28 March 2018 Vortex coronagraphs for the Habitable Exoplanet Imaging Mission concept: theoretical performance and telescope requirements
Author Affiliations +
Abstract
The Habitable Exoplanet Imaging Mission concept requires an optical coronagraph that provides deep starlight suppression over a broad spectral bandwidth, high throughput for point sources at small angular separation, and insensitivity to temporally varying, low-order aberrations. Vortex coronagraphs are a promising solution that performs optimally on off-axis, monolithic telescopes and may also be designed for segmented telescopes with minor losses in performance. We describe the key advantages of vortex coronagraphs on off-axis telescopes such as (1) unwanted diffraction due to aberrations is passively rejected in several low-order Zernike modes relaxing the wavefront stability requirements for imaging Earth-like planets from <10 to <100  pm rms, (2) stars with angular diameters <0.1 λ  /  D may be sufficiently suppressed, (3) the absolute planet throughput is <10  %  , even for unfavorable telescope architectures, and (4) broadband solutions (Δλ  /  λ  <  0.1) are readily available for both monolithic and segmented apertures. The latter make use of grayscale apodizers in an upstream pupil plane to provide suppression of diffracted light from amplitude discontinuities in the telescope pupil without inducing additional stroke on the deformable mirrors. We set wavefront stability requirements on the telescope, based on a stellar irradiance threshold set at an angular separation of 3  ±  0.5λ  /  D from the star, and discuss how some requirements may be relaxed by trading robustness to aberrations for planet throughput.
Ruane, Mawet, Mennesson, Jewell, and Shaklan: Vortex coronagraphs for the Habitable Exoplanet Imaging Mission concept: theoretical performance and telescope requirements

1.

Introduction

The Habitable Exoplanet Imaging Mission (HabEx) concept seeks to directly detect atmospheric biomarkers on Earth-like exoplanets orbiting sun-like stars for the first time.1 Accomplishing this task requires extremely high-contrast imaging over a broad spectral range using an internal coronagraph2 or external starshade.3 Sufficient starlight suppression may be achieved on an ultrastable telescope using an on-board coronagraph instrument (CGI) with high-precision wavefront control and masks specially designed to manage diffraction of unwanted starlight. Each of these critical technologies will be demonstrated in space with the WFIRST CGI at the performance level needed to image gas giant planets in reflected light with a 2.4-m telescope.4 Leveraging the advancements afforded by WFIRST, the HabEx mission concept makes use of a larger (>4  m) telescope whose stability specifications allow for the detection and characterization of Earth-like planets with planet-to-star flux ratios <1010.

The optimal coronagraph performance for a given mission depends strongly on the telescope design. The possible HabEx architectures currently under study in preparation for the 2020 Astrophysics Decadal Survey are a 4-m monolithic (architecture A) or 6.5-m segmented primary mirror (architecture B). For the purposes of this paper, we assume fully off-axis telescopes in both cases. The unobstructed, circular pupil provided by architecture A is ideal for coronagraph performance. In contrast, the segmented primary mirror of architecture B will introduce a number of additional complications owing to potential amplitude and phase discontinuities in the wavefront.

We present vortex coronagraph56.7 designs for each of the HabEx architecture and their theoretical performance. We demonstrate that the coronagraph may be designed to passively reject unwanted diffraction within the telescope in the presence of temporally varying, low-order aberrations as well as amplitude discontinuities (i.e., gaps between mirror segments). We set wavefront stability requirements on the telescope, including the phasing of the primary mirror segments in the case of architecture B, and discuss how some telescope requirements may be relaxed by trading robustness to aberrations for planet throughput.8

2.

Architecture A: 4-m Off-Axis, Unobscured, Monolithic Telescope

The first telescope architecture we analyze is a 4-m off-axis telescope with a monolithic primary mirror. The unobstructed pupil is conducive to highly efficient coronagraph designs, such as the vortex coronagraph, which provide sensitivity to weak planet signals at small angular separations, as demonstrated in the laboratory9 and observations with ground-based telescopes.1011.12.13.14 Figure 1(a) shows a schematic of a vortex coronagraph with dual deformable mirrors for wavefront control, a focal plane mask, and Lyot stop. The vortex focal plane mask is a transparent optic, which imparts a spiral phase shift of the form exp(ilϕ) on the incident field, where l is an even nonzero integer known as the “charge” and ϕ is the azimuth angle in the focal plane. Light from an on-axis point source (i.e., the star) that passes through the circular entrance pupil of radius a is completely diffracted outside of the downstream Lyot stop of radius b, assuming b<a, and one-to-one magnification within the coronagraph. In addition to ideal starlight suppression, the vortex coronagraph provides high throughput for point-like sources at small angular separations from the star [see Fig. 1(b)].

