Universe Today
The Nancy Grace Roman Space Telescope (NGRST) is on its way to its station at the Sun–Earth L2 point, where it will begin its projected 5-year mission. It has several goals, including measuring dark energy's effect on the Universe, the growth of structure, and general relativity's consistency over time. Gravitational lensing plays a big role in its work.
There are two kinds of gravitational lensing, strong and weak. In strong gravitational lensing, a massive foreground object like a galaxy cluster lines up with a background object and produces magnified and distorted images of the background object, making the background object open to observations. It can produce Einstein rings, arcs, and multiple images of the background object.
These distortions let scientists measure the mass of the foreground galaxies. Since visible matter isn't enough to cause the distortion on its own, scientists can do some fancy math to calculate the amount of dark matter that must be present. That, in turn, will help them understand dark energy, the growth of structure, and general relativity.
In this image, the red foreground galaxy produced two arcs of light that are distorted, multiplied images of a background galaxy; a large arc on the top and a smaller one on the bottom. With images like this, astronomers can measure the total mass in the foreground galaxy and the degree of magnification of the background galaxy. By comparing the two, they can determine the amount of dark matter present in the foreground galaxy. Image Credit: NASA, ESA, and X. Huang (University of San Francisco); Processing: Gladys Kober (NASA/Catholic University of America)
The Hubble is the Roman's older brother, in a way, and it works differently. It has something to offer the NGRST's mission. The Roman sees mostly in infrared, with a little bit of visible light. The Hubble sees in mostly visible and UV light, with a little infrared. But the pair of telescopes differ in another important way, too. While the Hubble can examine individual objects, that's not how the Roman works. It has a much larger field of view.
This image compares the Hubble's small, focused field of view and the Roman's much larger one. The difference is stark, but it lets Hubble do preparatory work that will help the Roman's weak gravitational lensing observations. Image Credit: Background image: Digitized Sky Survey and R. Gendler; Moon image: NASA’s Goddard Space Flight Center and Arizona State University
That means that while Hubble can examine individual gravitational lenses, the NGRST cannot. That's because the Roman is a wide-field survey telescope. Here's how the Hubble will help its more modern counterpart.
The Hubble examines a single strong gravitational lens at a time. Astronomers can use Hubble's detailed observations to understand the lenses better. That model will be used to help make sense of the Roman's gravitational lenses.
It comes down to what's called the point-spread function. Telescopes don't image a point source as a point. The source produces a pattern across the detector, and the PSF pattern always has aberrations, diffraction spikes, and other problems. Since the Hubble looks at each gravitational lens separately on a single detector, the PSF is easier to understand and account for.
The problem the Roman will have also concerns the point spread function. (If there's an astronomy paper that doesn't mention PSF, Universe Today hasn't seen it.) The Roman will detect most of its gravitational lenses in its High Latitude Wide Area Survey (HLWAS). The HLWAS will cover about 5,000 degrees2 of the sky (about 12% of the sky).
The Roman's High-Latitude Wide-Area Survey should detect about 160,000 gravitational lenses. But only about 500 of them are predicted to be good for follow-up observations that will reveal their substructure. Image Credit: NASA's Goddard Space Flight Center
That means Roman's images are 200 times larger than Hubble's, and spread across multiple detectors, each with a different focal-plane position. In a 2025 paperr, researchers forecast the number of strong gravitational lense the NGRST will detect. "For a fiducial case of single 146-second exposures, we predict around 160,000 detectable strong lenses in the HLWAS, of which about 500 will have sufficient signal-to-noise to be amenable to detailed substructure characterization," they wrote. The HLWAS is expected to detect 15 strong lenses per exposure.
That number, 15 strong lenses per exposure, makes understanding the PSF more difficult. Its images are detected across multiple detectors, each with a different focal plane. So it's harder for the Roman to deal with the PSF and do its job: measuring dark matter.
Astronomers are preparing for the Roman's gravitational lens detections by building a computer model with Hubble's observations. Hubble's PSFs are being used to help decipher the Roman's PSFs.
Hubble will help in another way, too, by performing follow up imaging of the lenses that the NGRST discovers.
This shows that even though the Hubble is technologically outclassed by newer telescopes like the JWST and the NGRST, it still plays an important role. It is, after all, still a powerful space telescope. It's as if the venerable space telescope is passing the baton to a new generation, even though it's still part of the team.
But while the Roman's mission is just about to begin, the Hubble Space Telescope is approaching its finish line. The Hubble is gradually losing its stabilizing gyroscopes, and with no repair mission planned, the telescope will probably die with its last gyroscope.
Nobody can say for sure when that will happen, but up until then, it still has important work to do by supporting its more modern counterparts.
