Gaia FAQ - Gaia
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.
For registered users who have questions on their Cosmos account and password resets, please visit the Cosmos FAQ.
If you can not find what you are looking for, please contact the Gaia Helpdesk.
|Question||Answer||Type||Last updated (dd/mm/yyyy)|
If you have used Gaia data in your research, please follow the credit and citation instructions as found here. When using Gaia DR1 data, we ask you to also cite certain Gaia Data Release 1 papers, as is indicated here. When using Gaia DR2 data, we ask you to also cite Gaia Data Release 2 papers, as indicated here. When using Gaia's Early Data Release 3 papers, we ask you to also cite Gaia's Early Data Release 3 papers as indicated here. When using Gaia Data Release 3 data, we ask you to also cite Gaia Data Release 3 papers, as indicated here. When using Gaia's Focused Product Release data, we ask you to cite the respective Gaia Focused Product Release papers as indicated here.
|Where will I be able to find all scientific papers that use Gaia data?||Gaia Data Release 1 papers, Gaia Data Release 2 papers, Gaia Early Data Release 3 papers, Gaia Data Release 3 papers, Gaia's Focused Product Release papers, publications in peer-reviewed journals or Gaia PhD Theses are all accessible through dedicated pages on the Gaia cosmos website.||General||21/08/2023|
|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. Other important moments in Gaia history can be found in the table given on this page.||General||21/08/2023|
|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. Gaia is also regularly observed as part of the ground-based orbital tracking (GBOT) operations. Gaia is visible in the sky most of the night due to its orbit around the second Lagrange point (with some variability in the elevation over the seasons). However, Gaia is not too bright (less than 20th magnitude) so not that easy to spot for an amateur astronomer. The telescopes used by GBOT have an aperture of about 2 metres.||General||-|
|What can amateur astronomers contribute to the mission?||Amateurs can contribute, for instance, through the Gaia-Groundbased Observational Service for Asteroids (Gaia-GOSA) or by participating in the Gaia solar-system alerts (Gaia FUN-SSO). Another opportunity is through the follow-up of Gaia photometric science alerts. A description of follow-up opportunities is given in this image of the week.||General||
|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.||Science||21/08/2023|
|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.||Science||-|
|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.||Science||-|
|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.||Science||-|
|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.||Science||-|
|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.||Science||-|
|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.||Science||21/08/2023|
|What does 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.||Science||-|
|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).||Science||-|
|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.||Science||-|
|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.||Science||-|
|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 350,000 objects in the Solar system. Most of these are main-belt asteroids but the sample will include some near-Earth asteroids and comets. Keep an eye out for our data release scenario to see when data will be released.||Science||-|
|I can not find data for a bright star. Why is that?||Some stars are too bright for Gaia to observe. Some of these bright stars are being observed with a special mode on board the spacecraft, requiring to predict when it passes the focal plane and then telling the on board computer to record the relevant pixels around the position of the star. These data should be processed eventually but cannot be handled automatically at this stage.||Science||-|
|The abstract of the Gaia DR1 release paper (Gaia Collaboration et al. 2016) states that "For the primary [TGAS] astrometric data set the typical uncertainty is about 0.3 mas for the ... parallaxes ... A systematic component of 0.3 mas should be added to the parallax uncertainties". This is confirmed at several places inside the paper. Is this really true? On the other hand, if a systematic error of 0.3 mas has been included already, how can many parallax standard errors in TGAS be close to 0.3 mas (or even be smaller)?||
Both the random and systematic errors have been included in the published "parallax_error" column in Gaia DR1 through the error "inflation" determined through a comparison with external data (see Equation 4 in Section 4.1 in Lindegren et al. 2016). So, what happened for TGAS stars with parallax standard errors below 0.3 mas? Did we forget to inflate their errors? No: some errors can be smaller than 0.3 mas because a different factor was applied for each star.
The confusion is related to the 0.3 mas magic number, which might not have been explained sufficiently well. At some point, two particular AGIS test solutions using different sets of input observations (a different half of the focal plane) were created. The residuals showed significant structure, i.e., correlated errors. The typical difference was about +/-0.1 mas, but was much higher (or lower) in particular regions of the sky. The magic number 0.3 mas takes these regions into account but should not be interpreted as an RMS value for the whole sky. The problem is that we cannot quantify (and thus remove) these errors more precisely. Further details can be found in Lindegren et al. 2016, Appendices B and C.
|What is the meaning of ICRS, ICRF, J2000.0 epoch, and J2000.0 equinox?||
Gaia DR2 astrometry consistently uses the ICRS reference system and provides stellar coordinates valid for epoch J2015.5 (roughly mid-2015, where J stands for Julian year). Equinox J2000.0 is a currently obsolete concept linked to former, dynamical reference systems such as FK5 which were tied to the celestial equator at a particular time.
