The Origin of CAPSCam - The Carnegie Astrometric Planet Search Camera

An example of an astrometric "wobble." The Sun wobbles about the center of mass of the Solar System, as a result of the motions of the eight planets, principally Jupiter and Saturn. This figure shows the location of the center of mass as viewed from the Sun’s point of view. The amplitude of the wobble is roughly the size of the Sun.
Tuesday, March 30, 2021 

How a theorist at EPL ended up leading the effort to build a specialized camera for the du Pont telescope at the Las Campanas Observatory in Chile. 

A real CAPSCam image. (Boss et al. 2009)

In 1938 Swarthmore astronomer Peter van de Kamp began taking photographs of Barnard's star, a red dwarf star only 6 light-years from the Sun. After 25 years, in 1963 van de Kamp announced that Barnard's star was orbited by a planet 1.6 times the mass of Jupiter. He deduced the presence of this unseen exoplanet by watching Barnard's star wobble as it moved across the sky, a tiny wobble induced by the star's motion around the center of the mass of the star-planet system.

Textbooks were rewritten to include the only planet known outside the Solar System. However, in 1973 George Gatewood published his analysis of Barnard's star, using a collection of photographs taken with a different telescope. Gatewood’s analysis showed no such wobble. Evidently, van de Kamp's detection was spurious, caused by changes in the telescope he had used. The planet around Barnard's star soon disappeared from textbooks.

Earth and Planets Laboratory theorist Alan Boss began serving as an advisor to NASA regarding the search for exoplanets in 1988. He and Gatewood served on several teams that planned to launch specialized space telescopes that would use van de Kamp’s technique, termed astrometry, to discover new worlds around nearby stars. When none of these proposed space telescopes came to life, Boss and Gatewood dreamed of moving Gatewood’s astrometric device from the 10-m Keck Telescopes in Hawaii to Carnegie’s 2.5-m du Pont telescope in Chile. While considerably smaller, the du Pont telescope has the advantage of being a highly stable telescope with a single primary mirror, compared to the 36 segments in the Keck primaries, which must be constantly re-aligned, limiting their astrometric accuracy.

When Boss proposed moving Gatewood’s instrument to the du Pont in 2001, Carnegie Observatories astronomer Ian Thompson suggested starting fresh and building a camera designed for the du Pont using the then-new Hawaii 2RG detectors. Along with Co-Investigators Gatewood, Thompson, and DTM’s Alycia Weinberger, Principal Investigator Boss was awarded $321K by the NSF and Carnegie in 2004 to build the Carnegie Astrometric Planet Search Camera.

CAPSCam was assembled by Thompson and his colleagues in Pasadena and shipped to Chile in 2007. CAPSCam observations began in March of that year and continued for an average of 28 nights each year until March 2020, when covid-19 closed LCO.

While other techniques have discovered thousands of exoplanets, astrometry has yet to find a single one. Boss and Alycia Weinberger are working hard to accomplish this historic goal: the first astrometric detection of an exoplanet. With thirteen years of CAPSCam observations analyzed so far, several intriguing wobbles have appeared, but Boss and Weinberger are being cautious about making claims that might prove to be incorrect. The shadow of Barnard’s star still looms over this promising but difficult technique.

Written by Alan Boss, Staff Scientist, EPL
Updated March 30, 2021

The bottom of the CAPSCam. Photo courtesy Alan Boss, DTM