Post on 17-Jan-2016
Instrument Performance Prediction 1
PACS SVR 22/23 June 2006
PACS Instrument Model andPerformance Prediction
A. Poglitsch
Instrument Performance Prediction 2
PACS SVR 22/23 June 2006
PACS Instrument Model
• Purpose of instrument model: provide best guess of in-orbit performance based on existing knowledge of the instrument and its subunits and the satellite– Incomplete knowledge about FM subunit / instrument /
system performance– Some knowledge of QM instrument performance, but with
degraded subunit and OGSE performance • Instrument model is a “living document” that has been
maintained since the early design phases of PACS and updated whenever new test results became available
• Parameters lacking experimental values have been assigned calculated or estimated values
• PACS instrument model is not a deliverable document (but has been used regularly as a reference for preparation and evaluation of tests and their results)
Instrument Performance Prediction 3
PACS SVR 22/23 June 2006
Spectrometer ModelModel strategy• Determine the background power reaching an individual
detector (pixel) and determine the NEP of an individual detector under this background (photon noise + detector/readout noise)
• Determine the coupling of an astronomical (point) source to the detector array– Telescope PSF– Vignetting / diffraction in instrument– Transmission of optical elements (mirrors, filters, grating)– Detector (quantum) efficiency– Effective number of pixels (spatial/spectral) needed for optimum
source/line extraction and resulting total noise and fraction of detected source flux
• Combine above results to calculate “raw” noise referred to sky
• Add overheads created by need for background subtraction (AOT-dependent)
Instrument Performance Prediction 4
PACS SVR 22/23 June 2006
Detector Performance• Relative spectral response
within modules• Relative photometric
response within modules• Absolute photometric peak
responsivity (moduleaverage)
0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200 250 300
wavelength [m]
rela
tive
spec
tral r
espo
nsiv
ity
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10 12 14 16
pixel number
rela
tive
re
spo
nsi
vity
Mean Responsivity of FM HS Detector Modules for Bias=70mV
0
10
20
30
40
50
60
70
80
FM
15
3
FM
15
8
FM
15
9
FM
16
0
FM
16
1
FM
16
2
FM
16
4
FM
16
5
FM
16
8
FM
16
9
FM
17
0
FM
17
1
FM
17
3
FM
17
4
FM
17
5
FM
17
9
FM
18
0
FM
18
1
FM
18
3
FM
18
4
FM
18
6
FM
18
7
FM
18
8
FM
18
9
FM
19
0
r [A
/W]
Instrument Performance Prediction 5
PACS SVR 22/23 June 2006
HS Detector Performance Model
• Observed noise consistent with photon noise + CRE noise (input-equivalent current noise density)
• Peak QE=0.26; QE() follows directly from rel. spectral responsivity• Average CRE noise 3.7x10-16 A/√Hz, average peak responsivity 45 A/W
b 20 80
20 30 40 50 60 70 800
0.5
1
1.5
2
2.5
bias [mV]
signal
4.7x10-15 W2.9x10-15 W
Circles: measured noiseDashed line: CRE noiseSquares: CRE noise subtr.Solid lines: combined modelDotted lines: background noise
b 20 80
20 30 40 50 60 70 800
0.002
0.004
0.006
0.008
0.01
0.012
noise
bias [mV]
Instrument Performance Prediction 6
PACS SVR 22/23 June 2006
LS Detector Model
• No reliable measurement of peak QE available; assumed to be the same as for HS detectors (by design)– Quite limited consequence for system performance –
CRE noise and responsivity dominate (see below)
• CRE noise same as for HS detector• Average peak responsivity 10…12 A/W
Instrument Performance Prediction 7
PACS SVR 22/23 June 2006
Spectrometer Model: Background Basics
BGP T em t( )2 h 3
c2
em t1
e
h
k T1
BGNEP T em t ( )2
h 2
c
2 em t
1
e
h
k T1
1 em t1
e
h
k T1