As of 1 January 1998, the International Celestial Reference System (ICRS) is the standard celestial reference system adopted by the International Astronomical Union (IAU). The ICRS is the set of prescriptions and conventions together with the modelling required to define, at any given time, a triad of orthogonal axes. The ICRS has its origin is at the barycentre of the Solar System, with axes that are space-fixed and kinematically non-rotating with respect to the most distant sources in the Universe. In practice, the ICRS is materialised by the International Celestial Reference Frame (ICRF) through the coordinates of a defining set of extra-galactic objects (quasars).
Prior to astronomers being able to define and use the ICRS and ICRF, dynamical reference systems were used based on observations of star positions tied in some way to moving objects in the Solar System. These reference systems refer to a mean equator and equinox at a given reference epoch (typically J2000.0), requiring precession/nutation models and corrections to deal with the time-variable fundamental plane. To within ~25 mas, mean J2000.0 equatorial coordinates are the same as ICRS coordinates such that, for "ordinary" applications, they can in practice be considered to be the same. For high-accuracy applications, the appropriate frame conversion shall be used.
|Where can I find information about upcoming data releases?||Information about all upcoming data releases is provided in the "Gaia Data Release Scenario".||Data||21/08/2023|
|Where can I find all relevant information on a specific Gaia data release?||Overview pages were created for each release. These are the respective pages: Gaia data release 1 overview page, Gaia data release 2 overview page, Gaia Early Data Release 3 overview page, Gaia Data Release 3 overview page, Gaia Focused Product Release overview page||Data||21/08/2023|
|Is data access limited? How is it regulated?||
Members of the scientific community will have access to Gaia data through intermediate catalogues, which will be released in the course of the mission (a 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.||Data||-|
|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||-|
|How is the Gaia data transferred to Earth?||Gaia produces a lot of data that all needs to be transferred from the satellite in orbit around L2 to a ground station on Earth. Gaia uses the large ESA ground stations: Cebreros, Malargue and New Norcia. Downlinking data needs some preparation: the sky Gaia is looking at is modeled in advance to get an indication of how much data to expect and also the link budget is modeled (this is how fast data can be downlinked based on antenna performance). Both combined give an indication of how much time is needed each day for downloading the data without wasting ground station time. On average it takes about 14 hours a day to get the Gaia data down to a ground station.||Data||-|
|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.||Data Processing||-|
|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.||Data Processing||-|
|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).||Spacecraft||-|
|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.||Spacecraft||-|
|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.||Spacecraft||-|
|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.||Spacecraft||-|
|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.).||Spacecraft||-|
|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.||Spacecraft||-|
|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.||Spacecraft||-|
|When will Gaia stop observing? What happens with the satellite afterwards?||The nominal, five-year mission ends in July 2019. The mission has been extended to the end of 2020, and has an indicative extension up to the end of 2022. 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.||Spacecraft||-|
|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).||Spacecraft||-|
|I have problems accessing the archive||
Planned maintenance to the Gaia Archive is announced through Gaia Cosmos and/or through a pop-up warning at the Gaia Archive. If you cannot access the Gaia Archive, and the unavailability of the Gaia Archive is not announced there, please contact us through the Gaia Helpdesk.
The first aid solution to most issues with the Gaia Archive GUI (from seemingly disappeared tables or strange behaviour after a query or problems signing in or out), is to close your browser, clear your cache and then retry. In most cases, this will solve things and you will be able to continue working in no time.
A second solution might be to switch browsers. Some browsers show more hickups than others in combination with the use of the Gaia Archive GUI.
|I exceeded my file quota||
Assumed is that you entered the Gaia Archive with a registered account. A registered account has the benefits of a dedicated user space with more space and larger quota. If you get an error message that you have exceeded your file quota, but you did not according to the information in your account, first check if you have jobs that failed to complete. You could see whether deleting the jobs that failed. solves the issue. Even though they seem to take up no space (0 bytes), sometimes they do and deleting them will free up space.
