CFHT MegaCam Two Deep Fields Imaging Survey (2DFIS) I: Overview

We present the Two Deep Fields Imaging Survey (2DFIS), a wide-field imaging program conducted with the Canada-France-Hawaii Telescope (CFHT) targeting two astrophysically distinct regions: one containing a repeating fast radio burst (FRB) source and another hosting a candidate of a rotating galaxy cluster. Achieving a depth of r~26mag, the survey enables a search for faint optical counterparts and environmental signatures associated with the FRB, while high-quality photometric and galaxy shape measurements in the cluster field support a weak-lensing analysis of its mass distribution. This paper describes the observing strategy and data processing methodology adopted for 2DFIS, including the use of the LSST Science Pipelines with survey-specific adaptations for CFHT/MegaCam data. We outline a complete workflow for transforming raw CFHT exposures into science-ready data products, including calibrated single-epoch images, multi-band coadded mosaics, and extensive source catalogs. These data products provide the foundation for ongoing and future studies of FRB host environments, cluster mass reconstruction, and related cosmological applications.

💡 Research Summary

The paper presents the Two Deep Fields Imaging Survey (2DFIS), a dedicated CFHT/MegaCam imaging program that simultaneously targets two astrophysically distinct regions: (1) the galaxy cluster RX CJ0110.0+1358, identified as a dynamically “rotating” or merging system at z≈0.058 with a halo mass log M200≈14.4, and (2) the repeating fast radio burst source FRB190417 (also known as FRB 20181017A) discovered by CHIME. The primary scientific goals are to obtain deep, multi‑band optical data that enable (a) a weak‑lensing mass reconstruction of the cluster, probing sub‑structure and the interplay between baryons and dark matter in a non‑relaxed environment, and (b) a search for faint optical counterparts, possible strong‑lensing configurations, and host‑galaxy candidates associated with the FRB, thereby exploring the FRB’s local environment.

Observations were carried out during the 2022B semester under photometric or near‑photometric conditions. Both fields were imaged in u, g, r, and i bands using the standard CFHT DP4 dither pattern scaled by a factor of 1.5, with four exposures per filter. The cluster field received a total integration of 3.8 h (u: 4 × 960 s, g: 4 × 420 s, r: 4 × 920 s, i: 4 × 960 s) designed to reach a 5σ point‑source depth of r≈26 mag. The FRB field was observed for 3.4 h (u: 4 × 900 s, g: 4 × 200 s, r: 4 × 600 s, i: 4 × 650 s), achieving i≈25.2 mag. Image quality constraints required r‑band seeing between 0.65″ and 0.80″, and airmass limits of <1.2 for the cluster and <2.0 for the FRB field. MegaCam’s 1 deg × 1 deg field of view (0.187″ pixel⁻¹) and its excellent optics provide a homogeneous photometric calibration across the full area.

Data reduction employs the LSST Science Pipelines (v19.0.0) as the core framework, supplemented by the LoV oCCS pipeline for specialized weak‑lensing and photometric‑redshift processing. Raw CFHT exposures and calibration frames (bias, flats) are ingested into the LSST Butler repository, with metadata stored in a SQLite registry. Reference catalogs from Gaia, SDSS, and Pan‑STARRS are used for astrometric and photometric calibration. Standard detrending (overscan, bias subtraction, flat‑fielding, illumination correction) is applied on a per‑CCD basis, followed by PSF modeling for each exposure. Source detection and initial photometry are performed on the single‑epoch images, after which the exposures are coadded to produce deep multi‑band mosaics. The coaddition step includes sky background modeling, artifact masking (satellite trails, cosmic rays), and weight‑map generation.

For the weak‑lensing analysis, the LoV oCCS pipeline takes the LSST catalogs and carries out refined star‑galaxy separation, magnitude cuts, photometric‑redshift estimation (template fitting combined with machine‑learning techniques), and shape measurement using algorithms such as ngmix and im3shape. The resulting shear catalog and photometric‑z catalog provide the essential inputs for constructing a mass map of the rotating cluster and for investigating the large‑scale structure around the FRB field.

Quality‑control metrics demonstrate that the survey meets or exceeds its design specifications. The r‑band depth reaches ≈26 mag (5σ), the galaxy number density is 17–19 arcmin⁻² (comparable to or slightly higher than CFHTLenS), PSF model residuals are below 0.5 %, astrometric residuals are ≈30 mas, and photometric uniformity is better than 1 % across the field. These performance figures confirm that the data are suitable for high‑precision weak‑lensing studies and for deep searches of faint optical counterparts.

In summary, the paper details the full end‑to‑end workflow—from survey design and observing strategy to data reduction, calibration, coaddition, and catalog generation—demonstrating how modern, community‑developed pipelines can be adapted to CFHT/MegaCam data. The 2DFIS data set provides a valuable resource for (i) probing the mass distribution and dynamical state of a candidate rotating galaxy cluster, (ii) searching for optical counterparts and possible strong‑lensing signatures of a repeating FRB, and (iii) broader cosmological investigations of dark matter, large‑scale structure, and the environments of transient phenomena.