|Astrometric Performance Photometric Performance Spectroscopic Performance|
Gaia will perform micro-arcsecond (μas) global astrometry for all ~1,000 million stars down to G ≈ 20 mag by linking objects with both small and large angular separations in a network in which each object is connected to a large number of other objects in every direction. Over the five-year mission lifetime, a star transits the astrometric instrument on average ~70 times, leading to ~630 CCD transits. Gaia will not exclusively observe stars: all objects brighter than G ≈ 20 mag will be observed, including solar-system objects such as asteroids and Kuiper-belt objects, quasars, supernovae, multiple stars, etc. The Gaia CCD detectors feature a pixel size of 10 μm (59 milli-arcsecond) in the scanning direction and the astrometric instrument has been designed to cope with object densities up to 750,000 stars per square degree. In denser areas, only the brightest stars are observed and the completeness limit will be brighter than 20th magnitude.
Photometric observations will be collected with the photometric instrument, at the same angular resolution as the astrometric observations and for all objects observed astrometrically, to:
- enable chromatic corrections of the astrometric observations, and
- provide astrophysical information for all objects, including astrophysical classification (for instance object type such as star, quasar, etc.) and astrophysical characterisation (for instance interstellar reddenings and effective temperatures for stars, photometric redshifts for quasars, etc.).
Spectroscopic observations will be collected with the spectroscopic instrument for all objects down to GRVS ≈ 16 mag, to:
- provide radial velocities through Doppler-shift measurements using cross-correlation (~150 million stars);
- provide astrophysical information, such as interstellar reddening, atmospheric parameters, and rotational velocities, for stars brighter than GRVS ≈ 12 mag (~5 million stars); and
- provide element abundances for stars brighter than GRVS ≈ 11 mag (~2 million stars).
The spectroscopic instrument has been designed to cope with object densities up to 36,000 stars per square degree. In denser areas, only the brightest stars are observed and the completeness limit will be brighter than 16th magnitude.
In the scientific performance assessments for Gaia presented below, all known instrumental effects are included under the appropriate in-flight operating conditions (temperature, CCD operating mode, etc.). The only exception is the slow, continuous contamination of the telescopes which has been discovered during the commissioning phase: the impact of this effect is not included in the current assessment. It is, however, believed that a few future de-contamination campaigns, to be scheduled in the second half of 2014, will be sufficient to reduce the contamination rates to acceptable levels (i.e., less than 10% transmission loss over the 5-year mission, which is comparable to the expected evolution of the light-collection efficiency resulting from radiation damage). All error sources are included as random variables with typical deviations (as opposed to best-case or worst-case deviations). All performance estimates include a 20% scientific contingency margin to cover, among others:
- "scientific uncertainties" in the on-ground data analysis, including uncertainties related to relativistic corrections, aberration corrections, and the spacecraft and solar system ephemeris;
- scientific effects such as the contribution to the astrometric error budget from the mismatch between the actual and the calibrating point spread function, estimation errors in the sky background and total detection noise values that need to be fed to the centroiding algorithm, etc.;
- the fact that the sky does not contain, as assumed for the performance assessments, "perfect stars" but "normal stars", which can be photometrically variable, have spectral peculiarities such as emission lines, have unrecognised companions, be located in crowded fields, etc.;
- other astronomical environmental factors such as, e.g., localised enhanced sky-background surface brightness, unrecognised small-scale sky-background-brightness gradients, unrecognised prompt particle events, etc.
The performance numbers presented below refer to the standard errors, i.e., they refer to the precision of the mission data. An assessment of potential (residual) systematic errors in the Gaia data, linking to its accuracy, is much more difficult to provide. For astrometry, a potential contributor to systematic parallax errors are unmodelled spin-synchronous basic-angle variations. The metrology data derived from the basic-angle monitoring device should allow, after careful calibration, to limit possible systematic effects to 1 μas. Work to achieve this goal is ongoing inside the Data Processing and Analysis Consortium (DPAC).
The astrometric standard errors are calculated following the recipe outlined in GAIA-JDB-022. The standard-error calculation includes all known instrumental effects, including the straylight levels as measured during the commissioning phase. For instrument-related residual calibration errors at ground-processing (DPAC) level, an appropriate calibration error is included. So-called residual "scientific calibration errors" (e.g., mismatch of the model point spread function, sky-background estimation errors, etc.), all of which result from the on-ground data processing, are not included. These latter errors are assumed to be covered by the 20% science margin.
