Issue 13 - April 2014
Coronagraph Concepts Selected for WFIRST-AFTA
By Ilya Poberezhskiy
A coronagraph is an optical instrument that suppresses light from bright objects in order to see much fainter ones nearby, whose light would normally be drowned out. It was invented to view the corona of the Sun – hence the name – by French astronomer Bernard Lyot in 1930. A different type of this instrument, called a stellar coronagraph, allows direct imaging of exoplanets around nearby stars. Compared to other methods of exoplanet detection, direct imaging allows us to measure an exoplanet’s spectrum, thus providing information about atmospheric composition, which can in turn give us clues about the presence of water and, potentially, life. Circumstellar disks are also coronagraph science targets.
To give you a rough idea about the challenge of building a coronagraph for imaging large known exoplanets around nearby stars, we must be able to see a planet that is about a billion times fainter than the star, when the angular separation between these two mismatched objects is comparable to them being just a few meters apart when observed from 4000 km away (the distance from New York to Los Angeles).
This is why NASA’s decision to add a coronagraph to the Wide-Field Infrared Survey–Astrophysics Focused Telescope Assets (WFIRST-AFTA) mission concept triggered a rigorous technology evaluation process during summer and fall 2013. The Exoplanet Exploration Program (ExEP) and the WFIRST-AFTA Study Office led the assessment of six submitted coronagraph technologies on parameters such as their science return (e.g., the number of known exoplanets whose spectra the coronagraph can measure in the allotted time), complexity, compatibility with the 2.4-meter AFTA telescope, and technical maturity.
The recommendation that emerged from this process was to pick a primary coronagraph design named an Occulting Mask Coronagraph that combines two technical approaches, Shaped Pupil and Hybrid Lyot, in one instrument. The Phase-Induced Amplitude Apodization Complex Mask Coronagraph (PIAA-CMC) was selected as the backup design. These technical approaches are briefly described below:
- The Shaped Pupil coronagraph, pioneered by Professor Jeremy Kasdin and his group at Princeton University, is a relatively simple and mature design that uses a carefully optimized binary pupil mask to diffract on-axis starlight in a way that directs the bulk of it to be blocked by a field stop in the focal plane. The slightly off-axis planet light is passed through the field stop and then reimaged to a detector.
- The Hybrid Lyot coronagraph, proposed by Dr. John Trauger and his collaborators at JPL, also utilizes a simple architecture and has achieved record starlight suppression levels in a laboratory testbed. In the heart of this coronagraph is a small focal plane mask with carefully optimized layers of nickel and dielectric which create a profile of intensity and phase transmission that stops the bulk of the starlight and sends the rest of the light toward a so-called Lyot stop in the pupil plane. The Lyot stop blocks the remaining starlight but passes the light from the planet.
- Finally, the backup PIAA-CMC is based on the PIAA concept invented about a decade ago by Professor Olivier Guyon of the University of Arizona and Subaru Telescope. This concept is also based on modifying, or “apodizing,” the telescope pupil. In contrast to the Shaped Pupil method which uses a binary amplitude mask, PIAA relies on reflections from two carefully shaped aspheric mirrors to achieve this goal. Together with Dr. Ruslan Belikov from NASA’s Ames Research Center, Dr. Guyon proposed a modified PIAA concept for the WFIRST-AFTA coronagraph that uses easier-to-make PIAA mirrors and adds a phase mask in the focal plane. The PIAA-CMC concept promises the greatest science return, but has less technical heritage and is somewhat more complex than the other two approaches.
All three coronagraphs work in conjunction with deformable mirrors that shape the light coming from the telescope to achieve the necessary starlight suppression.
It is interesting to note that generally these coronagraphs work best with a clear, unobscured pupil provided by an off-axis telescope. The AFTA 2.4-meter telescope, however, is taken “as is” and has a central obscuration consisting of the secondary mirror and the struts that support it.
The selected coronagraph design teams came up with effective solutions to accommodate this pupil shape, and going through this exercise will produce extra payoff down the line. The New Worlds telescope that is more than a decade away will need to have a significantly larger aperture than 2.4 meters in order to image Earth-like exoplanets that are even fainter and closer to the star than WFIRST-AFTA coronagraph targets. This telescope will thus likely be segmented, in which case it also will not have an ideal unobscured pupil. Thus, the technologies we will mature for the WFIRST-AFTA coronagraph are not just valuable for the science this mission will produce, but also as a stepping-stone to future missions that promise even greater discoveries.
The WFIRST-AFTA coronagraph team has drafted and started executing a plan to mature these coronagraph technologies and get them ready for flight. We expect to see some initial exciting results during 2014. Stay tuned!