Fig. 1

(a) Schematic of a vortex coronagraph with deformable mirrors DM1 and DM2, focal plane phase mask with complex transmittance exp(ilϕ), and circular Lyot stop. Starlight suppression is achieved by diffracting the stellar field outside of the Lyot stop. (b) Throughput performance of a vortex coronagraph for Lyot stop whose radius is 95% that of the geometric pupil (b/a=0.95). The horizontal lines indicate the maximum throughput in each case.

JATIS_4_1_015004_f001.png

2.1.

Ideal Coronagraph Throughput

We present two common throughput definitions in the literature (1) the fraction of planet energy from a planet that reaches the image plane and (2) the fraction of the planet energy that falls within a circular region-of-interest with radius r^λ/D centered at the planet position, where λ is the wavelength and D is the diameter of the primary mirror. The maximum throughput (at large angular separations) in each case is

(1)

ηp,max={(b/a)2,total energy(b/a)2[1J0(πr^ba)2J1(πr^ba)2],fraction with inr^λ/Dradius,
where J0() and J1() are Bessel functions of the first kind.15 For example, if b/a=0.95 and r^=0.7, the theoretical maxima for cases (1) and (2) are 90% and 58%, respectively. The latter value may also be normalized to the same quantity without the coronagraph masks. For example, in the case described above, 86% of planet energy remains within 0.7λ/D of the planet’s position in the image, a value referred to as the relative throughput. In the remainder of this section, we assume a typical value of b/a=0.95 for architecture A. In practice, the value of b/a will be selected based on the desired tolerance to lateral pupil motion and magnification. Definitions (1) and (2) are plotted for various values of l in Fig. 1(b) for angular separations up to 20λ/D using numerical beam propagation.

2.2.

Passive Insensitivity to Low-Order Aberrations

Detecting Earth-like exoplanets in practice will require a coronagraph whose performance is insensitive to wavefront errors owing to mechanical motions in the telescope and differential polarization aberrations, which both manifest as low-order wavefront errors. We describe the phase at the entrance pupil of the coronagraph as a linear combination of Zernike polynomials Znm(r/a,θ) defined over a circular pupil of radius a. An isolated phase aberration is written

(2)

P(r,θ)=exp[icnmZnm(r/a,θ)],ra,
where i=1 and cn,m is the Zernike coefficient. Assuming small wavefront errors (i.e., cnm1  radrms), the field in the pupil may be approximated to first order via its Taylor series expansion

(3)

P(r,θ)1+icnmZnm(r/a,θ),ra.

For convenience, we choose to use the set real valued of Zernike polynomials described by

(4)

Znm(r/a,θ)=Rn|m|(r/a)qm(θ),ra,
where Rnm(r/a) are the radial polynomials described in Appendix A and

(5)

qm(θ)={cos(mθ)m0sin(|m|θ)m<0.

The field transmitted through a vortex phase element of charge l, owing to an on-axis point source, is given by the product of exp(ilϕ) and the optical Fourier transform (FT) of Eq. (3)

(6)

Fnml(ρ,ϕ)[f00(ρ,ϕ)+icnmfnm(ρ,ϕ)]eilϕ,
where

(7)

fnm(ρ,ϕ)=ka2fJn+1(kaρ/f)kaρ/fqm(ϕ),
where ρ is the radial polar coordinate in the focal plane, k=2π/λ, λ is the wavelength, and f is the focal length, respectively. The field in the subsequent pupil plane (i.e., just before the Lyot stop), Elnm, is given by the FT of Eq. (6). The first term, f00(ρ,ϕ), is the common Airy pattern, which diffracts completely outside of the Lyot stop for all even nonzero values of l. In this case, the Lyot plane field becomes16

(8)

El,Airy(r,θ)={0raarR|l|11(ar)eilθr>a.

