Euclid Quick Data Release (Q1): From spectrograms to spectra: the SIR spectroscopic Processing Function

TL;DR

The paper presents the SIR spectroscopic Processing Function (PF) for Euclid’s slitless NISP-S data, detailing how raw spectroscopic exposures are transformed into calibrated 1D spectra for millions of sources in the Q1 release. It outlines a modular processing architecture with scientific pipelines (spectra extraction and spectra combination), calibration pipelines (maps, distortion and flux calibrations), and a validation stream, all interfacing with MER photometry and external data to produce robust spectra with accompanying metadata. The Q1 results show approximately $3.0\times 10^6$ validated spectra from red-grism observations, with wavelength accuracy within mission requirements and resolving powers in the $R\approx 500$–$700$ range, indicating high optical performance and effective contamination mitigation. The paper also discusses data quality control, validation against top-level requirements, and planned enhancements (persistence correction, improved 0th-order modeling, better background handling, and extended decontamination) to further improve reliability for future data releases and cosmological analyses.

Abstract

The Euclid space mission aims to investigate the nature of dark energy and dark matter by mapping the large-scale structure of the Universe. A key component of Euclid's observational strategy is slitless spectroscopy, conducted using the Near Infrared Spectrometer and Photometer (NISP). This technique enables the acquisition of large-scale spectroscopic data without the need for targeted apertures, allowing precise redshift measurements for millions of galaxies. These data are essential for Euclid's core science objectives, including the study of cosmic acceleration and the evolution of galaxy clustering, as well as enabling many non-cosmological investigations. This study presents the SIR processing function (PF), which is responsible for processing slitless spectroscopic data. The objective is to generate science-grade fully-calibrated one-dimensional spectra, ensuring high-quality spectroscopic data. The processing function relies on a source catalogue generated from photometric data, effectively corrects detector effects, subtracts cross-contaminations, minimizes self-contamination, calibrates wavelength and flux, and produces reliable spectra for later scientific use. The first Quick Data Release (Q1) of Euclid's spectroscopic data provides approximately three million validated spectra for sources observed in the red-grism mode from a selected portion of the Euclid Wide Survey. We find that wavelength accuracy and measured resolving power are within requirements, thanks to the excellent optical quality of the instrument. The SIR PF represents a significant step in processing slitless spectroscopic data for the Euclid mission. As the survey progresses, continued refinements and additional features will enhance its capabilities, supporting high-precision cosmological and astrophysical measurements.

Figures (14)

• Figure 1: Illustration of the various exposures entering the SIR pipeline for object ID 2684805874647806467, a $z=1.63$ galaxy. Upper left:;;50-cutout from the MER -band stack, centred on the object (green contour). Upper right: close-up on DET42 of preprocessed background-subtracted RGS000+0 spectroscopic exposure (pointing ID 11953) around the spectrogram of the same object (blue box). Bottom: zoom into the blue box in SIR coordinates; the effective extraction window is indicated as a green box, and the position of the reference wavelength $\lambda_{1}$ is marked with a star, while original pixels flagged as unusable are in grey. We note the bright (saturated) 0th-order spectrogram in the lower right, as well as low-level persistent traces of previously-observed tilted 1st-order and 0th-order spectrograms.
• Figure 2: Illustration of the application of the detector scaling product to a small section of the DET11 images: (a) the detector scaling image centred on the 'duck' structure (see Sect. \ref{['sec:cal-ff']}); (b) the same section of a dispersed image prior to correction, and (c) after the application of the detector scaling. The 'duck' structure has been successfully mitigated.
• Figure 3: Illustration of the basic decontamination procedure for a line-emitting source. The top panel shows the original RGS000+0 spectrogram of object ID 2709725257636288279 in SIR coordinates, i.e., the $y$-axis being the spatial direction and $x$-axis effectively the dispersion direction. The middle panel shows the model for the bright contaminant, in this case created from broadband photometry, which affects a part of the target spectrogram. The bottom panel shows the decontaminated spectrogram of the source of interest. The flux scaling of all panels is the same.
• Figure 4: Illustration of the different steps in the extraction of a spectrogram (right) of an extended source, along its photometry thumbnail (left). Top: original orientation in the FP. Middle: rotation to bring the dispersion direction to horizontal. Bottom: shear to bring the virtual slit along the cross-dispersion direction and minimise self-contamination. This illustration is a simplified case with no initial tilt or curvature in the spectral trace; furthermore, in practice, rotation and shear are performed in a single step to minimise correlations between resampled pixels.
• Figure 5: The four $5 \times 531$ pixels (corresponding to $\ang{;;1.5} \times 711.54nm$) resampled spectrograms for object ID 2684805874647806467 (RGS000+0, RGS180+4, RGS000-4 and RGS180+0 from top to bottom) from observation ID 2712. Resampled pixels flagged as unusable (NOT_USE) are in grey.