Another quick test to try and see if you can solve the exceeded file quota yourself, is to clear your cache (so close your browser and start a new private window) and then check again.
|What are the newest features of the Gaia Archive?||The Gaia Archive team is continuously working towards making the Gaia Archive more efficient and easy to use. New releases of the Gaia Archive come out at regular intervals. Service interruptions are announced beforehand in the GUI (one-time pop-ups). On the Gaia Archive "New Releases" page you can find information on the current and previous releases.||Archive||-|
|What are the celestial coordinates of my HEALPix identifier?||
This table provides a link between the HEALPix identifier (level 6, so Nside = 2^6 = 64) and equatorial, Galactic, and ecliptic coordinates. The text file has four header lines and 49152 lines, each with seven columns separated by commas:
The conversion from Gaia source_id to HEALPix number with nest pixel ordering is described here. The built-in Archive function GAIA_HEALPIX_INDEX(order, source_id) can be used to do the transformation within ADQL queries, more details at the Archive Help - ADQL syntax (part 3). Additional functions available. Read here about ring and nested HEALPix ordering.
|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 Gaia 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.||Archive||-|
|Where can I find the column descriptions and units of the data fields?||The Gaia Archive data model (column description, units, etc.) along with the extensive documentation of the data and its processing can be found here, with the opportunity to download the full data release documentation.||Archive||-|
|Why does my query time out after 90 minutes? Why is my query limited to 3 million rows?||
There are limits on how much data can be retrieved via ADQL (TAP) and how long a query can last. They are different for anonymous and registered (authenticated) users.
Anonymous access (as documented here):
Authenticated users (after login):
The DataLink maximum number of sources that can be retrieved is equal for registered and anonymous users: 5,000 per query.
Anybody can self-register for a Gaia ESA Archive user account through this page and then "SIGN IN" using the button in the top-right corner of the Gaia Archive, after which the reported user quota apply. In exceptional cases, upon demonstrated need, users can request (temporary) changes to their quota by sending a motivated request to the Gaia Helpdesk.
|I can not find a well known bright star in the Gaia Archive. Why?||These stars might be too bright for Gaia. They saturate the sensitive CCD detectors on-board of Gaia. The brightest object included in Gaia DR2 has a G magnitude (phot_g_mean_mag) of 1.71 but incompleteness is large until G ≈ 3. So practically, no star visible to the naked eye made it into Gaia DR1. More information on this topic can be found in the Gaia Data Release documentation or more specifically here.||Archive||-|
|Why do some sources have null 'parallax', 'pmra' and 'pmdec' values?||A five-parameter astrometric solution, including proper motions and parallaxes, has been computed for a sub-sample of about 1,3 billion sources of the total of 1,7 billion sources in Gaia Data Release 2; and has been computed for the TGAS sub-sample comprising circa 2 million sources in Gaia Data Release 1. More information can be found here.||Archive||-|
|Why are some parallaxes less than zero?||Negative parallaxes are caused by errors in the observations. Even if a negative distance has no physical meaning, there are a certain number of stars expected to have negative parallaxes just from an error propagation perspective. The negative parallax tail is a very useful diagnostic on the quality of the astrometric solution. Further details can be found here and here.||Archive||-|
|How can I extract Gaia data for my list of targets with RA and DEC?||
There are two ways to do this:
|I have a problem uploading a list of targets in the simple form. Why?||
The file to be uploaded to the
The input file has no header and one entry per line. Three different name resolvers are tried: Simbad, NED and Vizier. An intermediate job with the retrieved source properties (name, coordinates, parallax and proper motion, if available) is created and cross-matched against the Gaia Archive. A mixture of object names and positions in the sky (see previous FAQ) is allowed.
|The load_tables() method of astroquery.gaia.Gaia returns an SSL error. What is going wrong?||
This problem most likely originates from the way Python is installed. Native Python distributions (e.g. those downloaded from python.org) include an “Install Certificate” script that install a set of SSL certificates. However, this is not the case if the user is running the Python distribution that comes along with the Conda package. A workaround is to execute the following command in a terminal:
|Is it possible to delete all the jobs in my jobs list at once?||
The simplest way to do this is to manually select all the jobs that are listed in the Archive and then press the "Delete selected jobs" button on the bottom right corner of the Archive webpage. An alternative to this potentially tedious task is to use the Astroquery.Gaia Python module (link).
