Observatory: Hubble vs. Roman
Expanding Our View
NASA's Hubble and Roman space telescopes will work together to broaden our perspective of the universe.
Named for NASA’s first chief astronomer, the Nancy Grace Roman Space Telescope will build upon Hubble’s decades of discovery by providing us with deep, panoramic views of the cosmos. NASA’s newest telescope isn’t a successor to Hubble or the James Webb Space Telescope. Instead, it joins NASA’s family of astrophysical observatories in their ongoing partnership of exploration.
This team of space explorers will work in tandem, increasing our knowledge of the universe in ways we expect and those we don’t. Roman will survey wide swaths of sky while Hubble and Webb continue to focus in on specific objects or regions of space. Adding Roman to the mix will enable science that complements the more detailed observations that Hubble and Webb do. Roman will provide panoramic views of the cosmic landscape in some visible and near-infrared wavelengths, while Hubble continues to provide detailed, high-resolution views in ultraviolet, all of the visible spectrum, and near-infrared light. Webb sees red to mid-infrared light, delivering a zoom-lens into deep space as it captures light that traveled from the outer reaches of the early universe. Complementing this trio is NASA’s Chandra X-ray Observatory that detects the signatures of matter whirling into black holes at the heart of distant galaxies, feisty young stars trying to find their equilibrium, and other hot, high-energy phenomena that emit x-rays.
As Roman surveys the sky, its discoveries will guide the others toward targets that require a closer look or a specific wavelength of light to understand. Roman not only complements and builds upon the work of its siblings; it will forge new ground by revealing subtle changes in our Milky Way galaxy and across the night sky.
Design
Both Hubble and Roman are reflecting telescopes. They use curved mirrors instead of lenses to collect and "bend" light to their various instruments. Hubble is a Ritchey-Chrétien Cassegrain telescope that uses two hyperbolic mirrors to direct light to its instruments. Hubble’s large primary mirror sends incoming light to its secondary mirror that is centered directly above the primary mirror. The secondary mirror guides that light back to the primary mirror where it passes through a hole in the center and to Hubble’s scientific instruments, located behind the mirror. Such Cassegrain designs offer a long focal length (the length of the path that incoming light takes to come into focus) in a relatively compact package by "folding" the light path back and forth. The longer the focal length relative to the diameter of the main or primary mirror, the higher the magnification and depth of field.
Roman's design is similar to Hubble's, but it uses three curved mirrors instead of two. Its mirrors are shaped to a precise surface curve that gives the telescope a focal length that is roughly three times shorter than Hubble’s. The shorter the focal length relative to the diameter of the main or primary mirror, the wider the field of view. Roman’s Three-Mirror Anastigmat design achieves better optical performance over a significantly wider field of view compared to Hubble’s design.
Both Hubble and Roman have the same size primary mirror (7.9 feet or 2.4 meters wide), but thanks to newer technologies, Roman’s primary mirror is more than four times lighter — 410 pounds (186 kilograms) compared to Hubble’s hefty 1,825 pounds (828 kilograms). Despite its considerable weight as compared to Roman, it could have been much heavier. Fabricators used a lightweight, honeycomb structure for Hubble's primary mirror’s interior, which reduced its weight from about 8,000 pounds (3,636 kg). Both the primary and secondary mirrors are made of a high-silicon, Ultra-Low Expansion Glass developed by Corning Glass Works.
Their mirrored coatings differ as well. Although it sees visible through near-infrared light — 0.48 microns (a shade of blue-cyan in the visible) to 2.30 microns (in the near infrared) — Roman’s coating is optimized for near-infrared light (1.0 micron to 2.0 microns). The telescope sports a 400-nanometer thick silver coating, which is 300 times thinner than a human hair. Newer technologies also gave Roman’s primary mirror a smoother finish than Hubble’s. Roman’s mirror is so flat that the average bump on its surface would be about a quarter-inch high (0.635 cm) if the mirror were the size of Earth! By comparison, the largest bumps on the surface of Hubble’s primary mirror would be six inches (15 cm) tall.
Hubble’s mirrored coating is made of aluminum and magnesium fluoride, which allows it to capture ultraviolet, the entire visible spectrum, and near-infrared light — 0.1 microns (in the ultraviolet) to 2.4 microns (in the near infrared). The reflective layer of aluminum is 3.9 millionths of an inch (0.1-microns) thick and a protective layer of magnesium fluoride (0.025-microns) thick. The magnesium fluoride layer is overlayed on top of the aluminum to protect it from oxidation and to increase reflectivity of ultraviolet light.
Although both primary mirrors are the same size, Roman has a much shorter focal length than Hubble, helping its Wide Field Instrument achieve a view that is 100 times larger. That broader vista gives Roman the ability to survey large swaths of sky in a short time. With more than three decades of observations, Hubble has only seen one-tenth of one percent of the sky. During its first five years, Roman will image 50 times as much sky as Hubble captured in 30 years.
