Guillermo was at the Max-Planck Institute for Aeronomie (MPAe) from 1995-1996 as guest scientist, and in 1997-2000 as a PhD student. Part of his responsibilities during that time involved the design, development, and testing of the control software of one of the german solar telescopes (MICA: Mirror coronagraph for Argentina, similar in design to the LASCO/C1) installed at El Leoncito in the San Juan province, Argentina, as part of an international cooperation agreement between Germany and Argentina. The main topic of his thesis was the analysis of coronal images at several temporal scales.

After finishing his PhD he worked for the MPAe as a post-doc, primarily to develop image processing techniques for the SOHO/LASCO and EIT data, advisory of students, and management of the german-argentinean telescopes (until January 2003). Given the nature of the work, he distributed his time between the observatory in the mountains (El Leoncito), the Institute of Astronomy and Space Physics in Buenos Aires, Argentine, and the Max-Planck in Germany.

He started working at NASA (under contract of the Catholic University of America) in February 2003. He joined the SOHO team in January 2004. Primary responsibility: to serve as Operations Scientist for LASCO and EIT coronal imaging telescopes aboard the ESA/NASA SOHO space mission (and development of analysis techniques for the NRL SECCHI experiment).