The steps to do this are detailed below:
from astroquery.gaia import Gaia Gaia.login() jobs = [job for job in Gaia.list_async_jobs()] # To print all the jobs owned by the user: for job in jobs: print(job.jobid) # To remove all the jobs at once: job_ids = [job.jobid for job in jobs] Gaia.remove_jobs(job_ids)
|Is there a different behaviour when using integer or float division in the query?||Yes, when querying while using a division in the query, it makes a difference when one writes 1000/60 or 1000./60. The difference is that ADL does integer division in the first case and float division in the second case. Hence, results will be different!||Archive||-|
|Where can I find information on the Gaia Mission and Gaia data releases?||A dedicated website for the scientific community using Gaia data is in place: Gaia Cosmos. Gaia data release information is available from this website, as well as information on upcoming data releases. A set of frequently asked questions on the Gaia Mission and Gaia Data Releases is available from this list.||Archive||-|
|My question is not in this list. Who can help me?||
First check the internet and perform a search with your question. You probably find that the answer to your question is already around on one of the Gaia information pages.
Please also have a look at the Gaia Archive help pages which contains lots of helpful information and tutorials or perform a search through the Gaia Data Release Documentation and the Gaia Data Release papers (Gaia DR1, Gaia DR2). The question might be hidden in the information on some other Gaia Cosmos page or Gaia pages of our DPAC partners. When not sure what information is around for a certain data release, check out the overview pages for each release: Gaia Data Release 1, Gaia Data Release 2, Gaia Early Data Release 3, Gaia Data Release 3 and Gaia Focused Product Release.
If the internet search is not providing you with answers to your question, feel free to contact the Gaia Helpdesk. The Gaia Helpdesk helps out with questions on the Gaia Archive, use of the Gaia Archive, content of the Gaia Archive and more.
|How can I download the entire Gaia Source catalogue?||
Many of the catalogues hosted by the Gaia Archive (including the entire Gaia DR1, DR2, and eDR3 source catalogues) are available for bulk download in plain text (comma separated value, csv) format via the "Download" button visible in the landing page of the Archive. Because of the large volume of the data, the catalogue content is split in multiple files. To download all these files at once, it is suggested to use the "wget" command from a terminal. For example, the entire Gaia eDR3 source catalogue can be downloaded to the user local machine by typing the command:
|How can I install the latest version of Astroquery.Gaia?||
The Astroquery.Gaia Python module (included in Astroquery, an Astropy affiliated package) is under continuous development. When a new Astroquery version is released it can happen that, during a few weeks, the Astroquery.Gaia documentation is updated although its associated Python module remains the same when trying to install it via e.g.
$ pip install astroquery
In order to install the most up-to-date version of Astroquery you can type:
$ pip install --pre astroquery
If Astroquery is already installed in your machine, then make sure to include the --upgrade install option:
$ pip install --pre --upgrade astroquery
|Why are the column names of my uploaded table formatted to lowercase?||The ADQL language is case-insensitive (unless double quotes are used; see Sects. 2 and 2.1.7 of the ADQL 2.1 recommendation standard). Because of this, if a table uploaded by a user contains column names with uppercase letters, the Gaia ESA Archive will automatically convert them to lowercase letters. To learn how to upload a table to the Archive please take a look at this tutorial.||Archive||-|
|Are there any duplicated sources in the Gaia catalogue?||Duplicate sources are produced when the Gaia on-board detection software receives multiple triggers from one object. Although most multiple detections are successfully filtered out on ground, it can happen that the same celestial object has more than one source_id. The on-board detections originating from the same source are divided between the two sources, typically in such a way that one source has reliable astrometry with many transits whereas the other has suspect astrometry and a limited number of transits. Any duplicates with projected separation below 0.18” have been filtered out in Gaia EDR3, and for each duplicate only the source with the best astrometric solution has been kept in the catalogue. Although unlikely, it is possible that sources having a projected separation above 0.18” and nearly identical astrometry and photometry are duplicates. Would that be the case, we recommend to use quality indicators (like those indicated in Sect. 3.1 of the Gaia EDR3 summary article) to asses which one of the solutions contains the best astrometry.||Archive||-|
|Why are there cases with negative flux ratio F2/F1 in the binary_masses table?||
The binary_masses table is described in Gaia Data Release 3: Stellar multiplicity, a teaser for the hidden treasure (Gaia Collaboration, Arenou et al. 2022). Indeed, a few thousand entries in this table have fluxratio <= 0 and 2996 actually also have fluxratio_upper <= 0. Most of such cases are a natural outcome of the estimation process for fluxratio that has been used, analogous to the presence of negative parallaxes.
Symbolically speaking, with R denoting the flux ratio F2/F1, the (linear) semi-major axis of the astrometric photocentre equals a0= a (M2/(M1+M2) - F2/(F1+F2)) = a1 – a R / (1+R), where a denotes the relative semi-major axis. As a result, R is estimated using 1 / R = a / (a1-a0) – 1.