More generally, the full Lyot plane field is given by

(9)

Enml(r,θ)El,Airy(r,θ)+icnmgnml(r,θ),
where

(10)

gnml(r,θ)=ka2feilθ{(1)meimθWn+1l+m(r)+eimθWn+1lm(r)m0i[(1)m+1eimθWn+1l+m(r)+eimθWn+1lm(r)]m<0,
and Wpq(r) is a special case of the Weber–Schafheitlin integral (Appendix B)

(11)

Wpq(r)=Wp,q,0(r;kaρ/f,krρ/f)=0Jp(kaρ/f)Jq(krρ/f)dρ.
The solutions to Eq. (10) are shown in Fig. 2 and listed in Appendix C. In cases where all of the light is located outside of the geometric pupil, the source is extinguished by a Lyot stop with radius b<a. The constant term in Eq. (3) is completely suppressed for all nonzero even values of l. However, the first-order term is also blocked by the Lyot stop if |l|>n+|m|. A charge l vortex coronagraph is therefore passively insensitive to the l2/4 Zernike modes rejected by the Lyot stop.

Fig. 2

The low-order aberration filtering mechanism of a vortex coronagraph. The top row shows the wavefront at the entrance pupil of the coronagraph (“first pupil” in Fig. 1). The remaining rows show the amplitude distribution just before the Lyot stop (“second pupil” in Fig. 1). A vortex coronagraph is passively insensitive to modes, where the starlight appears outside of a Lyot stop whose radius b is less than the geometric pupil radius a and |l|>n+|m|.

JATIS_4_1_015004_f002.png

2.3.

Wavefront Stability Requirements

The wavefront error tolerances of a given coronagraph design depend on the aberration mode and/or spatial frequency content of the error. The coronagraph and telescope must be jointly optimized to passively suppress starlight and provide the stability needed to maintain suppression throughout an observation. We present telescope stability requirements for Earth-like exoplanet imaging with vortex coronagraphs in terms of low-order and mid-to-high spatial frequency aberrations.

2.3.1.

Low-order requirements: Zernike aberrations

Figure 3 shows the leaked starlight through the coronagraph (stellar irradiance, averaged over effective angular separations 3±0.5λ/D, and normalized to the peak value without the coronagraph masks) as a function of root mean square (RMS) wavefront error. Modes with n+|m||l| follow a quadratic power law and generate irradiance at the 1011 level for wavefront errors of 105 waves RMS. However, modes with n+|m|<|l| are blocked at least to first order at the Lyot stop, as described in the previous section. In these cases, the equivalent irradiance level (1011) corresponds to 100× of the wavefront error.

Fig. 3

Sensitivity of vortex coronagraphs to low-order aberrations. The stellar irradiance is averaged over effective angular separations 3±0.5λ/D, normalized to the peak irradiance without the coronagraph masks, as a function of RMS wavefront error in each Zernike aberration. As the vortex charge increases, larger errors may be tolerated on the lowest order aberrations, which typically dominate the dynamic wavefront error budget.

JATIS_4_1_015004_f003.png

We place requirements on the stability of the wavefront by setting a maximum allowable irradiance threshold on the leaked starlight at 3±0.5λ/D. This angular coordinate range corresponds to the separations, where a charge eight vortex coronagraph transmits 50% of the planet light. Here, the threshold is chosen to be 2×1011 per Zernike mode (dashed line in Fig. 3) to prevent any single low-order aberration from dominating the error budget. The corresponding wavefront error at λ=450  nm, likely the shortest and most challenging wavelength, is shown in Fig. 4 and listed in Table 1. Modes that are passively suppressed by the coronagraph have wavefront requirements >100  pmrms, whereas those that transmit tend to require <10  pmrms. The minimum charge of the vortex coronagraph may be chosen to preserve robustness to particularly problematic low-order aberrations as well as to relax requirements and reduce the cost of the overall mission. However, increasing the minimum charge has a significant impact on the scientific yield of the mission, especially since insufficient throughput at small angular separations (i.e., beyond the so-called “inner working angle”) will likely limit the number of detected and characterized Earth-like planets within the mission lifetime.18

Fig. 4

Wavefront error requirements in the Zernike mode basis. The maximum allowable RMS wavefront error generates a normalized irradiance of 2×1011 at an effective separation of 3±0.5λ/D for λ=450  nm. The Zernike modes are ordered by Noll index17 to conform to conventions in astronomy. However, the wavefront error tolerance depends more naturally on the sum of indices n+|m|. As the charge increases, large wavefront errors (>100  pmrms) may be tolerated on more of the low-order aberrations. We have emphasized the lowest 21 modes, many of which tend to dominate the wavefront error budget and may be readily suppressed by vortex coronagraphs. The requirements for noll indices >40 are roughly the same for all charges 10.

