Issue 14 - October 2014
By Ilya Poberezhskiy
WFIRST Coronagraph Technology Manager
In the previous edition of this newsletter, we described how the evaluation of various coronagraph technologies for the Wide-Field Infrared Survey Telescope/Astrophysics Focused Telescope Assets (WFIRST/AFTA) mission study resulted in the selection of the occulting mask coronagraph - an instrument that combines a Shaped Pupil Coronagraph (SPC) and a Hybrid Lyot Coronagraph (HLC). Since NASA’s decision of the selection was announced in December of 2013, the technology development team at JPL and partner institutions has been rapidly maturing the coronagraph technology, and achieved a number of significant advances. Several highlights of this progress are described below.
In the heart of any coronagraph are the so-called “masks”: shaped pupils for SPC and focal plane occulters for HLC. In each case, new designs had to be developed to achieve the required starlight suppression with the obscured 2.4 meter AFTA telescope. Both HLC and SPC teams presented viable designs in the fall of 2013, and during the course of 2014 the teams at JPL and Princeton University, respectively, have continued to improve their designs in order to increase the number of detectable exoplanets, reduce sensitivity to observatory pointing jitter, and make the masks easier to fabricate. As a result, exoplanet science that the WFIRST coronagraph can do under realistic conditions has improved significantly over the past year.
The new coronagraph mask designs contained features that had not been fabricated before, and in some cases were outright incompatible with the existing fabrication techniques. Thus new fabrication approaches had to be developed to make WFIRST/AFTA coronagraph masks. Over the past few months, both shaped pupil and hybrid Lyot mask fabrication teams were able to rise to this challenge and successfully produce both WFIRST/AFTA coronagraph masks.
The new SPC masks are of reflective rather than previously used transmissive type (Fig. 1a). While a portion of light in the pupil plane hits a highly reflective mirror surface, other light encounters specially processed “black silicon” regions. The black silicon process, performed at JPL’s MicroDevices Lab (MDL) and Caltech’s Kavli Nanoscience Institute, produces some of the darkest surfaces in the world, with specular reflectance of less than 1 part in 10 million. The quality of the produced reflective masks was initially verified through extensive characterization, modeling, and analysis, and later validated using the ultimate test – achieving high contrast (starlight suppression) on the coronagraphic testbed. Narrowband contrast of ~6x10-9 and broadband contrast of ~9x10-9 have been achieved between working angles of 4.4 and 11 λ/D, with contrast levels expected to further improve in the future (Fig. 1).
Circular HLC masks have also been fabricated for the first time at JPL using two different approaches. A mask made at MDL is shown in Fig. 2. Since this mask is in the focal plane, it is much smaller than the shaped pupil masks – the part that does the bulk of the starlight suppression is less than 1 mm in diameter! The HLC testbed is currently being commissioned in a vacuum chamber and should begin starlight suppression experiments soon.
Another key part of the WFIRST coronagraph instrument that has been making steady progress is the Low Order WaveFront Sensing and Control (LOWFS/C) subsystem. It uses rejected starlight to measure the pointing jitter of the telescope, caused by rotation of the reaction wheels used for spacecraft attitude control, as well as optical wavefront errors caused by telescope’s changing exposure to sunshine and Earthshine. After extensive analysis and modeling, the LOWFS team has selected Zernike wavefront sensing approach and is gearing up for its experimental demonstration. Together with the HLC design team, they verified for the first time the viability of co-designing the LOWFS mask with the HLC focal plane mask – a key advance that allows avoiding non-common path errors that would otherwise compromise the instrument’s stability.
We will continue to report our progress as we further mature the key coronagraph technologies that will enable WFIRST/AFTA to acquire direct images and spectra of exoplanets.
Fig. 1. (a) Reflective shaped pupil mask designed at Princeton University and produced at JPL; one-sided dark hole generated by the shaped pupil coronagraph in (b) narrowband light and (c) broadband light.
Fig. 2. (a) Hybrid Lyot coronagraph testbed moving into a vacuum tank for commissioning and starlight suppression experiments; circular hybrid Lyot coronargraph mask imaged under (b) optical and (c) atomic force microscope.