Frequently Asked Questions

Here you find answers to some of the most frequently asked questions about the Gaia mission, its science and its data. Additional FAQs are available on the ESA Gaia corporate website.

General

Science

How shall I acknowledge Gaia? What is Gaia's astrometric, photometric and spectroscopic performance?
Where will I be able to find all scientific papers that use Gaia data? How many objects in our solar system will Gaia detect?
When did Gaia's routine science operations phase start? What will be the most common type of star in the Gaia catalogue?
Which telescopes are being used to track Gaia from the ground? What is the Gaia scanning law and how does it work?
What can amateur astronomers contribute to the mission? Is there an input catalogue for Gaia? If yes, how are transient objects like supernova discovered?

Spacecraft

Is there any way to find out about transient objects immediately to do follow-up observations on-ground?
What is the difference between Gaia's three different CCD variants? What will the first Gaia catalogue look like?
Why does Gaia have two telescopes? What is the Robust Scatter Estimate (RSE)?
Why is there an angle of 106.5 degrees between the 2 telescopes? Where can I check how many times and when Gaia will observe my favourite object?
The spinning axis of Gaia seems to be fixed at 45 degrees with respect to the Sun. Is there any special reason why this value was chosen? I'm not interested in stars, I'm interested in minor bodies in the Solar System. When will you start releasing information about them? Will you calculate orbital elements for them?
What are the red and blue photometer used for? How can Gaia G, BP and/or RP values be transformed to V magnitudes?
How do you ensure the stability of the basic angle? I'm not interested in stars, I'm interested in galaxies. What can Gaia do for me?
What kind of mode do the CCDs operate in? What happens to newly detected solar system objects (Gaia asteroid observations)?
When will Gaia stop observing? What happens with the satellite afterwards?

Gaia Archive

How does the Gaia scanning law work? Where can I find the column descriptions and units of the data fields?

Gaia Data

Why do I find non-compliant units in FITS headers?

Where can I find information about upcoming data releases?

Why are there duplicate Hipparcos matches in table public.igsl_source_catalog_ids?
Is data access limited? How is it regulated? What are the celestial coordinates of my HEALPix identifier?
What types of data are being downlinked? Why does my query time out after 30 minutes? / Why is my query limited to return 3 million rows?
Are the data from the Gaia-ESO survey already public? If yes, where can I access them? I use the SAMP at the job result but nothing happens. Why?

DATA PROCESSING

 
What is DPAC?  

How does the on-board processing system work?

 
How do you mitigate the effects of radiation damage?  
What are the different steps in the Gaia data processing?  

 

General

How shall I acknowledge Gaia?
Credit and citation instructions can be found here.
 
Where will I be able to find all scientific papers that use Gaia data?
Please take a look here.
 
When did Gaia's routine science operations phase start?
Routine scientific observations commenced on 25 July 2014 with one month of Ecliptic Pole Scanning Law switching to Nominal Scanning Law.
 
Which telescopes are being used to track Gaia from the ground?
The following telescopes track Gaia from the ground: the Liverpool Robotic Telescope, the Faulkes Telescope North, the Faulkes Telescope South, and the ESO VLT survey telescope.
 
What can amateur astronomers contribute to the mission?
Amateurs can contribute, for instance, through the Gaia-Groundbased Observational Service for Asteroids (Gaia-GOSA) or through the follow up of Gaia photometric science alerts.
 

Gaia Archive

Where can I find the column descriptions and units of the data fields?
The Gaia Archive data model (column description, units, etc.) can be found here. More extensive documentation of the data and its processing can be found here.
 
Why do I find non-compliant units in FITS headers?
The headers of FITS files served from the Gaia Archive indeed contain constructs like TUNIT16 = 'Angular Velocity[mas/year]' which are not compliant with FITS standard 3.0. This will be corrected for Data Release 2 (DR2). Meanwhile, users can continue using the FITS files: their data contents are correct while the non-compliances in the headers may give warning messages (but not errors).
 
