Palomar Observatory Produces New and Improved "Rainbows" for Astronomers
On November 8, astronomers at Caltech's Palomar Observatory in the mountains above San Diego directed a brand-new spectrograph instrument to capture data from a newfound supernova. The resulting spectrum—a detailed look at the rainbow of different light wavelengths emanating from the erupting star—delighted many of the Caltech team members, who have been working on the instrument since 2017.
"It's the best kind of eye candy," says Mansi Kasliwal (PhD '11), a professor of astronomy at Caltech and the principal investigator of the instrument, called the Next Generation Palomar Spectrograph (NGPS).
The new "first-light" spectrum is proof of the instrument's ability to capture more detail and fainter targets than its predecessor, the Double Spectrograph, which was installed on the Hale Telescope more than 40 years ago. The new spectrum shows light from a supernova detected by the Zwicky Transient Facility (ZTF), a survey camera at the 48-inch Samuel Oschin telescope at Palomar. The wiggles in the data indicate chemical signatures that reveal the supernova belongs to a class called Type 1a, which occurs when a white dwarf orbiting a companion star undergoes a thermonuclear explosion.
"This new spectrograph is the most efficient of any comparable instrument working at other telescopes," Kasliwal says. "The old Double Spectrograph was Palomar's workhorse for more than 40 years and resulted in thousands of papers. But NGPS is more than three times more efficient and blows the old one out of the water."
NGPS will be used by astronomers to study everything from nearby asteroids and comets to more distant stars, galaxies, and supernovae. In fact, it will dramatically speed up the rate at which ZTF classifies newfound supernovae.
Recently, ZTF hit a milestone: The survey camera's detections led to the classification of more than 10,000 supernovae. Many of these exploding stars were observed by the Hale Telescope's old spectrograph. By obtaining spectra, researchers could confirm and characterize the exploding stars, learning their distance from Earth, as well as details about their chemistry, energetics, and the type of explosion that occurred. Now, supernovae detected by ZTF will be classified faster than before thanks to NGPS's improved efficiency.
"We were losing lots of light with the old spectrograph, but with this new one, more of an object's photons are making their way to the detectors. It's a major improvement," says Christoffer Fremling, a Caltech staff astronomer who helped develop and test the instrument.
The Hale Telescope first set its sights on the skies more than 75 years ago, but with new instruments like NGPS, it remains at the forefront of astronomy. "NGPS will keep Palomar relevant for years and years," Fremling says.
The instrument is a collaboration between partners in the US and China. On the US side, the partners are Caltech; the Jet Propulsion Laboratory, which Caltech manages for NASA; the National Science Foundation; and the Heising–Simons Foundation. The China partners are the Kavli Institute for Astronomy and Astrophysics at Peking University and the National Astronomical Observatories of China (NAOC).
Lin Yan (PhD '96), an astronomer with Caltech Optical Observatories, helped manage the partnership with China over the past several years, traveling back and forth between Pasadena and Beijing. Originally from Qinghai Province in the Northwestern corner of China, Yan helped manage the international collaboration.
"The first-light spectrum is a clear testament to our wonderful collaboration," she says. "Building the intricate design of the instrument required both engineering and fostering relationships between our partners."
The spectrograph's structure and optics were built in China then shipped to Caltech where they were integrated with cryogenically cooled detectors, an "image slicer" and slice-viewing cameras, a calibration system, an electronics cabinet, and instrument-control software.
The image slicer, an innovation refined at Caltech, collects and recombines light that would otherwise be lost at the edges of the slit—the thin window in the instrument where observations are made. The slit width can be automatically adjusted, which lets astronomers control the amount of resolution in the instrument versus sensitivity. The automated variable width also helps in responding to different weather conditions.
Another upgrade is the use of grating rather than a prism to break light apart into its constituent wavelengths.
According to Rob Bertz, NGPS project manager at Caltech, "Volume-phase holographic, or VPH, gratings dramatically reduce the light loss that happens when light is dispersed into different wavelengths. The VPH gratings, along with the four-channel instrument design and state-of-the-art optical coatings result in NGPS having three times the optical efficiency of its predecessor. This improved efficiency allows shorter exposure times and results in more science results each night."
In the future, the team plans to install two additional detectors to the instrument, allowing it to see bluer wavelengths of light and complete the coverage of the visible spectrum. They will also automate the spectrograph, such that it will autonomously process a pre-determined list of stars and other cosmic objects and capture their spectra.
"The combination of much higher photon efficiency and rapid target acquisition and spectrum production will produce a massive gain in the number of celestial objects observable every night," says Chris Martin, the Edward C. Stone Profess of Physics at Caltech and the director of the Caltech Optical Observatories (COO), which includes Palomar.
The NGPS project began in October 2017 after Shri Kulkarni, the George Ellery Hale Professor of Astronomy and Planetary Science, who was then the COO director, signed a memorandum of understanding with the Chinese partners. Evan Kirby, formerly an assistant professor of astronomy at Caltech and now at the University of Notre Dame, served as the principal investigator (PI) of the instrument from 2018 to 2021. Jonas Zmuidzinas, the Merle Kinglsey Professor of Physics and then COO Director served as PI from 2021 to 2023. Kasliwal has served as a co-PI since 2018 and has been the PI since 2024.
Members of the Caltech-NGPS team are pictured here with the new instrument at Caltech, before it was shipped to Palomar Observatory. Back row, from left to right: Hector Rodriguez, Chaz Shapiro (who works for JPL), Matt Matuszewski, Roger Smith, James Brugger, David Hale, Christoffer Fremling, Rob Bertz, and Dan Reiley. Front row, from left to right: Lin Yan, Lauren Fahey, and Rich Dekany.
Credit: Lauren Fahey/Caltech
NGPS as it is lifted by crane to the observing deck at Palomar Observatory.
Credit: Greg van Idsiga/Caltech
The NGPS instrument is seen here being installed in the Cassegrain focus cage of the 200-inch Hale Telescope a Palomar Observatory. This is the area where light collected by the telescope is focused to a plane accessible by interchangeable instruments such as NGPS.
Credit: Rob Bertz/Caltech
