Gaia: algorithms for the external...

Gaia: algorithms for the external calibration

Montegriffo P., Cacciari C., Ragaini S.

Thursday, February 18, 16

Roma & Teramo(Obs., ASDC)

Edinburgh (Royal Observatory)

Cambridge (Institute of Astronomy)(CU5 leadership)

Leiden (Observatory)

Bologna (INAF-OABO)

Barcelona(Universitat de Barcelona)

Bologna


Photometry Measurement Concept

Figures courtesy Anthony Brown

RP spectrum of M dwarf (V = 17.3 mag)Red box: data sent to ground

White contour: sky-background levelColour coding: signal intensity

During 5 years of mission each source is observed on average 80 times all over the

focal plane


Star motion in 10 s

Astrometric Field CCDs

Blue Photom

eter CC

Ds

Sky Mapper CCDs

Red Photom

eter CC

Ds

Radial-Velocity Spectrometer CCDs

Basic Angle

Monitor

Wave Front Sensor

Basic Angle

Monitor

Wave Front Sensor

Focal PlaneFigure courtesy Alex Short


Star motion in 10 s


Blue Photom

eter CC

Ds

Sky Mapper CCDs

Red Photom

eter CC

Ds


Basic Angle

Monitor

Wave Front Sensor

Basic Angle

Monitor

Wave Front Sensor

Focal PlaneFigure courtesy Alex Short

• PSF/LSF variation

BP - FoV Preceding - ROW7

-15 .0 -12 .5 -10 .0 - 7 . 5 - 5 . 0 - 2 . 5 0.0 2.5 5.0 7.5 10.0 12.5 15.0Sample position

0.001

0.01

0.1

LSF


-15 .0 -12 .5 -10 .0 - 7 . 5 - 5 . 0 - 2 . 5 0.0 2.5 5.0 7.5 10.0 12.5 15.0Sample position

0.001

0.01

0.1

LSF


Star motion in 10 s


Blue Photom

eter CC

Ds

Sky Mapper CCDs

Red Photom

eter CC

Ds


Basic Angle

Monitor

Wave Front Sensor

Basic Angle

Monitor

Wave Front Sensor

Focal Plane

• PSF/LSF variation• Dispersion & geometry


Star motion in 10 s


Blue Photom

eter CC

Ds

Sky Mapper CCDs

Red Photom

eter CC

Ds


Basic Angle

Monitor

Wave Front Sensor

Basic Angle

Monitor

Wave Front Sensor

Focal Plane

• PSF/LSF variation• Dispersion & geometry• Small scale (flat fields...)


Star motion in 10 s


Blue Photom

eter CC

Ds

Sky Mapper CCDs

Red Photom

eter CC

Ds


Basic Angle

Monitor

Wave Front Sensor

Basic Angle

Monitor

Wave Front Sensor

Focal Plane

• PSF/LSF variation• Dispersion & geometry• Small scale (flat fields...)• Background (stray-light)


Star motion in 10 s


Blue Photom

eter CC

Ds

Sky Mapper CCDs

Red Photom

eter CC

Ds


Basic Angle

Monitor

Wave Front Sensor

Basic Angle

Monitor

Wave Front Sensor

Focal Plane

• PSF/LSF variation• Dispersion & geometry• Small scale (flat fields...)• Background (stray-light)

Figure courtesy Giorgia Busso


Star motion in 10 s


Blue Photom

eter CC

Ds

Sky Mapper CCDs

Red Photom

eter CC

Ds


Basic Angle

Monitor

Wave Front Sensor

Basic Angle

Monitor

Wave Front Sensor

Focal Plane

• PSF/LSF variation• Dispersion & geometry• Small scale (flat fields...)• Background (stray-light)• Large scale response (QEs, FoVs, filter coating...)


Star motion in 10 s


Blue Photom

eter CC

Ds

Sky Mapper CCDs

Red Photom

eter CC

Ds


Basic Angle

Monitor

Wave Front Sensor

Basic Angle

Monitor

Wave Front Sensor

Focal Plane

• PSF/LSF variation• Dispersion & geometry• Small scale (flat fields...)• Background (stray-light)• Large scale response (QEs, FoVs, filter coating...)• Linearity (gates)• Flux loss• CTI mitigation• Decontamination• Deblending


Bologna, 18,19 February 2016 P. Montegriffo

Calibration strategy

• Internal calibrationThe goal is to provide an internally consistent flux scale all through the mission, across the focal plane, and for bright and faint sources. This is achieved by calibrating the relative variations of the instrument through the comparison of observations at different positions of the focal plane and different epochs for a set of reference sources.