Why are there duplicate Hipparcos matches in table public.igsl_source_catalog_ids?
This is the unavoidable consequence of using the Initial Gaia Source List (IGSL) for bootstrapping the cross-match procedures for data release 1. The IGSL is a collection of multiple, incomplete pre-Gaia catalogues, known to contain spurious and duplicate sources. To avoid dropping Hipparcos stars in the IGSL, those entries not matched were added as a "fake" Tycho-2 star with Tycho-2 identifier idTYCHO = 9999999000000+idHIP. Because of a bug in the matching procedures, approximately 12000 Hipparcos stars have been entered twice. These can be identified as objects with auxHIP = 1 and idTYCHO > 9999999000000 and should not be used. 
 
What are the celestial coordinates of my HEALPix identifier?
This table provides a link between the HEALPix identifier (level 6, so Nside = 26 = 64) and equatorial, Galactic, and ecliptic coordinates. The text file has four header lines and 49152 data lines, each with eight columns separated by commas:
  1. HEALPix identifier [ring scheme, Gaia default]
  2. HEALPix identifier [nest scheme]
  3. Right ascension [deg]
  4. Declination [deg]
  5. Galactic Longitude [deg]
  6. Galactic Latitude [deg]
  7. Ecliptic Longitude [deg]
  8. Ecliptic Latitude [deg]
The conversion from Gaia source_id to HEALPix number with ring pixel ordering is described here (read here about ring and nested pixel ordering).
 
Why does my query time out after 30 minutes? / Why is my query limited to return 3 million rows?
When using the TAP service for queries in anonymous mode, these limits indeed exist (as documented here). Users can, however, self-register through the "SIGN IN" button in the top-righter corner of the Archive, after which the reported user quota apply. Upon demonstrated need, users can request (temporary) changes to their quota by sending a motivated request to the Gaia Helpdesk.
 
I use the SAMP at the job result but nothing happens. Why?

There may be several reasons for this. First of all the SAMP hub must be up and running. For instance, if TOPCAT is being used a SAMP hub is automatically started.

Then there is a known limitation of current SAMP standard related to the use of HTTPS connections. Current implementations of SAMP hubs does not support HTTPS connections. If you are using the Gaia Archive in HTTPS mode this can be the cause of the inability to connect to the SAMP hub. There are two possible workarounds for this.

First workaround: allow your browser to connect to insecure content. This will allow connections to the SAMP hub using HTTP from your secure Gaia Archive application. Second workaround: use the Gaia Archive HTTP version. Be aware though that the latter implies a security risk as the communication between your browser and the Gaia Archive server is not being encrypted!

 

Science

What is Gaia's astrometric, photometric and spectroscopic performance?
A summary of key astrometric, photometric and spectroscopic performance figures can be found here. An extended overview is available on the Science Performance pages.
 
How many objects in our solar system will Gaia detect?
Gaia will detect and observe all objects brighter than 20 mag that are not too extended, which means not too large in size and not too fast moving. This covers, among others, the largest moons in the solar system, comets, Kuiper-Belt / trans-Neptunian objects, main-belt asteroids, near-Earth asteroids, and Trojan companions of planets. All in all, Gaia will observe some 350,000 objects in the Solar system, the vast majority of which are main-belt asteroids.
 
What will be the most common type of star in the Gaia catalogue?
Gaia provides an unbiased survey of the Milky Way, down to 20 mag. This covers all types of stars, including rare ones and stars with short-lived evolution phases. Whereas Gaia only observes a few hundred, hot, massive stars with spectral type O, it sees a few hundred million solar-type and less-massive G- and K-type stars. The vast majority of stars seen by Gaia are normal ('adult') main-sequence stars with only 10-20% of all stars being in their 'post-adult' giant phase.
 
What is the Gaia scanning law and how does it work?
The scanning law is the "law" describing how Gaia's fields of view scan the sky as function of time. The scanning is composed of two, independent, "superimposed" motions: (1) a rotation around the spacecraft spin axis with a period of 6 hours, and (2) a slow (~63-day-period) precession of the spin axis around the solar direction at a fixed solar-aspect angle of 45 degrees. Over the nominal 5-year mission, Gaia will complete 29 of these precession periods, leading to an optimally uniform sky coverage after 5 years.
 