At the time of the In-Orbit Commissioning Review (July 2014), the predicted end-of-mission parallax standard errors σπ, averaged over the sky for a uniform distribution, for unreddened B1V, G2V, and M6V stars are:
|Bright stars||5-14 μas (3 mag < V < 12 mag)||5-14 μas (3 mag < V < 12 mag)||5-14 μas (5 mag < V < 14 mag)|
|V = 15 mag||26 μas||24 μas||9 μas|
|V = 20 mag||600 μas||540 μas||130 μas|
All stars brighter than G = 3 mag will be observed with a special mode. Depending on how well this mode can be calibrated, end-of-mission parallax standard errors at the level of a few dozen μas could potentially be achieved for these stars. This remains the topic of further work. For stars fainter than G = 3 mag yet brighter than G = 12 mag, nominal yet shorter CCD integration times (through the use of TDI gates) will be used to avoid saturation. For these stars, the end-of-mission performance depends sensitively on the adopted TDI-gate scheme as well as on magnitude. This is reflected in the quoted performance range 5-14 μas.
The astrometric precision per single focal-plane crossing is roughly 4.3 times worse than the end-of-mission astrometric precision. Beware that the interpretation of single-crossing precisions is non-trivial: e.g., one transit does not provide a parallax measurement and, moreover, allows only a one-dimensional measurement.
For sky-averaged position and proper-motion errors, σ0 [μas] and σμ [μas yr-1], the following relations can be used, derived from scanning-law simulations:
|σ0||=||0.743 · σπ;|
|σα*||=||0.787 · σπ;|
|σδ||=||0.699 · σπ;|
|σμ||=||0.526 · σπ;|
|σμα*||=||0.556 · σπ;|
|σμδ||=||0.496 · σπ,|
where the asterisk denotes true arcs on the sky (σα* = σα · cos(δ), etc.).
The predicted standard errors vary over the sky as a result of the scanning law. The mean (ecliptic-longitude-averaged) variations with ecliptic latitude β are shown in this figure and given in this table, derived from scanning-law simulations. The (approximate) ecliptic latitude can be calculated from the equatorial coordinates (α, δ) or the galactic coordinates (l, b) using:
|sin(β)||=||0.9175 · sin(δ) - 0.3978 · cos(δ) · sin(α)|
|=||0.4971 · sin(b) + 0.8677 · cos(b) · sin(l - 6°.38).|
Gaia's photometry comprises:
- broad-band white-light G-band fluxes obtained in the astrometric instrument, and
- low-resolution spectro-photometry obtained in the Blue and Red Photometers (BP and RP).
The wavelength coverage of the astrometric instrument, defining the white-light G band, is ~330-1050 nm. The Sloan/Cousins/Johnson magnitude/colour transformations presented in 2010A&A...523A..48J allow estimation of the broad-band Gaia G magnitude as a function of V and V-IC valid over a wide colour interval. For ease of reference, we repeat from 2010A&A...523A..48J the relation to convert Johnson V and Johnson-Cousins V-IC to Gaia G:
G = V - 0.0257 - 0.0924 · (V-IC) - 0.1623 · (V-IC)2 + 0.0090 · (V-IC)3,
where the fit error is 0.05 mag. For relations using V-RC, RC-IC, or B-V or for relations linking Sloan magnitudes (g or r) and colours (g-r, g-i, or r-i) to Gaia G magnitudes, please see 2010A&A...523A..48J.
The spectral dispersion of the photometric instrument is a function of wavelength and varies in BP from ~3 to ~27 nm pixel-1 covering the wavelength range ~330-680 nm. In RP, the wavelength range is ~640-1050 nm with a spectral dispersion of ~7 to ~15 nm pixel-1. The 76%-energy width of the line-spread function along the dispersion direction varies along the BP spectrum from 1.3 pixels at 330 nm to 1.9 pixels at 680 nm and along the RP spectrum from 3.5 pixels at 640 nm to 4.1 pixels at 1050 nm. Whereas the G-band data are particularly useful for stellar variability studies, the BP/RP spectra allow the derivation of astrophysical parameters, such as interstellar extinctions, surface gravities, etc., needed for the scientific exploitation of the astrometric data. Over the five-year mission lifetime, a star transits the photometric instrument on average ~70 times, leading to ~70 transits in BP and ~70 transits in RP (the dependence on ecliptic latitude is summarised in this table).
