1. Coronagraph for NRO-WFIRST: A Way Forward
Wesley Traub
Chief Scientist, NASA’s Exoplanet Exploration Program
Jet Propulsion Laboratory, California Institute of Technology
NRO Meeting
Princeton University
4-6 September 2012
Ref.: Traub & Oppenheimer, Chapter on “Direct Imaging”,
in “Exoplanets” book, S. Seager, editor, 2011.
Copyright 2012 California Institute of Technology. Government sponsorship acknowledged.

2. NWNH “ ~ … carry out technology development for a direct imaging
mission to detect and characterize exoplanets in the 2020s … “
“ ~ … ultimate goal is to be able to detect Earth-like planets
in their habitable zones, and search for signs of life … “
Direct imaging instruments include:
coronagraphs, starshades, & interferometers.
NB: Direct imaging requires nearby targets (N ~ stars),
so transit method is not applicable (p ~ 0.5 % for HZ planet).
Next: 3 charts on exoplanets
2 charts on coronagraphs
1 chart on a plan forward

3. Already known planetary systems 1-4 planets per star (o)
136 Prime target stars (x)
2321
Kepler
planets
(cone) 17
Corot
(pencils)
471
RV, transit
& imaging,
+36 HAT
+13 WASP
(cloud)
prime hunting grounds for direct imaging
& astrometry
missions

4. 183,000 Kepler target stars K G F A B M I & III V VII 122,000 sample

5. Kepler planets: period distribution
Area under red curve gives ratio
(planet population)/(star population)
= 0.47 ± 0.01
for periods P < 135 days.
Extrapolation to HZ gives pl.pop./star.pop. = 0.10
2011 data release
2012 data release
Ref., left box: Traub, ApJ 745 (2012)

6. Nominal coronagraph capability
Based on current lab experience, I expect that
an NRO-WFIRST coronagraph should have:
contrast (planet/star) per snapshot ~ 10-9
~ Jupiter at 5 AU
~ Neptune at 2 AU
but can go deeper (~10-10) in sub-FOV area
~ Super-Earth at 2 AU
bandwidth per snapshot ~ 20 %
( 2 bands needed for color, 3 bands desirable)
( IFU with ~ 2 % resolution desirable)
FOV ~ (3 – 32) λ/D
~ – 2.5 arcsec radius
~ 2.5 – 25 AU at 10 pc
targets: planets, exozodi, & debris disks

7. What type of (internal) coronagraph?
Simple instrument
 acceptable cost
But disjoint pupil (secondary & spider arms)
 limited number of internal coronagraph designs
Current throughput capability (personal ballpark guesses):
shaped pupil: A = 0.2 x 6/6, Ω = small, BW = 20 %
band-limited: A = (2 to 3)/6, Ω = large, BW = 20 %
vector vortex: A = tbd, Ω = large, BW = 10 %
PIAA: A = tbd, Ω = large, BW = 1 %
visible nuller: A = 4/6, Ω = small, BW = 1 %
IFU spectroscopy module:
can be added to any of above
needed to characterize planet A: Ω:

8. Strawman plan for a 6-month coronagraph study
By April 2013, generate a community white paper addressing 3 major issues (below),
for each candidate type of coronagraph for NRO-WFIRST
(e.g., shaped pupil, band-limited, vector vortex, PIAA, visible nuller):
1. Provide a strawman instrument layout.
Assuming that each nearby star system has such a target
(e.g., a Jupiter at 5 AU, a Neptune at 3 AU, and a zodi like ours),
state how many could be discovered and characterized
in a given fraction (e.g., 20%) of the mission lifetime.
3. Estimate effort needed to get to TRL 6 by end of 2016.
This plan is similar to the already ongoing discussions of filters, gratings, cameras, etc.,
for the wide-field survey elements (e.g., dark energy, microlensing, GO) of NRO-WFIRST,
however for the coronagraph we have much more new ground to cover.

9. End This research has made use of the NASA Exoplanet Archive,
which is operated by the California Institute of Technology,
under contract with the National Aeronautics and Space Administration
under the Exoplanet Exploration Program.