I'm not interested in stars, I'm interested in galaxies. What can Gaia do for me?
Although Gaia is foremost a star-mapper mission aimed to observe the Milky Way, it also observes external galaxies brighter than 20 mag. For the nearest galaxies, like M31, individual stars are resolved and observed but a more typical case is an unresolved galaxy at a larger distance. Some one million of these, mostly elliptical galaxies, are observed by Gaia. In addition, Gaia observes around half a million quasars. All these objects undergo astrophysical classification and parametrisation, for instance morphology and star-formation history for galaxies and redshift for quasars.
 
Is there an input catalogue for Gaia? If yes, how are transient objects like supernova discovered?
Unlike the Hipparcos mission, which selected its targets for observation based on a pre-defined input catalogue loaded on board, Gaia will perform an unbiased survey of the sky. Since an all-sky input catalogue at the Gaia spatial resolution complete down to 20th magnitude does not exist, there has essentially been no choice but to implement on-board object detection, with the associated advantage that transient sources (supernovae, near-Earth asteroids, etc.) will not escape Gaia’s eyes.
 
Is there any way to find out about transient objects immediately to do follow-up observations on-ground ?
Gaia Photometric Science Alerts are published here. There is also a Gaia Follow-Up Network for Solar System Objects.
 
What will the first Gaia catalogue look like?
The data is contained in a database and can be extracted using various methods, for instance through a command-line interface based on the Table Access Protocol (TAP) or a web-based interface using simple query forms or using customised queries in Astronomical Data Query Language (ADQL). The Gaia Archive is accessible here.
 
What is the Robust Scatter Estimate (RSE)?
The “Robust Scatter Estimate” (RSE) is defined as 0.390152 times the difference between the 90th and 10th percentiles of the distribution of the variable. For a Gaussian distribution, it equals the standard deviation. Within the Gaia community, the RSE is used as a standardised, robust measure of dispersion (see, for instance, Lindegren et al. 2012).
 
How can Gaia G, BP and/or RP values be transformed to V magnitudes?
Pre-launch relations between Johnson, Gaia, and Sloan filters are published in:
http://adsabs.harvard.edu/abs/2010A%26A...523A..48J (for instance Table 3). A post-launch discussion is available here.
 
What happens to newly detected solar system objects (Gaia asteroid observations)?
Finding out whether an asteroid has a chance to hit our planet is an international effort: Gaia asteroid observations are sent to a central collection place called the Minor Planet Center (MPC). Position measurements from all observers on our planet are collected there and a first orbit estimate is done at MPC using a combination of all observations. A NASA-funded team at JPL in the US and a team in Pisa, working with ESA, then perform detailed computations to find possible impactors. Rules on informing affected countries are in place in some nations; at the UN-level this is the task of the recently formed IAWN. ESA is currently putting a warning mechanism in place addressing the needs of their member countries.
 
Where can I check how many times and when Gaia will observe my favourite object?
You can use the Gaia Observation Forecast Tool for this, which is available here.
 
I'm not interested in stars, I'm interested in minor bodies in the Solar System. When will you start releasing information about them? Will you calculate orbital elements for them?
Gaia will observe around 250,000 objects in the solar system. Most of these are main-belt asteroids but the sample will include some near-Earth asteroids and comets. According to the current data release scenario, these data are up for release in 2019.
 

Gaia Data

Where can I find information about upcoming data releases?
Information about all upcoming data releases is provided in the "Gaia Data Release Scenario".
 
Is data access limited? How is it regulated?

Members of the scientific community will have access to Gaia data through intermediate catalogues, to be released in the course of the mission (a preliminary release scenario is available).

Formal 'data rights' (for example, through a Call for Proposals) will not be assigned to any scientist involved in any aspects of the mission, including those scientists who participate in the data processing. Early access to the reduced data could, however, be awarded to individuals and groups participating in the data analysis, its validation, and documentation, according to procedures to be established by the Gaia Science Team, in consultation with the AWG and the executive committee of the Data Processing and Analysis Consortium.

The Gaia Data Rights are defined in the Gaia Science Management Plan ESA/SPC(2006)45 (SMP).