The photometric standard errors of the integrated G-band, BP-band, and RP-band are calculated following the recipe outlined in GAIA-JDB-022. The standard-error calculation includes all known instrumental effects, including straylight as measured during the in-orbit commissioning phase, as well as a 20% science margin. At the time of the In-Orbit Commissioning Review (July 2014), the following end-of-mission photometric errors, in units of milli-magnitude, would be reached with an assumed calibration error of 30 milli-magnitude at CCD-level:
The Gaia photometric data, sometimes in combination with the astrometric and the spectroscopic data, allow to retrieve astrophysical parameters of objects and to classify them. The accuracy of the estimation of the astrophysical parameters depends on G magnitude and on the value of the parameters themselves. The investigations reported in 2012MNRAS.426.2463L, based on simulated BP and RP data, indicate that, for stars at G = 15 mag with less than two magnitudes extinction, effective temperature Teff can be estimated to within 1%, surface gravity log(g) to 0.1-0.2 dex, and metallicity [Fe/H] (for FGKM stars) to 0.1-0.2 dex. Performance degrades at larger extinctions, but not always by a large amount. Extinction can be estimated to an accuracy of 0.05-0.2 mag for stars across the full parameter range with a priori unknown extinction between 0 and 10 mag. Performance degrades at fainter magnitudes, but even at G = 19 mag, log(g) can be estimated to better than 0.2 dex for all spectral types, and [Fe/H] to within 0.35 dex for FGKM stars, for extinctions below 1 mag. The strong and ubiquitous degeneracy in effective temperature and extinction (e.g., 2011MNRAS.411..435B) will limit the accuracy with which either parameter can be estimated at the faintest magnitudes.
Gaia's spectroscopic instrument, the Radial-Velocity Spectrometer (RVS), is an integral-field spectrograph with resolving power ~11,500 covering the wavelength range 845-872 nm. Over the five-year mission lifetime, a star transits the spectroscopic instrument on average ~40 times, leading to ~120 CCD transits.
Radial-velocity robust formal errors are calculated following the recipe outlined in GAIA-JDB-022. The calculation methodology prescribes, for all stars and magnitudes, that a single end-of-mission composite spectrum is first reconstructed by co-addition of all spectra collected during all CCD crossings throughout the five-year mission lifetime. A single mission-averaged radial velocity is then extracted from this end-of-mission composite spectrum by cross correlation with a template spectrum. The spectroscopic performance numbers reported below refer to this prescribed procedure, although it is foreseen in the a posteriori on-ground data analysis by DPAC to actually derive also single-field-of-view transit spectra, and to extract associated epoch radial velocities, whenever this proves possible in practice.
The robust-formal-error calculation includes all known instrumental effects, including straylight as measured during the in-orbit commissioning phase. For instrument-related residual calibration errors at ground-processing (DPAC) level, an appropriate calibration error is included. So-called residual "scientific calibration errors" (e.g., template-mismatch errors, residual errors in the derivation of the locations of the centroids of the reference spectral lines used for the wavelength calibration, etc.), all of which result from the on-ground data processing, are not included. These latter errors are assumed to be covered by the 20% science margin.
At the time of the In-Orbit Commissioning Review (July 2014), the predicted end-of-mission radial-velocity robust formal errors σvrad, averaged over the sky for a uniform distribution, for unreddened B1V, G2V, and K1III-MP (MP = metal-poor) stars are:
|Spectral type||V [mag]|| |
Radial-velocity error [km s-1]
The bright-star performance is limited by a systematic 1 km s-1 radial-velocity error up to the spectral-type-dependent magnitude limit shown in the table above. Investigations are currently ongoing to change the on-board acquisition and windowing strategy of the RVS spectra. Upon successful implementation of the required software changes, the magnitudes at which the 15 km s-1 error level is achieved could be 0.3 mag fainter.