JATIS_4_1_015004_f004.png

Table 1

Low-order wavefront error requirements for Earth-like exoplanet detection with vortex coronagraphs on future off-axis, monolithic, space telescopes.

AberrationIndicesAllowable RMS wavefront error per mode (nm)
Nollnml=4l=6l=8l=10
Tip-tilt2,31±11.15.91426
Defocus4200.814.61226
Astigmatism5,62±20.0071.10.94.6
Coma7,83±10.0060.660.825
Trefoil9,103±30.0070.0060.570.67
Spherical11400.0050.510.736.3
Second astig.12,134±20.0080.0070.670.73
Quadrafoil14,154±40.0080.0080.0060.54
Second coma16,175±10.0040.0050.690.85
Second trefoil18,195±30.0050.0060.0040.72
Pentafoil20,215±50.0050.0050.0050.005
Second spherical22600.0030.0030.841.1
Third Astig.23,246±20.0020.0040.0030.82
Second quadrafoil25,266±40.0030.0030.0030.004
Hexafoil27,286±60.0030.0030.0030.004

The requirements given in Fig. 4 and Table 1 may be scaled to any wavelength by simply multiplying the reported RMS wavefront error by a factor of λ/(450  nm). Although a higher charge (e.g., charges 6 or 8) may be used for the shortest wavelengths to improve robustness, using a lower charge (e.g., charge 4) at longer wavelengths would allow exoplanets detected near the inner working angle of the visible coronagraph to be characterized in the infrared, where the wavefront error requirements are naturally less strict. In that case, the infrared coronagraph would drive requirements in some of the lowest order modes, which would be relaxed by a factor of 2 with respect to high-order requirements driven by the visible coronagraph.

2.3.2.

Mid-to-high spatial frequency requirements

Although the coronagraph design provides degrees of freedom for controlling robustness to low-order aberrations, high throughput coronagraphs are naturally sensitive to mid- and high-spatial frequency aberrations. In fact, any coronagraph that passively suppresses midspatial frequency aberrations must also have low throughput for off-axis planets. This is an outcome of the well-known relationship between raw contrast and the RMS wavefront error in Fourier modes.19 The pupil field associated with a single spatial frequency is given by

(12)

P(r,θ)=exp[i22πωsin(2πxaξ)],ra,

(13)

1+i22πωsin(2πxaξ),ra,
where r2=x2+y2, ξ is the spatial frequency in cycles per pupil diameter, and ω is the RMS phase error in waves where we have assumed ω1. The corresponding field just before the focal plane mask is

(14)

F(ρ,ϕ)=f00(ρ,ϕ)+2πω[f00(ρ,ϕ)f00(ρ+,ϕ)],
where ρ2=x2+y2, ρ2=(xξλF#)2+y2, ρ+2=(x+ξλF#)2+y2, and F#=f/(2a). The coronagraph completely rejects the f00(ρ,ϕ) term. Thus, at position (x,y)=(ξλF#,0) after the coronagraph

(15)

F(ξλF#,0)=2ηpπω[f00(0,0)f00(2ξλF#,0)],
where ηp is the coronagraph throughput and F#=f/(2b). Solving for the normalized stellar irradiance, ηs, we find

(16)

ηs=ηp2(πω)2|f00(0,0)f00(2ξλF#,0)|2|f00(0,0)|2.
Therefore, for ξ1, the raw contrast at (x,y)=(ξλF#,0) is

(17)

C=ηs/ηp2(πω)2.

For example, a 1 pm RMS midspatial frequency wavefront error described by the vector ξ=ξxx^+ξyy^ generates a change in raw contrast of 1010 at λ=450  nm in the corresponding image plane location (x,y)=(ξxλF#,ξyλF#). This implies a stability requirement of 1  pmrms per Fourier mode for midspatial frequency wavefront errors. The stellar irradiance as a function of spatial frequency and charge is shown in Fig. 5.

Fig. 5

Stellar irradiance (log scale) due to a sinusoidal phase error as a function of spatial frequency, ξ, and charge. An error in a single Fourier mode generates a speckle at the corresponding position in the image plane. For example, if ωλ=100  pm, the raw contrast at x=±ξλF# is C2(πω)2=9.7×107 at λ=450  nm regardless of the charge.

