PACS Instrument Model and Performance Prediction

Instrument Performance Prediction 1

PACS SVR 22/23 June 2006

PACS Instrument Model andPerformance Prediction

A. Poglitsch



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)



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)



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]



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]



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



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)



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



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



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.)



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



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)



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



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]



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


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



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)



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)



Spectral Resolving Power

• Simple calculation, requiring some fine tuning– Pixel sampling– Exact grating

illumination(physical optics)

Wavelength [µm]

Reso

lvin

g p

ow

er



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



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)



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



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



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



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



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



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



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



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



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



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

PACS Instrument Model and Performance Prediction

Documents

Transcript of PACS Instrument Model and Performance Prediction