
lock-in amplifier*
*Phase sensitive detection established a signal over 103
times the dark signal. Long time constants ( ~300 ms) help
smooth the noise.
The detector for the UV measurements was a silicon
photodiode. For the IR, it was a PbS photoconductor
thermoelectrically cooled to –20°C.
p polarization
0.8
Preliminary time series reveal degradation, or the fractional decrease of transmission over time.
1.02
0.7
s polarization
0.6
0.5
0.4
700
orientation of the
two polarizations
Dark counts are
also recorded.
time (minutes)
800
900
1000
1100
1200
1300
UV measurements, p polarization
UV measurements, s polarization
IR measurements, p polarization
uncertainty in p transmission
IR measurements, s polarization
uncertainty in s transmission
1400
1500
1600
1700 1800 1900 2000
wavelength (nm)
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
on-orbit degradation fraction
0
IR configuration was used from 0.85 to 3.0 µm
turntable
with detector
transmission coefficient (fraction)
0.03
chopper
wheel*
To Prism B
Focusing
Beamsplitter
InGaAs
Photodiode
0.08
0.06
To Periscope B
To Prism A
To Prism B
1000
Data was taken in 5 nm increments from 200 to 400 nm, then every 10 nm until 1000 nm. Uncertainties
were less than 300 ppm (1σ), smaller than the plotting symbols above.
0.07
From Prism A
To Periscope B
Entrance
Slit B
UV measurements, horizontal (P) polarization
uncertainty in horizontal (P) transmission
UV measurements, vertical (S) polarization
uncertainty in vertical (S) transmission
IR measurements, horizontal (P) polarization
IR measurements, vertical (S) polarization
MgF2 rochon
(polarizer)
From Periscope
Spectrometer A
From Periscope
Spectrometer A
Periscope
s polarization
Next, the mirror rotates, intercepting the transmitted beam
from the prism (red ray). Also, the prism under test turns,
sweeping the transmitted beam across the mirror.
Folding Mirror
TurnTable
0.65
0.55
SIM B
The reference intensity is measured when the mirror reflects
light directly into the photodiodes. (red ray)
0.45
1200 groove/mm
UV grating
SIM A
diode signal (volts)
The detector turns to measure transmission
Prism A degradation
1
325 nm
0.98
0.96
0.94
0.92
Preliminary data shows that the prism transmission
drops above 2.5 microns.
60
70
80
90
100
110
120
130
140
degradation data points
uncertainty in degradation fraction (1 sigma)
interpolated degradation curve
150
160
170
180
190
200
210
day of year 2003
220
230
240
250
260
270
280
290
300
310
320
330