Gaia: algorithms for the external calibration

Gaia: algorithms for the external calibration Montegriffo P., Cacciari C., Ragaini S.

Edinburgh (Royal Observatory) Cambridge (Institute of Astronomy) (CU5 leadership) Leiden (Observatory) Bologna (INAF-OABO) Bologna Roma & Teramo (Obs., ASDC) Barcelona (Universitat de Barcelona)

Photometry Measurement Concept RP spectrum of M dwarf (V = 17.3 mag) Red box: data sent to ground White contour: sky-background level Colour coding: signal intensity During 5 years of mission each source is observed on average 8 times all over the focal plane Figures courtesy Anthony Brown

Figure courtesy Alex Short Focal Plane Wave Front Sensor Wave Front Sensor Blue Photometer CCDs Red Photometer CCDs Radial-Velocity Spectrometer CCDs Basic Angle Monitor Basic Angle Monitor Star motion in 1 s Sky Mapper CCDs Astrometric Field CCDs

Focal Plane Figure courtesy Alex Short BP - FoV Preceding - ROW1 PSF/LSF variation.1 LSF.1 Wave Front Sensor Wave Front Sensor Blue Photometer CCDs.1 Red Photometer CCDs -15. -12.5-1. -7.5-5. -2.5. 2.5 5. 7.5 1. 12.5 15. Sample position BP - FoV Preceding - ROW7 Radial-Velocity Spectrometer CCDs Basic Angle Monitor.1 Basic Angle Monitor LSF.1 Star motion in 1 s Sky Mapper CCDs Astrometric Field CCDs.1-15. -12.5-1. -7.5-5. -2.5. 2.5 5. 7.5 1. 12.5 15. Sample position

Focal Plane PSF/LSF variation Dispersion & geometry Wave Front Sensor Wave Front Sensor Blue Photometer CCDs Red Photometer CCDs Radial-Velocity Spectrometer CCDs Basic Angle Monitor Basic Angle Monitor Star motion in 1 s Sky Mapper CCDs Astrometric Field CCDs

Focal Plane PSF/LSF variation Dispersion & geometry Small scale (flat fields...) Wave Front Sensor Wave Front Sensor Blue Photometer CCDs Red Photometer CCDs Radial-Velocity Spectrometer CCDs Basic Angle Monitor Basic Angle Monitor Star motion in 1 s Sky Mapper CCDs Astrometric Field CCDs

Focal Plane PSF/LSF variation Dispersion & geometry Small scale (flat fields...) Wave Front Sensor Background (stray-light) Wave Front Sensor Blue Photometer CCDs Red Photometer CCDs Radial-Velocity Spectrometer CCDs Basic Angle Monitor Basic Angle Monitor Star motion in 1 s Sky Mapper CCDs Astrometric Field CCDs

Focal Plane PSF/LSF variation Dispersion & geometry Small scale (flat fields...) Wave Front Sensor Background (stray-light) Wave Front Sensor Large scale response (QEs, FoVs, filter coating...) Basic Angle Monitor Blue Photometer CCDs Red Photometer CCDs Radial-Velocity Spectrometer CCDs Basic Angle Monitor Star motion in 1 s Sky Mapper CCDs Astrometric Field CCDs

Focal Plane PSF/LSF variation Dispersion & geometry Small scale (flat fields...) Wave Front Sensor Background (stray-light) Wave Front Sensor Large scale response (QEs, FoVs, filter coating...) Linearity (gates) Flux loss Basic Angle Monitor CTI mitigation Basic Angle Monitor Decontamination Deblending Sky Mapper CCDs Astrometric Field CCDs Blue Photometer CCDs Red Photometer CCDs Radial-Velocity Spectrometer CCDs Star motion in 1 s

Calibration strategy Internal calibration The 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 calibration The aim of the external calibration is to determine the characteristics of the mean instrument by using a suitable number of spectrophotometric standard stars (SPSS) whose absolute spectral energy distributions (SEDs) are known with great accuracy from ground observations Bologna, 18,19 February 216

Calibration strategy Internal calibration 14, The goal is to provide an internally consistent flux scale all through 13, 12, the mission, across the focal plane, and for bright and faint sources. 11, 1, This 9, is achieved by calibrating the relative variations of the 8, 7, instrument through the comparison of observations at different 6, 5, positions of the focal plane and different epochs for a set of reference 4, 3, sources. 2, flux [photons/s/nm] 1, 5 1 15 2 Predictions 25 3 35 4 45 5 55 6 Sample position 14, 13, External calibration 12, 11, The aim of the external calibration is to determine the characteristics 1, 9, of the 8, mean instrument by using a suitable number of spectrophotometric standard stars (SPSS) whose absolute spectral energy 7, 6, 5, 4, distributions (SEDs) are known with great accuracy from ground 3, 2, observations 1, flux [photons/s/nm] 5 1 15 2 25 3 35 4 45 5 55 6 Sample position Purpose: - provide calibrated spectra in physical units - give feedback to CU8 for AP classification Bologna, 18,19 February 216

XP spectra formation General formulation of the XP instrument: f(u) = L (u + (1/, ), ) R(, ) s( ) d 2 u 4 3 samples wavelengths 5 ACf ieldangle Bologna, 18,19 February 216

