By 孙敏 on Wed, 2025-01-08 09:49

Figure 1:  The 200-inch Hale Telescope at Palomar Observatory in California was the largest telescope in the world from 1949-1993 and still plays a prominent role in many high-impact science areas.  Figure obtained from https://en.wikipedia.org/wiki/Palomar_Observatory and credited to Gerard T. van Belle.

On November 8, 2024, astronomers at Caltech's Palomar Observatory directed a brand-new spectrograph instrument, the Next Generation Palomar Spectrograph (NGPS), to capture data from a newfound supernova. The resulting spectrum from the erupting star was a delight for the many team members in the USA and China, including at Peking University, who have been working on the instrument since 2017. The new first-light spectrum demonstrates new instrument's ability to capture more detail, and fainter targets, than its predecessor, the Double Spectrograph, which was installed on the historic Hale Telescope more than 40 years ago.

“With its innovative design, the NGPS is more powerful than any other spectrograph of its kind, with 50% more sensitivity than even the X-Shooter spectrograph on the Very Large Telescope,” says Luis Ho, Director of the Kavli Institute for Astronomy and Astrophysics at Peking University (PKU) and the principal investigator of the PKU effort.  “PKU astronomers are excited to use NGPS to better understand supermassive black holes, tidal disruption events, and many other phenomena that rule our universe.”

The 200-inch Hale Telescope was the world’s effective largest telescope from 1949-1993 and is still one of the world’s most productive telescopes.  "The old Double Spectrograph was Palomar's workhorse for more than 40 years and resulted in thousands of papers.” says Mansi Kasliwal, professor at Caltech and the principal investigator of the instrument. “NGPS is more than three times more efficient and blows the old one out of the water.”

NGPS was developed by a collaboration between the Kavli Institute for Astronomy and Astrophysics at Peking University, the National Astronomical Observatories of China (NAOC) and Nanjing Institute of Astronomical Optics & Technology (NIAOT) in China and Caltech and the Jet Propulsion Laboratory in the US, with additional funding support from the National Natural Science Foundation of China, the US National Science Foundation and the Heising-Simons Foundation.  

Figure 2:  NGPS installed on the Hale Telescope.

“The NGPS project demonstrates the value of international collaboration between Chinese astronomical institutions and the California Institute of Technology. This cooperation has been built on the mutual desire to actively develop a long-term partnership.” says Suijian Xue, former Deputy Director of NAOC and Executive Officer of the NGPS Collaborative Board.  “The design and fabrication of NGPS leveraged expertise and strengths in both China and the US.” Prof. Zhongwen Hu, Deputy Director of NIAOT, shares his thoughts on the collaboration: "This international partnership has been a resounding success, culminating in the excellent performance of the NGPS during its trial runs. The results have been met, and it brings us great joy to see the fruition of our collective efforts."

The spectrograph structure and optics were built in China, then shipped to Caltech where it was 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 collects and recombines light that would otherwise be lost at the edges of the slit—the thin window in the instrument where the observations are made. The volume phase holographic, or VPH, gratings minimize light loss, when compared to more traditional prisms.  “The design and fabrication of this world-leading instrument demonstrates the innovative ability of the engineers at NAOC and other institutes in the Chinese Academy of Science system,” says Xue.

Figure 3:  The first-light spectrum of NGPS, which led the science team to classify Supernovae ZTF24abrfcq as a Type 1a supernova caused by the explosion of a white dwarf.  The NGPS splits the light into two different bands to optimize sensitivity.  The blue and orange colors are spectra from the r and i-band detectors.  An upgrade is planned to extend the wavelength coverage to 3500 A.

Chinese Project Manager Hangxin Ji from NIAOT reflects on the team's journey, "In the years leading up to this triumph, our team embarked on a challenging yet exhilarating journey to develop the NGPS, which is set to become the most powerful optical spectrograph in the realm of time domain astronomy. It has not been an easy path, but each team member has played a critical role. The process has been deeply enjoyable for everyone involved, particularly our younger members who have thrived and grown professionally.”

NGPS will be used by PKU astronomers to study everything from the oldest and youngest stars in the Milky Way to distant galaxies, supermassive black holes, supernovae, and tidal disruption events.  “China has many state-of-the-art facilities, such as Einstein Probe and WFST, and will soon have the Chinese Space Station Telescope (CSST),” says Ho.  “These facilities will all discover many exciting objects that will need optical spectroscopy for the scientific breakthroughs.”

PKU astronomers played a leading role in defining the scientific requirements and the design and development of NGPS.  “PKU astronomers are excited to use the instrument for many science cases, including as follow-up for CSST,” said Prof. Xue-Bing Wu, Chair of the Department of Astronomy at PKU.  “Our role in NGPS was facilitated by strong support from PKU’s discipline construction program, which provided the initial seed funding for the entire project.  The initial investment is always the most important funding to turn a project from an idea into a reality. Later we also obtained important financial support to the NGPS from a major instrumentation project of the National Natural Science Foundation of China.”

In the future, the team plans to install two additional detectors to the instrument, which will see bluer wavelengths of light. NGPS was designed to work in tandem with the ground-layer adaptive optics system SIGHT, now under development, to further optimize sensitivity by correcting for atmospheric turbulence and thereby reducing the size of an unresolved object on the detector.

This press release is based on a press release for NGPS originally written by Whitney Clavin.

Caltech's website:

https://www.caltech.edu/about/news/palomar-observatory-produces-new-and-improved-rainbows-for-astronomers