: etendue of optical train (conserved by optics, except for detector light cones): optical frequency: detected optical bandwidth (around given frequency)T: temperature of emitterem: emissivity of emittert: transmission of all optics between respective emitter and detector: detector quantum efficiency
(Calculated for groups/ elements along optical train from telescope to detector)
Instrument Performance Prediction 8
PACS SVR 22/23 June 2006
Spectrometer Model: Background Etendue
1.652
9.4
3600
180
2
(3.3m eff. diameter, 9.4"x9.4" pixels)
throughout optical train, for all contributions from outside of the 5K environment [deviations are lumped into effective emissivities]
cold := 4
correcting for the effective cone acceptance angle seeing the 5K optics
Instrument Performance Prediction 9
PACS SVR 22/23 June 2006
Spectrometer Model: Background/Straylight Temperatures and
Emissivities
Telescope
Baffle Shield Cold optics
T 80 K 60 K 23 K 5.5 K
4% 1% 1% 15%
• PACS-external contributions dominant
Instrument Performance Prediction 10
PACS SVR 22/23 June 2006
Spectrometer Model: Background/Straylight Transmission to
Detector and Bandwidth
Wavelength [µm] Wavelength [µm]B
ack
gro
und o
pti
cal
bandw
idth
/pix
el [H
z]
Tele
scope b
ack
gro
und
transm
issi
on
• Bandwidth same for all background contributions except 5K post-grating (which is negligible)
• Effective transmission for background contributions varies slightly (pupil aperture sizes etc.)
Instrument Performance Prediction 11
PACS SVR 22/23 June 2006
Spectrometer Model: Transmission Breakdown
Wavelength [µm] Wavelength [µm]
Filt
er
transm
issi
on
• Filter transmission based on RT FTS measurements of FM filters
• Dichroic will be replaced before ILT
Calc
ula
ted g
rati
ng e
ffici
ency
Instrument Performance Prediction 12
PACS SVR 22/23 June 2006
Spectrometer Model: Transmission Breakdown
Wavelength [µm]
Calc
ula
ted s
licer
effi
ciency • Slicer efficiency: vignetting
of diffraction side lobes by optics
• Additional transmission factors– Lyot stop efficiency: 0.9
(diffraction by field stop plus telescope/instrument alignment tolerances)
– Mirror train: 0.85(dissipation, scattering)
Instrument Performance Prediction 13
PACS SVR 22/23 June 2006
Spectrometer Model: Additional Optical Efficiencies Relevant for Source Coupling
Wavelength [µm] Wavelength [µm]
• Telescope efficiency: fraction of power received from point source measured in central peak of PSF
• Pixel efficiency: inverse number of pixels (spatial/spectral) needed to retrieve power of unresolved spectral line in central peak of PSF
Est
imate
d t
ele
scope m
ain
beam
effi
ciency
(diff
ract
ion +
WFE
)
Calc
ula
ted p
ixel effi
ciency
Instrument Performance Prediction 14
PACS SVR 22/23 June 2006
Background Power and BLIP Noise per Pixel
• HS detector QE based on measurement (peak QE + relative spectral responsivity)
• LS detector QE based on assumed, same peak QE + relative spectral responsivity measurement
Back
gro
und p
ow
er
[W]
BLI
P N
EP [
W/√
Hz]QE
Wavelength [µm] Wavelength [µm] Wavelength [µm]
Instrument Performance Prediction 15
PACS SVR 22/23 June 2006
BLIP Noise vs. Readout Noise
NEI [A/√Hz] = NEP [W/√Hz] x responsivity [A/W]
NEI [A
/√H
z]
Wavelength [µm]
• BLIPNEP converted to electrical noise (solid lines)
• Readout noise (dashed line)• BLIPNEP (and, therefore,
QE) significant/dominant noise source in “red” band (1st order, HS)
• BLIPNEP (and, therefore, QE) not dominant noise source – readout noise and responsivity relevant
Instrument Performance Prediction 16
PACS SVR 22/23 June 2006Total Noise at Detector and Coupling to
Sky
Wavelength [µm]
• Total NEP at detector: BLIP NEP and electrical readout-noise equivalent power
• Coupling correction: inverse of all optical efficiencies; factor 2 forbackground subtraction; chopper duty-cycle of ≥0.8