In the processing for Gaia DR3, R has been estimated using a0 from the astrometric processing of non-single stars, a1 = a1 sini / sini from the spectroscopic and astrometric processing, a from the period, M1 from the Hertzsprung-Russell diagram, and M2 from the spectroscopic mass function. Clearly, R is an estimated quantity with uncertainties (fluxratio_lower and fluxratio_upper). Although the true R is physically positive, estimates of it can be negative within the quoted uncertainties. Extreme cases, meaning when the flux ratio and its uncertainties are very unlikely compatible with being positive, point at special systems (for instance ternary systems) and/or inconsistencies in the astrometric and spectroscopic Gaia DR3 data processing of the object (for instance, when a0 >> a1, R would be negative, and in such cases, the spectra of the two components would not necessarily be resolved so that the “spectrocentre” variations could be measured instead). Such cases should obviously be treated with care.
|Why are there no inclinations for astrometric or astrometric-spectroscopic orbits in the nss_two_body_orbit table?||The nss_two_body_orbit table is described in Gaia Data Release 3: Stellar multiplicity, a teaser for the hidden treasure (Gaia Collaboration, Arenou et al. 2022). Indeed, in this table, the inclination is provided explicitly only for eclipsing binaries. However, for astrometric and combined astrometric-spectroscopic orbits, the Thiele-Innes orbital elements (A, B, F, G) rather than the Campbell elements (a0, i, Ω, ⍵) are provided. In these cases, the inclination (plus the two other angles as well as the semi-major axis of the photocentre) can be computed using the Python (or R) code provided on https://www.cosmos.esa.int/web/gaia/dr3-nss-tools.||Archive||-|
Indeed, all non-integer, non-Boolean, and non-character fields in all tables in the Gaia ESA Archive contain data that are stored and displayed to their full numerical precision, i.e., without taking into account the number of significant digits warranted by the uncertainties on the quantities. Although this seems not in line with common scientific practice, where measurements are typically provided with a number of significant digits that is compatible with their uncertainty estimates plus one extra digit, this has been a deliberate choice that has been made after careful consideration.
The key requirements in the discussion have been that data shall be immutable against the selected storage format (binary in the Archive database and ASCII in the bulk-download repository) and that data shall be stored with full numerical precision. In theory, usage of "numeric" datatypes at database level could support storage of data with a number of significant digits that varies from column to column but such a solution would have a strongly negative indexing performance impact and would produce quantisation effects in plots – especially when zooming in over a small dynamic range of a rounded quantity – without significant advantages, for instance through reduced storage needs or increased data transfer rates.
That still leaves the option to round the display resolution of the data on the fly while keeping the data stored in full numerical precision. Such rounding, however, cannot be applied on a per-column basis but would have to operate on a per-value-error pair (one row, two columns) since the rounding depends on the relative uncertainty of the measurement which often varies with source brightness. This, in turn, would require a complex (and risky) workflow which would require significant architectural changes in the Archive and which would come at a high performance price. More importantly, such a concept would always have a fundamental flaw in the area of user tables (or compound fields computed by users from published columns) by allowing inconsistencies in the resolution of catalogue data saved in the user area in comparison with the results of a query to the catalogue.
All in all, it has been decided to store and to display all measurements with full numerical precision and leave it up to the users, and their particular science cases, to apply the appropriate rounding.
|Why does my small (<150 MB) table not fit into my user space?||At first sight, one may be surprised to discover that the storage space for a table uploaded from a local machine to the Archive user space can be easily a number of times larger than expected, possibly raising an error with the message "UWS error [DB quota exceeded ]". In other words, a table of size 150 MB on a laptop can easily inflate to occupy ~900 MB in the Gaia Archive user space. The reason for this significant size increment is that the Gaia Archive uses a relational database to store the vast amount of data hosted in it. Tables in a relational database contain indexes that speed up the access to their content, and these indexes occupy significant disk space. An example of this index is the "<table_name>_oid" extra column that is automatically added by the Archive to each table uploaded to a user space.||Archive||20/11/2023|
The abstract of the Gaia DR1 release paper (Gaia Collaboration et al. 2016; http://adsabs.harvard.edu/abs/2016A%26A...595A...2G) states that "For the primary [TGAS] astrometric data set the typical uncertainty is about 0.3 mas for the ... parallaxes ... A systematic component of 0.3 mas should be added to the parallax uncertainties". This is confirmed at several places inside the paper. Is this really true? On the other hand, if a systematic error of 0.3 mas has been included already, how can many parallax standard errors in TGAS be close to 0.3 mas (or even be smaller)?