JATIS_4_1_015004_f005.png

Rejecting starlight with midspatial frequency phase errors and proportionally reducing the coronagraph throughput at the position of interest degrades performance in the photon-noise-limited regime, where the signal-to-noise ratio (SNR) for planet detection scales is defined as ηp/ηs. Provided an optimal coronagraph maximizes the SNR, a coronagraph that is passively robust to spatial frequencies, where (x,y)=(ξxλF#,ξyλF#) is in the region of interest (i.e., dark hole), is not desirable.

2.4.

Sensitivity to Partially Resolved, Extended Sources

The fraction of energy from a point source that leaks through the coronagraph as a function of angular separation, α, may be approximated for small offsets (i.e., αλ/D) through modal decomposition of the source.20 The transmitted energy is given by Tα=τl(παD/λ)l, where τl is a constant [see Fig. 6(a)]. Integrating over an extended, spatially incoherent, stellar source of angular extent, Θ, the expression becomes TΘ=κl(πΘD/λ)l, where κl is a constant [see Fig. 6(b)]. The theoretical values for τl and κl are given in Table 2. Higher charge vortex coronagraphs are far less sensitive to small tip-tilt errors and sources of finite size. For example, charge six vortex coronagraphs sufficiently suppress light from stars with angular diameters up to 0.1λ/D or 2  mas for a 4-m telescope at λ=450  nm.

Table 2

Coefficients for analytical approximations of transmitted energy from point sources at small angular separations, Tα=τl(παD/λ)l, and extended sources TΘ=κl(παD/λ)l.

Chargeτlκl
l=21/81/64
l=41/1921/9216
l=61/92161/2359296
l=81/7372801/943718400

Fig. 6

Sensitivity of the vortex coronagraph to (a) tip-tilt, (b) stellar angular size, and (c) unresolved dust rings. (a)–(b) The total energy leaked versus (a) the angular separation of a point source and (b) the angular size of the star. The coefficients for each power law are given in Table 2. (c) Stellar irradiance, averaged over source positions 3±0.5λ/D and normalized to the peak of the telescope PSF, owing to an unresolved ring of dust at astrophysical contrast of ε=1%.

JATIS_4_1_015004_f006.png

An often overlooked potential source of leaked starlight is the presence of an unresolved disk of dust around the star. Figure 6(c) shows the stellar irradiance that appears in the image plane due to scattered light from the debris ring at astrophysical contrast of 1% and angular separation of 3λ/D from the star as a function of the size of the ring. For example, imaging an Earth-like planet at 3λ/D around a star with a dust ring of radius 0.2λ/D requires at least a charge six coronagraph. We note that over the last 10 years, long baseline near infrared interferometric observations21,22 has suggested that 10% to  20% of nearby main sequence stars have such rings of small hot dust grains concentrating near the sublimation radius, and contributing about 1% of the total solar flux in the near infrared. Assuming that we are looking for a 300 K planet at 3λ/D separation, 1500 K dust grains would be located 25× closer, i.e., at 0.12λ/D. In addition to being more resilient to low-order wavefront aberrations, higher charge vortex coronagraphs are also less sensitive to astrophysical noise sources in the inner part of the system, such as bright dust rings that may be fairly common.

2.5.

Effect of Adding an Opaque Spot to the Vortex Mask

Manufacturing processes will limit the minimum size of the central defect in a vortex phase mask. A small opaque occulting spot may be introduced to block the central region, where the phase shift deviates from the ideal vortex pattern [see Fig. 7(a)]. The maximum allowable size of this mask depends on the charge of the vortex. Figure 7(b) shows the stellar irradiance at 3±0.5λ/D as a function of the mask diameter for various vortex charges. For charges 6 and 8, the occulting mask can be as large as 1λF# and 1.7λF#, respectively, while maintaining sufficient suppression for imaging of Earth-like planets. In each case, the opaque mask does not significantly degrade the planet throughput.

Fig. 7

The influence of an opaque spot at the center of the focal plane mask. (a) Phase shift imposed by a charge 6 vortex mask. Light is blocked within the mask at the center (shown in black). (b) Stellar irradiance, averaged over source positions 3±0.5λ/D and normalized to the peak of the telescope PSF as a function of the mask diameter. Larger masks may be used with higher charges, which alleviate some manufacturing challenges and allow for a reflective low-order wavefront sensor.

