June 7, 2023

Instrument measurements

Euclid’s instruments will, first and foremost, acquire images and spectra with an extremely high level of sensitivity from light emitted by far-off galaxies. The mission’s goal is to:

Study the spatial distribution of galaxies over the ages by measuring the distances between them using baryonic acoustic oscillations (BAOs). This will be the task of the Near Infrared Spectro Photometer (NISP).
Precisely image galaxy distortions due to weak gravitational lensing (WL) at different distances and epochs. This will be the task of the VISible Instrument (VIS).

In most domains of astrophysics, from planets to the outer reaches of the universe, analysing light reveals the properties and physical mechanisms at work. In our desire to understand the universe, the challenge is to collect light with Earth-based and space telescopes. For the Euclid mission, sending a telescope into space is the only choice for two main reasons due to a single culprit: Earth’s atmosphere, which has the unfortunate tendency to absorb infrared light. If Euclid were on Earth, its NISP instrument would see only blurred images or be completely blind.

What’s more, movements of air masses in the atmosphere would, like an aircraft flying through turbulence, cause light from the stars to quiver and thus blur imagery. While adaptive optics technologies now exist to limit these undesirable effects in tiny portions of the sky for Earth-bound telescopes, the precision required to measure the shape of galaxies over a third of the whole sky means that we need to be totally free of the atmosphere’s obscuring effects, which is why we have to send a satellite into space.

Spectral domains in which Euclid’s instruments will measure: visible (550 nm to 900 nm) for the VIS instrument and infrared (900 nm to 2,000 nm) for the NISP instrument. Credit: Patrice Amoyel

The two instruments will acquire images in two dimensions. To add the third dimension, along the line of sight, we need to know the distance between each light source and the telescope. To do that, it’s vital that we measure a third parameter: redshift, noted z, so called because wavelengths of radiation are stretched—or “shifted”—on their journey through an expanding universe. Redshift provides a very easy way to calculate the speed at which a galaxy is moving away from us. This is how Edwin Hubble established his famous law by measuring distance using standard candles, the Cepheid variables discovered by the astronomer Henrietta Leavitt in 1908. Since Hubble, and using standards candles—type 1a supernovae and even quasars—that enable us to calculate much greater distances, astronomers have extended the law to distances as far as modern telescopes can see. This law, which serves to validate the standard cosmological model, therefore allows us to determine distance when the redshift is known.

Distance as a function of redshift. The distance D is given on a logarithmic scale called the distance module, noted μ (where μ = 5.log10(D) – 5, with the distance D in parsecs). From Suzuki et al. (2012).

But the image of a galaxy travels at the speed of light, so the image we see is of light emitted in the past, and the further it comes from, the further we can look back in time. Taking into account the speed of light and the expansion of the universe derived from the standard cosmological model, we can estimate when the light we see today was emitted. So the light sources—mostly galaxies—imaged by Euclid’s instruments will be associated with a redshift value. It will therefore be possible, according to the z value and the epoch, to determine the distribution and shape of galaxies, to measure their brightness and many other useful parameters to gain new insights into our universe, its components and its evolution over time.

Epoch (in billions of years) as a function of redshift. Credit: Georg Feulner (University of Munich).

We can see, for example, that a redshift z=2 corresponds to about 10 billion years in the past.

Images from the two instruments will therefore have to be combined with redshift measurements.

Studying the distribution of galaxies as a function of time requires relatively precise redshift measurements and excellent spectral resolution. Euclid’s Near Infrared Spectro Photometer (NISP) will measure redshift by spectrophotometry, in other words, by dispersing collected light into spectra. These spectra will reveal, at certain characteristic wavelengths, emission lines that will allow us to identify the atoms and molecules making up galaxies, particularly hydrogen atoms (as galaxies consist primarily of stars and interstellar clouds that are 90% hydrogen).

Determining redshift by spectrophotometry. After dispersing light with a prism or grating, we can identify the redshift of a characteristic emission line λ0 that hence takes the value λ. The redshift z is equal to (λ-λ0)/λ0 . Credit: A. Debus

Depending on the expansion velocity, and therefore the distance (according to Hubble’s law), and, as we have just seen, the epoch, the spectrum is shifted towards longer wavelengths and therefore red. In practice, it is relatively easy to work with the Hα visible spectrum line in the Balmer series (656.3 nm in the red). This redshift is calculated simply as the variation in wavelength of a line relative to its reference value λ0.

NISP will acquire both images and spectra using slitless spectrometry, imaging light sources and at the same time dispersing their light in different directions. Images and spectra will subsequently be processed on the ground during the science operations phase. The aim is to acquire images and spectra of 35 million galaxies.

Slitless spectrometry. Images of light sources (galaxies, white dots) and their spectra (red oval) are shown in the same acquisitions. Credit: Euclid Consortium/Audrey Le Reun/ Yannick Copin

The VISible Instrument (VIS) is aiming to image 1.5 billion galaxies. While image resolution must be excellent, redshift measurement precision and resolution do not need to be as good as for NISP. It will be determined photometrically by measuring the brightness of each light source in specific spectral bands, using filters. The band in which visible images are acquired by VIS, between 550 nm and 900 nm, is too wide and, most of all, does not cover enough wavelengths. Redshifts will therefore be determined from images acquired in two ways:

In the infrared by NISP, which as well as acquiring spectral images will generate images in the same field of view as VIS through three IR filters: YE (950 nm-1,212 nm), JE (1,168 nm-1,567 nm) and HE (1,522 nm-2,021 nm).
In the visible with ground telescopes acquiring images and measurements through five filters: u (311 nm-397 nm), g (398 nm-552 nm), r (566 nm-714 nm), i (699 nm-854 nm) and z (849 nm-1,002 nm)

Determining redshift (z) by photometry. The redshift (red curve) means that the mean brightness values measured in each filtered spectral band are no longer the same (white curve). Redshift is therefore determined according to libraries of brightness curves and values. Credit: A. Debus

Euclid will therefore work with ground-based telescopes to improve the determination of redshift in photometric spectra, and to model point spread functions (PSF) of individual galaxies.

Eight telescopes at three sites in Spain, Hawaii and Chile will therefore be taking part in the Euclid project. Of these, six will be involved in wide-field photometry surveys:

Canada-France-Hawaii (CFHT)
Javalambre Space Telescope (JST) in Spain, operated by CEFCA (Centro de Estudios de Física del Cosmos de Aragón)
Pan-STARRS (Panoramic Survey Telescope And Rapid Response System) in Hawaii
Blanco Telescope at the Cerro-Tololo Inter-American Observatory (CTIO) in Chile
LSST (Legacy Survey of Space and Time) at the Vera Rubin Observatory, also in Chile
HSC (Hyper Suprime-Cam) mounted on the prime focus of the Japanese Subaru Telescope in Hawaii

The two other telescopes will be used for calibrating photometric redshifts from spectroscopic redshifts: