Euclid preparation: The NISP spectroscopy channel, on ground performance and calibration

This paper details the construction and ground testing of the flight model for ESA's Euclid mission's NISP spectroscopy channel, confirming its performance specifications across three operational grisms that provide diffraction-limited slitless spectroscopy over two wavelength intervals.

The Gist

Imagine the Euclid mission as a giant, cosmic detective sent by the European Space Agency to solve the universe's biggest mystery: Dark Energy and Dark Matter. To do this, Euclid needs to take a massive "family photo" of billions of galaxies, measuring how far away they are and how they are moving.

To take these photos, Euclid carries a special camera called NISP (Near-Infrared Spectrometer and Photometer). Think of NISP as a high-tech prism that doesn't just take a picture, but breaks the light from every galaxy into a rainbow (a spectrum). By analyzing these rainbows, scientists can tell exactly how fast the universe is expanding.

This specific paper is the "pre-flight checkup" for the most critical part of that camera: the grisms.

What is a Grism?

A grism is a clever piece of glass that acts like a hybrid between a prism and a diffraction grating.

• The Analogy: Imagine a prism is like a glass wedge that bends light. A diffraction grating is like a comb with tiny teeth that splits light into colors. A grism is a prism that has been carved with those tiny "comb teeth" directly onto its surface.
• The Job: When light hits a grism, it splits the light into a rainbow and bends it so that the rainbow lands perfectly on the camera's sensor. This allows Euclid to see the galaxy's image and its rainbow spectrum at the same time, without needing a slit (a narrow opening) to block out other light.

The Story of the Ground Tests

Before Euclid could launch into space, the team had to test the NISP instrument in a giant, super-cold vacuum chamber on Earth (in Marseille, France). They wanted to make sure the grisms worked perfectly under space-like conditions.

Here is what they found, broken down into three main chapters:

1. The "Super-Sharp" Vision (Optical Quality)

The team wanted to know: If we shine a tiny, perfect point of light through these grisms, how sharp will the resulting rainbow be?

• The Test: They shone a laser through the system and looked at the resulting "smear" of light on the detector.
• The Result: The images were incredibly sharp. The "blur" (called the Point Spread Function) was so small it was almost as perfect as the laws of physics allow.
• The Metaphor: Imagine trying to read the fine print on a newspaper from a mile away. Most cameras would make the letters blurry. These grisms are so good that they make the letters look like they are right in front of your face. They passed the "sharpness test" with flying colors.

2. The "Broken Compass" (The RGS270 Problem)

The instrument has four different grisms. Three of them are "Red" (for longer wavelengths) and one is "Blue" (for shorter wavelengths).

• The Issue: One of the red grisms, named RGS270, was built with a slight mistake. The engineers had to orient the "comb teeth" on the glass in a specific direction to correct for the tilt of the telescope. Unfortunately, the factory built it with the teeth facing the wrong way.
• The Consequence: When they tested it, the rainbow from RGS270 got blurry and out of focus, especially at the red end of the spectrum. It was like trying to read a book where the pages were slightly curled and out of focus.
• The Fix: The team couldn't swap the part in space (it's glued in!). Instead, they came up with a clever workaround. They decided to rotate the other two working red grisms slightly (by 4 degrees). This creates a new angle for the rainbows, allowing them to untangle overlapping galaxy spectra just as well as the broken one would have. It's like realizing you can't use your left hand to open a jar, so you just rotate the jar and use your right hand instead.

3. The "Rainbow Map" (Dispersion Calibration)

To measure distances, scientists need to know exactly which color corresponds to which position on the camera.

• The Test: They used a special light source (a Fabry–Pérot etalon) that creates a series of perfectly known "ticks" or lines across the spectrum, like the markings on a ruler.
• The Result: They mapped out exactly how the light bends across the entire camera. They found that the "ruler" is incredibly accurate. The spacing between the colors is consistent and predictable.
• The Metaphor: Think of the grism as a mapmaker. They needed to ensure that if you walk 10 steps on the map, you actually travel 10 miles in reality. The tests confirmed that Euclid's mapmaker is precise to within a fraction of a pixel.

The Final Verdict

The paper concludes that the NISP instrument is ready for space.

• The Good News: The three working grisms (BGS000, RGS000, and RGS180) are performing better than the scientists even hoped. They are sharper and more precise than the requirements demanded.
• The Workaround: Even though one grism (RGS270) is broken, the team has a solid plan to work around it without losing any scientific data.
• The Goal: With these tools, Euclid will be able to measure the distances of 25 million galaxies, helping us understand why the universe is expanding faster and faster.

In short, this paper is the "quality control report" that says: "The camera is built, the lenses are perfect, the map is accurate, and we have a backup plan for the one broken piece. Euclid is ready to go!"

Technical Summary

1. Problem and Context

The European Space Agency's (ESA) Euclid mission aims to map the geometry of the dark Universe by measuring the expansion history and growth of cosmic structures. A critical component of this mission is the Near-Infrared Spectrometer and Photometer (NISP), which performs slitless spectroscopy to obtain redshifts for approximately $2.55 \times 10^7$ galaxies.

