Image of the Week

Gaia DR2 Passbands


The coloured lines in the figure show the revised passbands for G, GBP and GRP (green: G; blue: GBP; red: GRP), defining the Gaia DR2 photometric system. The thin, grey lines show the nominal, pre-launch passbands published in Jordi et al. 2010, used for Gaia DR1.


The photometry of a celestial source is generally performed by recording the incoming light on a detector after it has passed through an optical filter that selects a range of wavelengths. A set of filters, in practice, allows us to take different slices of the spectrum of a source (e.g., star or a galaxy). This is a simple and fast way to obtain information on the shape of the spectrum, and is especially useful for faint sources.

The combined effect of the filter, the transmission, the optical path, and the detector quantum efficiency, as a function of wavelength, defines the passband, i.e. the function to be applied to the spectrum of a given source to obtain its corresponding magnitude in the passband defined by a given instrument. Clearly, this is an essential piece of information when transforming the output of theoretical models of stars, galaxies, asteroids, etc. into predictions of magnitudes to be compared with observed magnitudes from a given instrument.

The integrated photometry in the Gaia catalogue is no exception: an accurate determination of the three passbands, G, GBP, and GRP, is crucial for comparisons with theoretical models. The Gaia passbands are defined by a combination of factors, including the wavelength response of the prisms (including the filters on them) and the telescope mirrors and the quantum efficiency of the CCD detectors.

In principle, by comparing the observed photometry of an arbitrarily large set of sources with well-known spectral energy distributions (SEDs) with the synthetic photometry obtained assuming some mathematical model for the passband, it should be possible to infer the true shape of the passband. However, in practice, any such set of spectrophotometric calibrators will only represents a subspace of all possible SED shapes either because it is finite in size and mostly because there exists classes of astrophysical  objects that are intrinsically variable and hence cannot be properly used as calibrators. Hence, there will always be components of the passband that are not well constrained.

A way to mitigate this problem is to add priors to the passband model, in particular using the pre-launch knowledge of the instrument overall response. In the case of Gaia, the satellite manufacturer provided an extensive set of measurements and models of most of the factors contributing to the definition of the instrument response. This data has been used to build the nominal, pre-launch passband curves that were published in Jordi et al. 2010.

There are many reasons to expect significative differences between nominal passbands and the “true” passbands in flight corresponding to the Gaia DR2 photometric system. For example, the contamination effect described in Gaia collaboration et al. 2016 introduces a flux loss term that is wavelength dependent and that changes significantly and occasionally abruptly with time (as a result of de-contamination campaigns). The effect of contamination has been largely compensated by the internal photometric calibration (for details, see Riello et al. 2018, available as of 25 April 2018).

To account for possible differences between the nominal and "true" passbands, a model based on the nominal curves has been fitted with a multiplicative correction factor to adapt synthetic and observed photometry for a set of 93 spectrophotometric standard sources extracted from the Gaia SPSS catalogue (see Pancino et al. 2012). Evans et al. 2018 (available as of 25 April 2018) provides more details on the computation of the passbands and the external calibration for Gaia DR2.

A first version for the three Gaia passbands defining the Gaia DR2 photometric system was released for internal DPAC use in July 2017. These passbands were used in the data processing for Gaia DR2 leading to the magnitudes to be published on 25 April. However, later analysis of BP/RP spectral data showed that a significant offset in the RP cutoff wavelength position was required to well describe the spectral data. A new set of revised passbands was subsequently derived in October 2017 to introduce this and a few other minor changes. These revised passbands are shown in the figure above as green, blue, and red solid lines for the G, GBP, and GRP bands, respectively. The thin grey lines show the nominal, pre-launch passbands published in Jordi et al. 2010.

Gaia DR2 passbands are offered to users as a record to understand how the Gaia DR2 magnitudes are computed, and to eventually reproduce analysis of data made by other DPAC Coordination Units. For other applications, for example the computation of synthetic fluxes or magnitudes, one might use the revised passbands. The passbands are available below for download. Please consider carefully which set to use with your data.

Credits: ESA/Gaia/DPAC, P. Montegriffo, F. De Angeli, C. Cacciari

[Published: 16/03/2018]


Image of the Week Archive

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
27/08: Quest for the Sun's siblings
Please note: Entries from the period 2003-2010 are available in this PDF document.