 
What types of data are being downlinked?
The data downlinked from Gaia comprises astrometry, photometry, and spectroscopy. Most data, except for the 1% of the very brightest stars, comes down in small images which have been compressed (binned) into one number (the intensity/brightness of the light) in the so-called across-scan direction. This allows accurate timing and hence location estimation of the images (astrometry) as well as flux measurements (photometry). For the spectro-photometry and spectroscopy, the dispersion direction is orthogonal to the binning direction such that wavelength information is preserved.
 
Are the data from the Gaia-ESO survey already public? If yes, where can I access them?
Gaia ESO survey data is public and is being served from here.
 

Data Processing

What is DPAC?

The Data Processing and Analysis Consortium (DPAC) is a large pan-European team of expert scientists and software developers. It is responsible for processing the Gaia data with the final objective of producing the Gaia catalogue.

DPAC has been in place since 2006 and has the task to develop the data processing algorithms, the corresponding software, and the IT infrastructure for Gaia. It also executes the algorithms during the mission in order to turn the raw telemetry from Gaia into the final scientific data products that will be released to the scientific community.

More information about the consortium and its structure is available here.

 
How does the on-board processing system work?
Gaia's focal-plane assembly contains 106 CCD detectors, comprising nearly a billion pixels, and operates in TDI mode with a line period of 1 milli-second. The enormous amount of data this detector system could continuously generate is a few orders of magnitude too large to be transmitted to ground. There are hence three on-board processes applied to the science data:
1: not all pixel data are read from the CCDs but only small areas, so-called windows, around objects of interest;
2: the two-dimensional images (windows) are, except for bright stars, binned in the direction orthogonal to the scanning direction;
3: the resulting along-scan intensity profiles are compressed on board without loss of information.
The resulting, compressed star packets are transmitted to ground, typically within 24 hours after they are created, where they enter the data processing in the science ground segment.
 
How do you mitigate the effects of radiation damage?
Mitigation takes places at two levels, in the spacecraft hardware and software and in the software for the data processing. The design of the spacecraft has several features to mitigate the impact of non-ionising radiation damage, including focal-plane shielding, a supplementary buried channel in the detectors, and a charge-injection mechanism for the detectors. In addition, the temperature and clocking characteristics of the detectors have been selected taking the impact of radiation damage into account. Also the on-board science software, in charge of placing the windows around the stars, takes the effect of radiation damage into account by allowing a non-centred, asymmetric placement. In the ground-processing software, radiation damage is calibrated using models inspired by and partially calibrated through extensive pre-launch laboratory test campaigns using flight-representative CCD detectors.
 
What are the different steps in the Gaia data processing?
The data processing is a complex task which is not easily summarised in a sentence or two. A concise description can be found in Section 7 of the Gaia mission paper.
 

Spacecraft

What is the difference between Gaia's three different CCD variants?
Gaia carries 106 charge-coupled-device (CCD) detectors. They come in three different types: the broad-band CCD, the blue(-enhanced) CCD, and the red(-enhanced) CCD. Each of these types has the same architecture (e.g. number of pixels, TDI gates, read-out register, etc.) but differ in their anti-reflection coating (and surface passivation), their thickness, and the resistivity of their Silicon waver. The broad-band and blue CCDs are both 16 micrometer thick and are both manufactured from standard-resistivity silicon; they differ only in their anti-reflection coating, which is optimised for short wavelengths for the blue CCD and optimised to cover a broad bandpass for the broad-band CCD. The red CCD, in contrast, is based on high-resistivity silicon, is 40 micrometer thick, and has an anti-reflection coating optimised for long wavelengths. The broad-band CCD is used in the star mapper (SM), the astrometric field (AF), and the wavefront sensor (WFS). The blue CCD is used in the blue photometer (BP). The red CCD is used in the basic-angle monitor (BAM), the red photometer (RP), and in the radial-velocity spectrograph (RVS).
 