Instruments
Roman’s Wide Field Instrument is the workhorse that will capture those large swaths of sky at an astonishing rate. It holds a 288-megapixel near-infrared camera with 18 detectors designed to survey vast chunks of sky, capturing a 0.8 by 0.4-degree field of view. That’s slightly more than the 0.5-degree width of the Full Moon in our night sky! In contrast, the Wide Field Channel of Hubble’s Advanced Camera for Surveys covers a 0.056 by 0.056-degree field of view. Its sensor array is also considerably smaller, only 16 megapixels, so it captures a much smaller segment of Hubble’s focal plane compared to the area Roman’s Wide Field Instrument captures.
Roman’s Wide Field Instrument will give us Hubble’s sensitivity and resolution (0.1 arcsec/pixel) at near-infrared wavelengths, while providing a panoramic view. However, in visible and ultraviolet light, no instrument outshines Hubble’s Wide Field Camera 3, which offers a high level of sensitivity and resolution (0.04 arcsec/pixel). Think of Roman as a wide-angle lens in infrared light while Hubble is the zoom lens in ultraviolet and visible light.
Like Hubble’s cameras (Wide Field Camera 3 and the Advanced Camera for Surveys), Roman’s Wide Field Instrument can also do spectroscopy across the instrument’s entire field of view. This capability, across such a large area of sky, will allow astronomers to better detect the motion of objects as well as their chemical composition, density, and temperature. That data will go a long way in helping us improve our understanding of the motions and evolution of the objects Roman observes.
Hubble has several instruments dedicated solely to doing spectroscopy. These include the Cosmic Origins Spectrograph (COS), the leading instrument for precision, high-resolution ultraviolet observations. COS increases Hubble's ultraviolet sensitivity by at least 10 times and up to 70 times when looking at extremely faint objects.
Astronomers also use Hubble’s Space Telescope Imaging Spectrograph (STIS) to obtain high-resolution spectra of resolved objects. STIS has the special ability to simultaneously capture spectra from many different points along a target, which allows astronomers to better understand the object’s composition, temperature, density, and motion.
The second of Roman’s two scientific instruments will capture images of distant planets. Roman’s Coronagraph holds a system of masks, prisms, detectors, and “deformable” mirrors designed to block starlight and capture images of faint planets orbiting other stars. The instrument’s advanced ability to adapt to conditions should provide researchers with 100 to 1,000 times the capability of previous coronagraphs.
Orbits
Hubble was designed for servicing by astronauts in space, so it needed to be in near-Earth orbit where astronauts could easily reach it with the space shuttle. Hubble’s current orbit is about 300 miles (483 kilometers) above Earth. Like the James Webb Space Telescope, Roman primarily sees infrared light. It needs a cold, thermally stable environment to see the faint heat signature of infrared light. Roman will orbit the Sun with Earth in the same area of space as Webb, roughly one million miles (1.5 million kilometers) away around the second Lagrange point or L2. Although both telescopes orbit L2, their orbits are enormous and far from each other or any other spacecraft orbiting L2.
There are several benefits of an L2 orbit, where the gravitational pull of the Sun and Earth balance, providing a stable orbit that uses very little fuel. This cold, stable orbit keeps Roman (and Webb) far from Earth's heat and light and the moon’s reflected sunlight, making it ideal for infrared astronomy.
The great advances in science usually result from new tools rather than from new doctrines.
Freeman John Dyson
British-American Theoretical Physicist and Mathematician
Science
Roman’s ability to capture enormous swaths of sky will expand and complement the partnership between NASA’s family of astrophysical observatories with its vistas, helping us answer questions that Hubble’s discoveries helped generate. Among these are dark matter and dark energy. In the 1990s, researchers using Hubble and other ground-based observatories discovered that the universe isn’t just expanding, but that the rate of expansion is accelerating instead of slowing down. This unseen force dominates the universe, yet we know so little about it. Roman’s broad view will collect a wealth of infrared light from objects both near and far. This data will help astronomers better understand the dynamics of the universe, seeing if dark energy changes over time.
Roman will also study elusive dark matter, by charting the motions of objects we can see. The large field of view of Roman’s Wide Field Instrument will reveal subtle changes in the motions of galaxies, galaxy clusters, and other visible matter. We can’t see dark matter, but Roman will detect its gravity by watching how visible matter moves through the universe. Hubble helped pioneer the study of dark matter, and Roman will carry that legacy forward.
Hubble’s sensitivity, high-resolution, and spectroscopic capabilities have uncovered many exotic worlds orbiting other stars. Hubble uses a technique called transit spectroscopy to see the spectral signature of starlight passing through an orbiting planet’s atmosphere as the planet passes in front of (transits) the star. Because different chemicals absorb specific wavelengths of light, Hubble can detect their presence in the atmospheres of these distant worlds. Roman’s Coronagraph will block a star’s light, revealing any planets that may be orbiting the star.
Roman’s ability to take high-resolution images across a wide field of view over a relatively short period of time will revolutionize how we see the universe. Its broad views will build upon and guide the tighter and deeper views Hubble and Webb are famous for. Hubble’s ever increasing 36-year data archive gives us a glimpse of subtle changes on a human time scale. Roman’s vistas captured over time will expand that knowledge exponentially. Its surveys will help us see changes in our galaxy and the universe. Along the way, Roman’s discoveries will help us answer some of our most pressing questions, but those discoveries will also generate new questions for future observatories to answer.
