After almost 4 years of operations, last January 20, 2021 was the last night of the APOGEE-2 project, whose first light occurred in March 2017, with regular operations since April of the same year. The second stage of this project operated at the du Pont telescope at Las Campanas Observatory since its inception, but due to the spread of Coronavirus around the world, it was halted between March and mid-October 2020.
APOGEE, which is mainly based on collecting infrared spectra of a large number of stars, is a spectroscopic survey of the Milky Way that seeks to answer some fundamental questions about the star formation and enrichment history of the Milky Way. It also investigates the dynamics of the disk, bulge and halo of our galaxy and the age distribution of stars, among other more specific objectives.
The APOGEE project began as APOGEE-1 at Apache Point Observatory. It was followed by APOGEE-2 and APOGEE-2S.
“One of the major milestones of APOGEE-2 was to obtain spectra of bulb stars, something very difficult or almost impossible to obtain in the north. In addition, the first spectroscopic census of the Magellanic Clouds was made, which is allowing us to understand their history and their interaction with our Galaxy. Many rare stars with exotic abundances have also been identified and many other things are on the way”, explains Andres Almeida, leader of observers of the project.
Almeida led a multidisciplinary and international team composed of nine observers, an engineer and several astronomers working on the selection of sources for the fields.
This survey had an infrared multi-spectrograph, fed by optical fibers. Cartridges, as the object in which these fibers are stored is called, were used for this purpose. The cartridges are created so that a plate with the design of a region of the sky can be installed on them. In it, there are holes where the fibers are connected to collect starlight.
“The area of the du Pont telescope and the particularity of having a large field of view make it possible to position many optical fibers in the focal plane and send the starlight to a spectrograph, positioned in a room below the observation plane”, emphasizes Francesco Di Mille, Technical Operations Manager of Las Campanas Observatory.
After resuming operations following the pandemic shutdown, about 130,000 spectra of individual stars were obtained. Together, APOGEE-1 and APOGEE-2 took about 2.6 million spectra over a 10-year period. In the first instance, members of the SDSS collaboration have access to these data but after a year, they are released to the entire scientific community. “This is one of the strengths of the SDSS: a lot of the science and advances have been achieved thanks to people who are not directly linked to the project,” stresses Almeida.
APOGEE-2 will be succeeded by the SDSS-V project. “In the case of APOGEE the fibers were positioned by hand on the cartridges that were mounted on the telescope at night. This work has a limitation in terms of observing efficiency. With the new survey, a robot is being developed that allows to position the fibers automatically and change the configuration many times at night,” Di Mille emphasizes.