The primary scientific challenge addressed in this paper is the ground-based validation and calibration of NISP's spectroscopic channel before launch. Specifically, the team needed to:

• Verify that the instrument's Point Spread Function (PSF) and optical quality meet the stringent requirement of $\sigma_z \leq 0.001(1+z)$, necessitating a spectral resolution $R \geq 380$ for 0.5" sources.
• Characterize the dispersion properties of the four flight grisms (three red, one blue) to ensure accurate wavelength-to-pixel mapping.
• Address a critical non-conformity discovered in one of the red grisms (RGS270) during testing, which threatened the mission's ability to decontaminate overlapping spectra.

2. Methodology

The authors conducted extensive ground testing of the NISP flight model in the ERIOS vacuum chamber at the Laboratoire d'Astrophysique de Marseille (LAM) in 2019–2020. The instrument was cooled to operational temperatures (~135–140 K for optics, ~85 K for detectors) to simulate space conditions.

Key Experimental Setup:

• Light Sources: A point-source simulator (pinhole + off-axis mirror) illuminated the instrument. Light was provided by a tunable monochromator, a Fabry–Pérot etalon (providing 64 emission lines for calibration), and an Argon spectral lamp for validation.
• Optical Quality Assessment: The team acquired hundreds of dithered exposures to over-sample the PSF. They analyzed the Encircled Energy (EE50 and EE80) radii using three different PSF models: asymmetric Gaussian, dual-Gaussian, and elliptical Moffat profiles.
• Dispersion Calibration:
  • The Fabry–Pérot etalon was projected onto 144 points across the focal plane.
  • Spectral traces were modeled using Chebyshev polynomials to map wavelength to pixel coordinates ($y, z$).
  • The 0th-order position was determined via template fitting to serve as a reference.
  • An Argon lamp was used to validate the derived dispersion laws independently.
• RGS270 Investigation: Upon discovering defocusing in the RGS270 grism, the team analyzed the optical design errors and proposed a survey strategy modification (rotating the grism wheel by $\pm 4^\circ$) to compensate for the loss of the third spectral orientation.

3. Key Contributions

• Comprehensive Ground Characterization: This paper provides the first complete on-ground performance report for the NISP spectroscopic channel, establishing the baseline for in-flight operations.
• PSF Modeling Validation: The study rigorously tested three PSF models against ground data, confirming that all provide robust estimates of image quality, with EE50 values significantly better than requirements.
• Dispersion Law Derivation: The authors derived a high-precision 2D mapping of the spectral dispersion across the entire field of view (FoV), accounting for optical distortions and curvature.
• Crisis Management for RGS270: The paper details the root cause analysis of the RGS270 failure (incorrect propagation of the optical reference frame to the mechanical frame) and the approved mitigation strategy involving a modified observation sequence using only RGS000 and RGS180 with rotational offsets.
• Data Products: The authors released calibration coefficients and throughput data (0th and 1st order) for the operational grisms.

4. Results

Optical Quality:

• The measured image quality is diffraction-limited and close to theoretical expectations.
• EE50 (radius containing 50% of energy) is approximately 0.6 times smaller than the scientific requirement ($<0.3''$ at 1500 nm).
• Testing confirmed that rotating the grism wheel by $\pm 4^\circ$ (to mitigate RGS270 issues) does not degrade image quality.

Dispersion and Resolution:

• Dispersion Values:
  • Blue Grism (BGS000): $\approx 1.24$ nm/pixel.
  • Red Grisms (RGS000/RGS180): $\approx \pm 1.37$ nm/pixel (opposite directions).
• Spectral Resolution ($R$):
  • For a 0.5" source, the resolving power ranges from $R \approx 440$ to 690 for the blue grism and $R \approx 550$ to 740 for the red grisms.
  • These values significantly exceed the minimum scientific requirements ($R \geq 380$ for red, $R \geq 260$ for blue).
• Calibration Precision:
  • The dispersion calibration error is $< 0.04$ pixels in the cross-dispersion direction and $< 0.06$ pixels in the dispersion direction.
  • Validation with Argon lines confirmed predicted positions fall within $\approx 0.5$ pixels of measured positions.

RGS270 Status:

• RGS270 was found to be partially defocused due to a manufacturing/design reference frame error.
• Decision: RGS270 will remain on board for mass balance but will not be used for science. The survey strategy was modified to use RGS000 and RGS180 with $\pm 4^\circ$ offsets to generate the necessary spectral orientations for decontamination.

5. Significance

This paper confirms that the NISP instrument is fully capable of meeting Euclid's ambitious cosmological goals.

• Mission Assurance: The validation of image quality and dispersion laws ensures that the mission can achieve the required redshift accuracy ($\sigma_z \leq 0.001(1+z)$) for galaxy clustering and baryon acoustic oscillation (BAO) measurements.
• Operational Readiness: The successful derivation of the dispersion law and the mitigation plan for RGS270 allow the mission to proceed with a robust observation strategy, ensuring the collection of high-quality spectroscopic data for $\sim 14,000$ deg$^2$ of the sky.
• Scientific Impact: By confirming that the spectral resolution and optical quality exceed requirements, the paper validates the potential for Euclid to provide precise constraints on dark energy parameters ($w_p, w_a$) and the nature of dark matter.

The data products and calibration coefficients described in this work are essential for the Euclid data processing pipeline and will be used to process the massive dataset expected from the mission's 6-year survey.