Visible Nulling Coronagraphy for Exoplanets

Richard G. Lyon  (NASA/Goddards Space Flight Center)

Three of the completed NASA Astrophysics Strategic Mission Concept
(ASMC) studies addressed the feasibility of a Visible Nulling
Coronagraph (VNC) as the prime instrument for starlight suppression
for exoplanet science. The VNC is a modified Mach-Zehnder nulling
interferometer that works with filled, segmented and sparse or diluted
aperture telescope systems and thereby spans the range of possible
future exoplanet missions. It has both bright and dark (nulled) output
channels, such that the sum total of the photons from both is a
constant independent of the state of the instrument. This has
ramifications for wavefront control since the sensing and control now
becomes independent of the instruments contrast, allowing closed-loop
control at a bandwidth set by the brightness of the target star, with
the net effect of relaxing the tolerances on the stability of the
collecting telescope. NASA/Goddard Space Flight Center (GSFC) has a
well-established effort to develop VNC technologies and has developed
an incremental sequence of VNC testbeds to advance this approach and
the technologies associated with it.

The VNT is a NASA Technology Development Exoplanet Mission (TDEM)
funded effort to achieve a sequence of three milestones of contrasts
of 10^8 (2010) 10^9 (2011) and ultimately 10^10 at inner working
angles approaching ~2*lambda/D and culminate in spectrally broadband
(>20%) high contrast imaging. This is achieved with a hex-packed MEMS
based deformable mirror, optically mapped to a coherent fiber bundle,
achromatic phase shifters and wavefront sensing and control. Discussed
will be the configuration, laboratory results, critical technologies
and null sensing and control. The VNT has also led to the design of a
'compact nuller' - a dimensionally stable athermal textbook sized
nuller that will be discussed as time permits.

BRIEF BIOGRAPHY Rick Lyon is an optical scientist working primarily on
imaging interferometry and coronagraphy for exoplanet detection and
characterization. His research interests are challenging optical
problems that require precision sensing and control and he has been
involved in the Hubble Space Telescope and proposed phase retrieval
for it as a backup prior to launch and employed phase retrieval to
diagnose and assess the problems with it along with a number of other
groups. Due to these efforts phase retrieval was chosen as the method
of choice for the James Webb Space Telescope and he led the way with
proposing it in the mid 1990's for JWST.  Most recently he has
developed a sequence imaging interferometry testbeds for Michelson
wide-field imaging interferometry, Fizeau sparse aperture
interferometry and nulling interferometry and coronagraphy.