Wavelength [µm]
Tota
l N
EP [
W/√
Hz]
Couplin
g c
orr
ect
ion
Instrument Performance Prediction 17
PACS SVR 22/23 June 2006
Predicted Sensitivity
• Calculated for (off-array) chopping• Wavelength switching could have advantage (spectral line
always within instantaneous coverage) or disadvantage (off-line switching and likely need for off-position observation)
Wavelength [µm]Poin
t so
urc
e c
onti
nuum
sensi
tivit
yper
spect
ral re
solu
tion e
lem
ent
[Jy]
(5,
1h)
Wavelength [µm]
Poin
t so
urc
e lin
e s
ensi
tivit
y[W
/m2]
(5,
1h)
Instrument Performance Prediction 18
PACS SVR 22/23 June 2006
Operation/Performance under p+ Irradiation
Simulated chopped observation with one ramp/chopper plateau.For each bias value, 5 ramp lengths tested: 1s, 1/2 s, 1/4 s, 1/8s, 1/16 s.The detector was in its high responsivity plateau, ~2 hours after the last curing.
Instrument model value,based on lab measurementswithout irradiation
NEP as a function of detector/readout setting
• With optimum bias setting (lower than in lab!) and ramp length / chopping parameters, NEP close to lab values possible in space
• Curing may be necessary only after solar flare, or once per day (self- curing under telescope IR background sufficient)
Instrument Performance Prediction 19
PACS SVR 22/23 June 2006
Spectral Resolving Power
• Simple calculation, requiring some fine tuning– Pixel sampling– Exact grating
illumination(physical optics)
Wavelength [µm]
Reso
lvin
g p
ow
er
Instrument Performance Prediction 20
PACS SVR 22/23 June 2006
Main Limitations of Spectrometer Model
• No “systematics”/ higher-order effects and their implications for AOTs considered– no real instrument simulator
• No end-to-end test of instrument in representative high-energy radiation environment
• Limited feed-back from QM ILT– Serious uncertainty about detector responsivity makes
evaluation of instrument optical efficiency difficult– Defocus, low transmission and high/inhomogeneous
window emissivity have hampered PSF determination– Lack of laser source (or adequate gas cell) – no
unresolved, strong lines available
Instrument Performance Prediction 21
PACS SVR 22/23 June 2006
Photometer Model
Model strategy• Determine the background power reaching an individual
detector (pixel) and determine the NEP of an individual detector under this background (photon noise + detector/readout noise)
• Determine the coupling of an astronomical (point) source to the detector array– Telescope PSF– Vignetting / diffraction in instrument– Transmission of optical elements (mirrors, filters)– Detector (quantum) efficiency– Effective number of pixels needed for optimum source
extraction and resulting total noise and fraction of detected source flux
• Combine above results to calculate “raw” noise referred to sky
• Add overheads created by need for background subtraction (AOT-dependent)
Instrument Performance Prediction 22
PACS SVR 22/23 June 2006
Bolometer Performance• Pixel yield ~98%• NEP “blue” ~1.7...2 x nominal
– Small variation with BG power– But 1/f noise– Best NEP only for fast
modulation (chopping/ scanning)
• NEP “red” slightly higher
“blue” BFP
NEP vs.backgroundpower
Instrument Performance Prediction 23
PACS SVR 22/23 June 2006
Photometer Model: Background Basics
BGP T em t( )2 h 3
c2
em t1
e
h
k T1
: etendue of optical train (conserved by optics, except for detector light cones): optical frequency: detected optical bandwidth (around given frequency)T: temperature of emitterem: emissivity of emittert: transmission of all optics between respective emitter and detector: detector quantum efficiency
(Calculated for groups/ elements along optical train from telescope to detector)
Crude approximation for largebandwidth of photometer!(But it doesn’t matter.)
