Earth's Atmosphere and the Need for Space Telescopes
Telescopes on the Earth must contend with both atmospheric turbulence and atmospheric absorption. Over the last few decades, astronomers have developed sophisticated Adaptive Optics (AO) systems that can be used to sharpen the images of ground-based telescopes. However, such systems only work over relatively small fields, are effective only at wavelengths longer than ~0.5 μm, and often come at the cost of spatial and time varying "point spread functions" (PSFs). For most scientific studies, it is vitally important to have sharp and stable PSFs.
Ground-based images in the visible region are blurry because the Earth's atmosphere absorbs and scatters starlight. At red-optical and infrared wavelengths (i.e., longer than λ~0.8 μm), observations are possible only in discrete and usually narrow "windows" — at least until one reaches the broad radio window at ~1 cm. In the ultraviolet (UV) region (i.e., at wavelengths shorter than ~0.3 μm), the atmosphere is almost entirely opaque due to the combination of ozone absorption and Rayleigh scattering.
Observing from space offers both unimpeded access to the UV region and a much sharper view in the optical than is possible from the ground. This was the original motivation for the Hubble Space Telescope (HST) whose razor-sharp images in the UV, optical and IR regions have revolutionized astronomy and delighted the public for more than a quarter century.
Atmospheric absorption in the UV, visible, near-IR, and mid-IR wavelength regions. The Euclid mission and Roman space telescope focus on the red-optical and near-IR regions, whereas CASTOR would operate in the blue-optical and UV regions, which on the ground are strongly affected by molecular oxygen and ozone absorption.
Importance of the UV/Blue-Optical Region in Astronomy
Astronomers sometimes speak of the "UVOIR" region — the portion of the electromagnetic spectrum that runs from the Lyman limit at ~0.091 to ~5 μm, the approximate upper boundary of the near-IR region, above which ground-based observations become nearly impossible due to strong water absorption in the Earth's atmosphere. This UV-Optical-IR region holds a special significance in astronomy as it contains a uniquely high density of astrophysical information and is critically important for understanding both stars and plasmas up to temperatures of ~one hundred thousand Kelvin — arguably the two most important physical tracers in modern astrophysics. Within the UVOIR wavelength range, the blue-optical and UV region is particularly important for the study of stars, galaxies, planets, the intergalactic and interstellar medium, active galactic nuclei (AGNs). Uninterrupted coverage over the entire UVOIR, at high angular resolution and high sensitivity, is thus necessary for a full understanding of nearly all astronomical objects — from exoplanets to cosmology.
In the next decade, a fleet of new space telescopes (JWST, Euclid, Roman) is poised to open a new window into the red-optical and IR universe. At shorter wavelengths, astronomers will need to rely on ground-based telescopes and the Hubble Space Telescope (HST), which provides direct access to the UV region that is unobservable from the ground. However, ground based telescopes must contend with atmospheric absorption and turbulence, as well the bright nighttime sky. Meanwhile, HST is limited by its small field of view. And since it relies on a number of critical subsystems, such as batteries and gyroscopes, HST may fail sometime in the next decade.
Operating high above the blurring effects on the Earth's atmosphere, CASTOR would survey huge swaths of the sky at blue-optical and UV regions, with a resolution comparable to the Hubble Space Telescope (and at least five times better than is possible from the ground at these wavelengths). In a single exposure, CASTOR would cover an area roughly one hundred larger than HST, and do so simultaneously in three bands that fully cover the UV/blue-optical region from 0.150 to 0.55 μm.
At a distance of 2.5 million light years, Andromeda is the giant galaxy nearest to the Milky Way and has historically been a favourite target of large observing campaigns with both ground- and space-based telescopes. The most ambitious study of Andromeda to date was carried out with the Hubble Space Telescope which covered roughly a third of its disk with 414 separate pointings, shown in the panel to the left, each of which was imaged six times in six different filters over a 40-day campaign — one of the largest surveys ever undertaken by Hubble. Thanks to its revolutionary optical design, CASTOR could cover the entire disk of Andromeda in only six pointings, as shown in the right panel. Furthermore, each pointing would produce simultaneous images in three different filters, giving us, by default, a panchromatic view of the galaxy. Hubble’s and CASTOR’s vastly different fields of view are superimposed on the full moon, which is shown to scale at the centre of this image.
A portion of the COSMOS field — one of the deepest and most intensely studied fields in the sky — is shown in the left panel as seen by NASA’s GALEX satellite which was launched in 2003 and deactivated in 2013. On the right, the same field is shown as it would be seen by CASTOR, whose resolution matches that of the legendary Hubble Space Telescope but covers a field that is roughly 100 times larger.
Legacy Science from CASTOR
CASTOR would rely on an operations model that has been shown by many previous missions to address ambitious and fundamental questions in science, while at the same time having the flexibility to address the needs of diverse research communities. Through a combination of legacy surveys and Guest Observer programs, CASTOR would have an impact spanning a huge range of astrophysics. Representative science drivers include:
Cosmological surveys focusing on the mass power spectrum, the distribution of dark matter, and tests of gravity on cosmological scales.
Unique high-sensitivity UV access to a vast array of astrophysical transients ranging from tidally disrupted stars to gravitational wave events.
The evolution of cosmic star formation, on sub-galactic scales, including the connection between the growth of stellar mass to the assembly of dark matter halos.
Echo mapping of AGNs to probe the geometry, kinematics and physical conditions of photo-ionized gas in active galaxies.
The discovery of new Galactic satellites and streams, and the outermost structure of our Milky Way galaxy.
The UV/blue-optical properties of stars of all sorts, ranging from young and hot stars, degenerate objects and even chromospheric activity in M dwarfs.
The star formation and chemical enrichment histories of nearby galaxies and clusters.
Characterization of exoplanet atmospheres from time series transit photometry and spectroscopy, and phase curve analyses.
The identification of the smallest and/or most remote objects in the outer solar system, as well as the surface chemistry of small bodies from their UV to IR spectral energy distributions.
Depth of wide-field imaging surveys as function of wavelength. Results are shown for Rubin Observatory's LSST survey, the Euclid Wide survey, the Roman Space Telescope's High Latitude Survey (HLS) and CASTOR's primary survey. The labels under each filter indicate image quality (EE50 radius ~ 0.6*FWHM) for each survey.
CASTOR is currently base-lined for a five-year mission lifetime, with a possible launch as soon as the mid-2020s, to maximize synergies with the JWST, Rubin, Euclid and Roman telescopes, as well as many other multi-wavelength facilities covering the full electromagnetic spectrum, from gamma rays to radio wavelengths.