XP spectra formation General formulation of the XP instrument: f(u) = L (u + (1/, ), ) R(, ) s( ) d 2 u 4 3 samples wavelengths 5 ACf ieldangle Observation LSF Dispersion Response SED Bologna, 18,19 February 216

XP spectra formation General formulation of the XP instrument: f(u) = L (u + (1/, ), ) R(, ) s( ) d 2 u 4 3 samples wavelengths 5 ACf ieldangle Observation LSF Dispersion Response SED BP - FoV Preceding - ROW4.1 LSF.1.1-2. -17.5-15. -12.5-1. -7.5-5. -2.5. 2.5 5. 7.5 1. 12.5 15. 17.5 2. Sample position Bologna, 18,19 February 216

XP spectra formation General formulation of the XP instrument: f(u) = L (u + (1/, ), ) R(, ) s( ) d 2 u 4 3 samples wavelengths 5 ACf ieldangle Observation LSF Dispersion Response SED BP - FoV Preceding - ROW4 BP - FoV Preceding - ROW4.1.1 LSF.1 LSF.1.1.1-2. -17.5-15. -12.5-1. -7.5-5. -2.5. 2.5 5. 7.5 1. 12.5 15. 17.5 2. Sample position -2. -17.5-15. -12.5-1. -7.5-5. -2.5. 2.5 5. 7.5 1. 12.5 15. 17.5 2. Sample position Bologna, 18,19 February 216

XP spectra formation General formulation of the XP instrument: f(u) = L (u + (1/, ), ) R(, ) s( ) d 2 u 4 3 samples wavelengths 5 ACf ieldangle Observation LSF Dispersion Response SED BP - FoV Preceding - ROW4 BP - FoV Preceding - ROW4.1.1 LSF.1 LSF.1.1.1-2. -17.5-15. -12.5-1. -7.5-5. -2.5. 2.5 5. 7.5 1. 12.5 15. 17.5 2. Sample position -2. -17.5-15. -12.5-1. -7.5-5. -2.5. 2.5 5. 7.5 1. 12.5 15. 17.5 2. Sample position Γ1 Γ2 Γ3 Bologna, 18,19 February 216

XP spectra formation General formulation of the XP instrument: f(u) = L (u + (1/, ), ) R(, ) s( ) d 2 u 4 3 samples wavelengths 5 ACf ieldangle Observation LSF Dispersion Response SED Sample BP - FoV Preceding - ROW4 BP - FoV Preceding - ROW4.1.1 LSF.1 LSF.1.1.1-2. -17.5-15. -12.5-1. -7.5-5. -2.5. 2.5 5. 7.5 1. 12.5 15. 17.5 2. Sample position -2. -17.5-15. -12.5-1. -7.5-5. -2.5. 2.5 5. 7.5 1. 12.5 15. 17.5 2. Sample position Γ1 Γ2 Γ3 Bologna, 18,19 February 216

XP instrument model f(u) = L (u + (1/ )) R( ) s( ) d Bologna, 18,19 February 216

XP instrument model f(u) = L (u + (1/ )) R( ) s( ) d...discretize f(u j )= X i L (u j + (1/ i )) R( i ) s( i ) i Bologna, 18,19 February 216

XP instrument model f(u) = L (u + (1/ )) R( ) s( ) d...discretize f(u j )= X i L (u j + (1/ i )) R( i ) s( i ) i! f = I! s Bologna, 18,19 February 216

XP instrument model f(u) = L (u + (1/ )) R( ) s( ) d...discretize f(u j )= X i L (u j + (1/ i )) R( i ) s( i ) i! f = I! s Calibrate s by solving a linear system of equations Bologna, 18,19 February 216

Source SED model Express source SEDs as a linear combination of a suitable set of basis functions: s( )= X i b i B i ( )... or in matrix notation! s = B! b! f =(I B)! b External calibration means solve for SED shape parameters Bologna, 18,19 February 216

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) = L (u + (1/ )) R( ) s( ) d Constrained solution: response and the effective LSFs as linear combinations of ad hoc basis functions L (u) =H (u, )+ R( )= n R X j= n L X i=1 r j R j ( ) h i H i (u, ) (1/ )=d + d 1 (1/ ) Bologna, 18,19 February 216

Integrated photometry calibration f ' f(u) du = L (u + (1/ )) du R( ) s( ) d Bologna, 18,19 February 216

Integrated photometry calibration f ' f(u) du = L (u + (1/ )) du R( ) s( ) d f ' R( ) s( ) d Bologna, 18,19 February 216

Integrated photometry calibration f ' f(u) du = L (u + (1/ )) du R( ) s( ) d f ' 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 Bologna, 18,19 February 216

Schedule 1993 1994 1995 1996 1997 1998 1999 2 25 21 215 22 221 222 Proposal Definition 24 23 22 21 Concept & Technology Study Mission Selection Re-Assessment Study Phase B1 29 28 27 26 214 213 212 211 219 218 217 216 Selection of Prime Contractor (EADS Astrium SAS) Implementation Phase B2 Phase C/D Launch December 213 Operation Data Processing Studies Scientific operation Software Development (DPAC) Data Processing Mission Products Intermediate Final Figure courtesy Michael Perryman and François Mignard Today