Release notes of ERO data for the Euclid Consortium

Euclid Early Release Observations (EROs)

The ERO programme is an initiative of the Euclid Science Team / ESA to collect 1 day of observations to showcase the Euclid mission and its capabilities before the start of the nominal survey. These observations are not part of the nominal survey, addressing legacy science, and the data are meant to be released to the general community as soon as possible after the programme is completed.

A call for proposals was issued for Euclid observations with both communication/outreach and scientific merit, with priority to the communication/outreach merit. The call was addressed to the Euclid Science Collaboration consisting of the Euclid Consortium and the Independent Legacy Scientists. A total of 6 proposals were selected covering observations including a variety of objects at different distances covering different scales in the Universe. The teams supporting these proposals will carry out the first analyses of the data based on the science proposed.

Selected ERO Projects

The projects and best feasible targets were selected by the ERO programme committee:

  • A first glance at free-floating baby Jupiters with Euclid - Lead scientist: E. Martín (Instituto de Astrofísica de Canarias)
  • Euclid view of Milky Way globular clusters - Lead scientist: D. Massari (INAF-OAS Bologna)
  • A Euclid showcase of nearby galaxies - Lead Scientist: L. Hunt (INAF-AO Arcetri, Firenze)
  • A galaxy cluster seen with Euclid - Lead scientist: A. Lancon (Strasbourg Observatory)
  • A cluster of galaxies - Lead scientist: J.-C. Cuillandre (CEA, AIM, Université Paris-Saclay)
  • A Glimpse Into Euclid’s Universe Through a Giant Magnifying Lens - Lead scientist: H. Atek (Institut d’Astrophysique de Paris)

Data Collection

The ERO proposals and initial observations were selected before Euclid launch assuming the pre-launch schedule for the commissioning and the start of the nominal survey. The actual schedule had to account for the update of the Fine Guidance Sensor (FGS) software update, causing the start of the Performance Verification Phase in which the EROs should have been collected, several weeks later than planned.

Before the re-start of the Performance Verification Phase, the operations teams managed to schedule a number of observations before they would move out of the visibility window of Euclid. A subset of these observations could be executed successfully without guiding errors, and are used as ERO showcases. However, the majority of the successful ERO observations have been collected during the Performance Verification Phase in the period from 29 September until 3 December 2023.

Data Products and usage

The imaging data have been processed using processing pipelines developed by J.-C. Cuillandre (CEA, AIM, Université Paris-Saclay) based on the Euclid Level 1 data. The detailed description of the ERO processing will be published as a separate paper which will be part of the set of the ERO papers to be released in early 2024.

The ERO pipeline is designed from the ground up for diffuse emission science (low surface brightness Universe, LSB) and standard point/compact source science. It is a direct legacy from proven imaging pipelines developed for CCD and FPA mosaics at the CFHT over the past two decades with several steps relying on E. Bertin's AstrOmatic toolbox. It is fully independent from the processing functions developed by the Euclid Science Ground Segment, which are optimised for the generation of data products supporting the core cosmology.

The FITS files made available here are only accessible to all active Euclid Consortium members who are interested in assessing and possibly exploring the first Euclid data. Please take the following instructions into account:

  • Do not share the data with persons who are not part of the Euclid Collaboration until the public data release of the EROs in 2024
  • In case you wish to carry out research with these data, first consult the relevant EC Portal project page where all on-going projects are listed. If you are interested in joining the ERO team to contribute to an existing project or develop your own program by accessing its internal advanced data products, you should contact the proposal lead scientist. Alternatively, if joining the ERO team is not the preferred option after contacting the proposal lead scientist, you can provide a description of your project in the EC project portal.
  • Do not share images (JPEGs, PNGs TIFF, etc) before the embargo date imposed by ESA communications, on social media, mass media, or other media (e.g. slack).

ERO Internal Release 1: 2 Nov 2023

These data are associated with the communications products released during the media briefing on 7 Nov 2023. The data sets are processed stacks of the images taken in the VIS, Y, J, and H Euclid bands.

ERO Internal Release 2: 21 Dec 2023

ERO internal Release 2 covers the observations collected during the Euclid Performance Verification phase, except for three observations executed by the end of November and one set of stacks obtained from a collection of partly successful observations of the same field. The exceptional fields are still being validated by the Lead Scientists and will be released at a later stage. Besides the stacks of the images in the four Euclid bands, Release 2 includes source catalogues in the VIS and NISP bands. The fields of Release 1 have been reprocessed.


ERO internal Release 3 (ERO DR3) is a homogeneous reprocessing of the previously released fields with the inclusion of 4 new ERO fields. The VIS and NISP images from this release form the data set of which the first batch of ERO publications will refer to. ERO DR3 covers image stacks and validation catalogues of fields of all 6 selected ERO projects adding up to a total of 16 fields on the sky. A total of 10 million unique sources were extracted from the VIS images, and typically NISP is only 30% behind on each of the three NISP bands. The availability of the reprocessed data does not imply that the analyses derived with previous versions of the stacks have become invalid: a careful check that results are compatible on a small area ought to suffice. The detailed release notes describing the updated/improved processing steps as welll as field specific comments are provided below.

Instruction on access to the data files:

  • Register for access by opening a ticket at the ESA Euclid Helpdesk:
    • Please select ‘General’ as the department and ‘Euclid ERO access’ with your COSMOS username in the subject.
    • If you do not have a COSMOS account yet or do not have the account details, please open a ticket in the ‘Accounts’ department.
  • Access the data : You will need to log in with your COSMOS username and password.

We anticipate a high level of interest in this release. In order keep help everybody please download only the data which you are immediately interested in; the files will be available through this website for the indefinite future. If you are working at an institute with other Euclideans, please download the data files and distribute locally and let others know. Thanks!

Please use the #early-release-observations channel on the Euclid Consortium Slack workspace for questions, prompt updates will be posted there.