• External calibrationThe aim of the external calibration is to determine the characteristics of the mean instrument by using a suitable number of spectro-photometric standard stars (SPSS) whose absolute spectral energy distributions (SEDs) are known with great accuracy from ground observations






•Purpose: - provide calibrated spectra in ‘physical units’






•Purpose: - provide calibrated spectra in ‘physical units’- give feedback to CU8 for AP classification






•Purpose: - provide calibrated spectra in ‘physical units’- give feedback to CU8 for AP classification

Predictions

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 5 5 6 0Sample position

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

flux

[pho

tons

/s/n

m]

Predictions

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 5 5 6 0Sample position

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

flux

[pho

tons

/s/n

m]



f(u) =Z 1

0L�(u + �(1/�, ⇣), ⇣) · R(�, ⇣) · s(�) d�

XP spectra formation

General formulation of the XP instrument:2

4u � samples� � wavelengths⇣ � ACfieldangle

3

5



f(u) =Z 1

0L�(u + �(1/�, ⇣), ⇣) · R(�, ⇣) · s(�) d�




3

5

Observation LSF Response SEDDispersion




-20 .0 -17 .5 -15 .0 -12 .5 -10 .0 - 7 . 5 - 5 . 0 - 2 . 5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0Sample position

0.001

0.01

0.1

LSF

f(u) =Z 1

0L�(u + �(1/�, ⇣), ⇣) · R(�, ⇣) · s(�) d�




3

5





-20 .0 -17 .5 -15 .0 -12 .5 -10 .0 - 7 . 5 - 5 . 0 - 2 . 5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0Sample position

0.001

0.01

0.1

LSF

f(u) =Z 1

0L�(u + �(1/�, ⇣), ⇣) · R(�, ⇣) · s(�) d�




3

5



-20 .0 -17 .5 -15 .0 -12 .5 -10 .0 - 7 . 5 - 5 . 0 - 2 . 5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0Sample position

0.001

0.01

0.1

LSF




-20 .0 -17 .5 -15 .0 -12 .5 -10 .0 - 7 . 5 - 5 . 0 - 2 . 5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0Sample position

0.001

0.01

0.1

LSF

f(u) =Z 1

0L�(u + �(1/�, ⇣), ⇣) · R(�, ⇣) · s(�) d�




3

5



-20 .0 -17 .5 -15 .0 -12 .5 -10 .0 - 7 . 5 - 5 . 0 - 2 . 5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0Sample position

0.001

0.01

0.1

LSF

!1 !2 !3




-20 .0 -17 .5 -15 .0 -12 .5 -10 .0 - 7 . 5 - 5 . 0 - 2 . 5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0Sample position

0.001

0.01

0.1

LSF

f(u) =Z 1

0L�(u + �(1/�, ⇣), ⇣) · R(�, ⇣) · s(�) d�




3

5



-20 .0 -17 .5 -15 .0 -12 .5 -10 .0 - 7 . 5 - 5 . 0 - 2 . 5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0Sample position

0.001

0.01

0.1

LSF

!1 !2 !3

Sample



XP instrument model

f(u) =Z 1

0L�(u + �(1/�)) · R(�) · s(�) d�



XP instrument model

f(u) =Z 1

0L�(u + �(1/�)) · R(�) · s(�) d�

...discretize

f(uj) =X

i

L�(uj + �(1/�i)) · R(�i) · s(�i) ��i



XP instrument model

�!f = I⇥�!s

f(u) =Z 1

0L�(u + �(1/�)) · R(�) · s(�) d�

...discretize

f(uj) =X

i

L�(uj + �(1/�i)) · R(�i) · s(�i) ��i



XP instrument model

�!f = I⇥�!s

f(u) =Z 1

0L�(u + �(1/�)) · R(�) · s(�) d�

...discretize

f(uj) =X

i

L�(uj + �(1/�i)) · R(�i) · s(�i) ��i

Calibrate s by solving a linear system of equations



�!s = B⇥�!b

Source SED model

s(�) =X

i

biBi(�)

•Express source SEDs as a linear combination of a suitable set of basis functions:

.... or in matrix notation

�!f = (I⇥B)⇥

�!b

External calibration means solve for SED shape parameters



Instrument update process

•Use SPSS to constraint instrument model I

�!f = I⇥�!s

•Each model component depends on a (small) number of adjustable parameters

f(u) =Z 1

0L�(u + �(1/�)) · R(�) · s(�) d�

L�(u) = H0(u, �) +nLX

i=1

hi Hi(u, �)

R(�) =nRX

j=0

rj Rj(�)

•Constrained solution: response and the effective LSFs as linear combinations of ad hoc basis functions

�(1/�) = d0 + d1 �0(1/�)



f 'Z 1

0f(u) du =

Z 1

0

Z 1

0L�(u + �(1/�)) du · R(�) · s(�) d�

Integrated photometry calibration



f 'Z 1

0f(u) du =

Z 1

0

Z 1

0L�(u + �(1/�)) du · R(�) · s(�) d�


f 'Z 1

0R(�) · s(�) d�



f 'Z 1

0f(u) du =

Z 1

0

Z 1

0L�(u + �(1/�)) du · R(�) · s(�) d�


•External calibration of integrated G, GBP, GRP photometry achieved by fitting the actual shape of the passband through SPSS usage

•Only a zeropoint is needed (no color terms) to link to the absolute flux scale

f 'Z 1

0R(�) · s(�) d�


Schedule

ProposalConcept & Technology Study

Mission Selection

Re-Assessment StudyPhase B1

Scientific operation

Launch December 2013

Final

Studies

Data Processing

Implementation

Data Processing

Definition

Operation

Mission ProductsIntermediate

Selection of Prime Contractor (EADS Astrium SAS)

Phase B2Phase C/D

Software Development (DPAC)

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2021

TodayFigure courtesy Michael Perryman and François Mignard

2022


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