Euclid Q2 Release Documentation

The Q2 release of the Euclid Galactic Bulge Survey (EGBS) provides calibrated VIS imaging data, associated maps, calibration catalogues and analysis tools to support scientific exploration. The EGBS data were acquired between 23–24 March 2025 during a dedicated one-time survey of the Galactic Bulge.

Key Contents

• Calibrated VIS frames (DET)
  Multi-extension FITS files including science images, flag maps, and noise maps
• Background maps (BKG)
  Estimated sky background for each exposure
• Weight maps (WGT)
  Pixel-level exposure weights based on detector response
• Calibration source catalogues (CAT)
  Detected sources with associated metadata (FITS + XML)
• PSF models
  PSFEx-based models for image analysis and calibration

Observations

• The release is based on three observation sequences:
  • 067068 – PSF calibration field (16 exposures)
  • 067070 – Main science field (144 exposures)
  • 067071 – PSF calibration field (16 exposures)


• The calibrated frames and catalogues provided in Q2 correspond to:
  • Science data: obsid = 067070
  • PSF calibration data: (067068, 067071) used to derive PSF models (not distributed as calibrated frames)


• Coverage organised into 9 fields, each sampled across multiple dithers

Calibration source catalogues

• Catalogues are available for each VIS frame in .xml and .fits format.

Please note: The calibration source catalogues are intended for calibration purposes and are not suitable for statistical analyses. For further details, please refer to the Q2 paper describing the data products.

File Naming Convention

All files follow a consistent naming pattern:

{product}-{obsid}-{dither}-{expnum}

• obsid – Observation identifier
• dither – Sequential dither number
• expnum – Exposure number within the dither (always 1)

Data Products

The Q2 release includes the following VIS data products:

• Calibrated Frames (DET) - EUC_VIS_SWL-DET-{obsid}-{dither}-{expnum}__*.fits
  VIS calibrated frame that includes the science image, the map of flags and the noise map. See below how to split all these images in different files.


• Background Maps (BKG) - EUC_VIS_SWL-BKG-{obsid}-{dither}-{expnum}__*.fits
  Background image of the corresponding calibrated frame, generated by GnuAstro NoiseChisel.


• Weight Maps (WGT) - EUC_VIS_SWL-WGT-{obsid}-{dither}-{expnum}__*.fits
  VIS calibrated exposure weight map. It's the master PRNU image with all invalid pixels set to zero.


• Source Catalogues (CAT) - EUC_VIS_SWL-CAT-{obsid}-{dither}-{expnum}__*.fits
  Calibrated frame catalogue.


• Source Catalogues (CAT) - EUC_VIS_SWL-CAT-{obsid}-{dither}-{expnum}__*.xml
  Metadata associated to the VIS calibrated frame catalogue stored in the Euclid Archive System (EAS).

PSF Products

• PSF models are provided in the PSFEx_model/ directory
• Derived from dedicated PSF calibration observations (obsid = 067068, 067071)

Data Organisation

Dither Strategy and Field Coverage

Each field is observed multiple times using a dither pattern:

• 16 exposures per field
• Observed at 4 slightly offset positions, repeated 4 times

Dither indices correspond to spatial fields as follows:

Detector Geometry

Each exposure corresponds to the VIS focal plane:

• 36 CCDs, each divided into 4 quadrants
• Total: 144 quadrants per exposure

It is possible to determine the field of observation based on the dither number. As illustrated in the image below:

Table 1: Euclid Galactic Bulge Survey field centres

Field number	RA (Deg, J2000)	Dec (Deg, J2000)
1	267.425	-30.019
2	267.441	-29.259
3	267.456	-28.499
4	268.248	-28.610
5	268.237	-29.369
6	268.227	-30.129
7	269.030	-30.236
8	269.036	-29.476
9	269.041	-28.716

Each field (a focal-plane array) is divided into 144 quadrants (36 CCDs, each made of 4 quadrants), as shown in the image below:

From Euclid Quick Data Release (Q1) : VIS processing and data products (Euclid Collaboration: H. J. McCracken et al. 2025)