BGNEP T em t ( )2
h
c
2 em t
1
e
h
k T1
1 em t1
e
h
k T1
Instrument Performance Prediction 24
PACS SVR 22/23 June 2006
Photometer Model: Background Etendue
throughout optical train, for all contributions from outside of the 5K environment [deviations are lumped into effective emissivities]
cold := 10
correcting for the effective detector/baffle acceptance cone seeing the 5K optics
1.652
6.4
3600
180
2
1.652
3.2
3600
180
2
“Red” Photometer: 6.4”□ pixels “Blue” Photometer: 3.2”□ pixels
Instrument Performance Prediction 25
PACS SVR 22/23 June 2006
Photometer Model: Background/Straylight Temperatures and Emissivities
Telescope
Baffle Shield Cold optics
T 80 K 60 K 23 K 5.5 K
4% 1% 1% 15%
• PACS-external contributions dominant
Instrument Performance Prediction 26
PACS SVR 22/23 June 2006
Photometer Model: Background/Straylight Transmission to Detector and Bandwidth
• Bandwidth same for all background contributions except 5K post-grating (which is negligible)
• Filter transmission based on RT FTS measurements of FM filters• Dichroic will be replaced before ILT
LyotStop
MirrorTrain
FilterChain
Total
“Red”130-
210µm
0.9 0.85 0.5 0.383 2.2
“Green”
85-130µm
0.9 0.85 0.45 0.344 2.4
“Blue”60-85µm
0.9 0.85 0.35 0.268 2.9
Instrument Performance Prediction 27
PACS SVR 22/23 June 2006
Photometer Background Power andNoise per Pixel
• QE assumed to be 0.8 (from bolometer absorber structure reflectivity measurement) for BLIPNEP
• Realisation of “measured NEP” requires modulation near 3 Hz (1/f noise)
BG[pW]
BLIPNEP[W/√Hz]
Measured NEP
[W/√Hz]
“Red”130-210µm
5.2 1.24 x 10-16 3 x 10-16 tbc
“Green”85-130µm
2.9 1.15 x 10-16 2.5 x 10-16
“Blue”60-85µm
3.4 1.54 x 10-16 2.5 x 10-16
Instrument Performance Prediction 28
PACS SVR 22/23 June 2006
Photometer Model: Additional Optical Efficiencies Relevant for Source Coupling
• Telescope efficiency: fraction of power received from point source measured in central peak of PSF
• Pixel efficiency: inverse number of pixels needed to retrieve power in central peak of PSF
eff_tel eff_pix
“Red”130-
210µm
0.77 0.181
“Green”
85-130µm
0.73 0.113
“Blue”60-85µm
0.64 0.162
Instrument Performance Prediction 29
PACS SVR 22/23 June 2006
Total Coupling to Sky
• Coupling correction– inverse of all optical
efficiencies– factor 2 for
background subtraction
– chopper duty-cycleof ≥0.8
Coup_corr
“Red”130-210µm
17.7
“Green”85-130µm
26.3
“Blue”60-85µm
32.2
Instrument Performance Prediction 30
PACS SVR 22/23 June 2006
Predicted Sensitivity
• Point source sensitivity equivalent to mapping speed of ~10’ x 10’ in 1 day
on-array chopping/line scanning
off-position chopping
wavelength [µm]
[mJy
] (p
oin
t so
urc
e;
5/
1h
)
50 100 150 200
5
4
3
2
1
0 0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
50 100 150 200 250
PhotometricBands
wavelength [µm]
filt
er
transm
issi
on
Instrument Performance Prediction 31
PACS SVR 22/23 June 2006
Main Limitations of Photometer Model
• No “systematics”/ higher-order effects and their implications for AOTs considered– no real instrument simulator
• Origin of 1/f noise not clear– Is it driven thermally?– Will operation in PACS cryostat be representative for
in-orbit Herschel cryostat thermal (in)stability?
• Limited feed-back from QM ILT– Serious uncertainty about detector responsivity
makes evaluation of instrument optical efficiency difficult
– Defocus, low transmission and high/inhomogeneous window emissivity have hampered PSF determination