Why is there an angle of 106.5 degrees between Gaia's two telescopes?
The choice of the so-called basic angle of Gaia was a non-trivial one. On the one hand, it should be of order 90 degrees to allow simultaneous measurements of stars separated by large angles on the sky. On the other hand, it should not be a harmonic ratio of a 360-degree circle (e.g., 60 deg, 90 deg, or 120 deg). Taking these considerations into account, acceptable ranges for the basic angle are 96.8 +/- 0.1 deg, 99.4 +/- 0.1 deg, 100.5 +/- 0.1 deg, 105.3 +/- 0.1 deg, 106.5 +/- 0.1 deg, 109.3 +/- 0.1 deg, 109.9 +/- 0.1 deg, etc. Accommodation aspects identified during industrial studies subsequently favoured 106.5 deg as the value finally adopted for Gaia.
 
The spinning axis of Gaia seems to be fixed at 45 degrees with respect to the Sun. Is there any special reason why this value was chosen?
The parallax factor, which means the measurable, along-scan parallactic displacement of a star, is proportional to the sine of the so-called solar-aspect angle (the angle between the spin axis and the direction to the Sun). Therefore, the highest signal to noise for parallax measurements would be reached for a solar-aspect angle of 90 degrees. This, however, would mean that the sunshield would not point to the Sun so that solar-cell-driven power generation would be inhibited and, more importantly, that sunlight would enter the telescope apertures. In the other extreme case, i.e., with a zero solar-aspect angle, power generation would be optimal but parallaxes could not be measured. In practice, given design considerations on the required size of the sunshield to keep the payload in permanent shadow and to generate sufficient power, the final compromise for Gaia ended up at 45 degrees.
 
Why does Gaia have two telescopes?
The goal of the Gaia mission is to perform global astrometry over the entire celestial sphere. For this purpose, the satellite is equipped with two telescopes whose viewing directions are separated by a large, so-called basic angle (106.5 deg). This allows to make simultaneous measurements of star positions at small and large angular scales, i.e., inside each telescope field of view and between the two fields of view. Since the parallax factor differs between the two fields of view, global astrometry can be performed (as compared to relative astrometry offered by a single field of view). Wide-angle measurements also guarantee a distortion-free and rigid system of coordinates and proper motions over the whole sky.
 
What are the red and blue photometer used for?
Photometric observations are collected with the photometric instrument, at the same angular resolution as the astrometric observations and for all objects observed astrometrically. The purpose of the photometry is to enable chromatic corrections of the astrometric observations and to 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.).
 
How do you ensure the stability of the basic angle?
The stability of the basic angle is achieved through the design of the spacecraft and mission (profile), including the choice of material for most payload structural parts (silicon-carbide), the absence of moving parts in the spacecraft (except for the thruster valves), the thermo-mechanical decoupling between the service and payload modules, a constant power dissipation of the payload, and the spacecraft orbit and attitude (ensuring a constant solar illumination without eclipses by the Earth). In addition, the basic-angle monitor provides continuous feedback on short-term basic-angle variations (P < 12 h), with long-term variations (P > 12 h) being calibrated within the data processing.
 
What kind of mode do the CCDs operate in?
The CCD detectors operate in Time-Delayed Integration (TDI) mode. That means that the photo-electrons generated by the starlight are transferred through the CCD with the same speed as the star images move, as a result of the slow spin / rotation of the spacecraft, over the detector surface. It takes an object 4.4 seconds to cross a CCD detector.
 
When will Gaia stop observing? What happens with the satellite afterwards?
The nominal, five-year mission ends in July 2019. The consumables of the spacecraft have been sized to allow for an extension of at least one year. Extrapolating the current cold-gas propellant consumption suggests that the mission could be extended, provided this is approved by the ESA advisory bodies, up to 2023 (with an uncertainty of plus or minus one year). After science operations are terminated, the spacecraft will be put into a disposal orbit (away from L2) to comply with space-debris regulations.
 
How does the Gaia scanning law work?
Gaia scans the sky in a complex way. First, the telescopes rotate around the spin axis every six hours to sweep great circles on the sky (with a height of about 0.7 deg). At the same time, the spin axis precesses slowly around the solar direction (at a fixed, 45-degree angle), as the Sun moves over the ecliptic, with a period of 63 days. This optimises the uniformity of the sky scanning over five years and leads to, on average, 70 astrometric and photometric transits across the focal plane (and 40 for the RVS instrument).

 

 

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