JATIS_4_1_015004_f007.png

In addition to masking manufacturing errors, there are other potential benefits to introducing a central opaque spot. For instance, the reflection from the spot may be used for integrated low-order wavefront sensing, as recently demonstrated for the WFIRST CGI,23 potentially in addition to a reflective Lyot stop sensor.24,25 In the case of a charge 6 vortex coronagraph, 80% of the starlight would be available from the reflection off of the opaque mask for fast tip-tilt and low-order wavefront sensing. Combined with the natural insensitivity to low-order aberrations of vortex coronagraphs, this capability will help maintain deep starlight suppression throughout observations and extend the time between calibrations of the wavefront error and reference star images, thereby improving overall observing efficiency.

3.

Architecture B: 6.5-m Off-Axis, Unobscured, Segmented Telescope

The second potential telescope architecture we study for the HabEx mission concept is a 6.5-m off-axis segmented telescope. This arrangement introduces a few additional complications with respect to the monolithic version. First, a primary mirror with a noncircular outer edge generates diffraction patterns that are difficult to null. To remedy this, we insert a circular subaperture in a pupil plane just before the focal plane mask, which provides improved starlight suppression at the cost of throughput (see Fig. 8). Partial segments may also be introduced to form a circular outer edge. Second, the gaps between mirror segments must be apodized to prevent unwanted diffraction in the image plane from amplitude discontinuities. In this section, we present a promising vortex coronagraph design for the 6.5-m HabEx concept and address the associated telescope requirements.

Fig. 8

Schematic of an apodized vortex coronagraph. A gray-scale apodizer (see Fig. 9) prevents unwanted diffraction from the noncircular outer edge of the primary and gaps between mirror segments.

JATIS_4_1_015004_f008.png

3.1.

Apodized Vortex Coronagraph Design

Figure 9(a) shows a notional primary mirror with 37 hexagonal segments whose widths are 0.9  m flat-to-flat. The corresponding pupil masks used in the apodized vortex coronagraph are shown in Figs. 9(b) and 9(c). The apodizer clips the outer edge of the pupil to make it circular and imparts an amplitude-only apodization pattern on the transmitted or reflected field. Most of the starlight is then diffracted by the vortex outside of the Lyot stop. The small amount of starlight that leaks through the Lyot stop (2%) only contains high-spatial frequencies greater than a specified value of ξmax=20 cycles across the pupil diameter. Thus, in an otherwise perfect optical system, a dark hole appears in the starlight within a 20λ/D radius of the star position for all even nonzero values of the vortex charge l. We used the auxiliary field optimization method26 to calculate the optimal grayscale pattern.27,28

Fig. 9

An apodized vortex coronagraph for a 6.5-m HabEx. (a) The image of the primary mirror at the entrance pupil of the coronagraph, (b) the apodizer (squared-magnitude of the desired pupil field), and (c) the Lyot stop. The apodizer and Lyot stop diameters are 83% and 80%, respectively, of the pupil diameter (flat-to-flat).

JATIS_4_1_015004_f009.png

The throughput of the coronagraph with various focal plane vortex masks is shown in Fig. 10. We report both the absolute throughput ηp and relative throughput ηp/ηtel within a circular region of interest of radius r^λ/D centered on the planet position, where ηtel represents the throughput of the telescope with the coronagraph masks removed. After the coronagraph, 60% of the total energy from an off-axis source remains. Less than 30% of the total energy appears within 0.7λ/D of the planet position, including losses from the apodizer and broadening of the point spread function by the undersized pupil mask and Lyot stop. Approximately 50% of the planet light remains within 0.7λ/D compared with the point spread function with the coronagraph masks removed. Other than a loss in throughput, the apodized version shares most of the same performance characteristics as the conventional vortex coronagraph. Furthermore, a primary mirror that includes partial segments to create a circular outer boundary would allow for drastically improved coronagraph throughput.

Fig. 10

The throughput of the apodized vortex coronagraph with charge 4, 6, and 8 focal plane masks. (a) Absolute throughput. The fraction of total planet light that falls within r^λ/D of the planet position, assuming an otherwise perfect optical system. (b) Relative throughput. The fraction of planet light that falls within r^λ/D of the planet position compared with case with the coronagraph masks removed. Throughput losses originate from introducing a semitransparent mask and clipping the outer edge of the pupil to create a circular boundary (see Fig. 9).

JATIS_4_1_015004_f010.png

3.2.

Sensitivity to Low-Order Aberrations and the Angular Size of Stars

Assuming the telescope is off-axis and unobstructed, the leaked stellar irradiance in the presence of low-order aberrations appears identical to the monolithic case, up to a radius of ξmaxλF# (see Fig. 11). However, to maintain a fixed raw contrast threshold, the wavefront error requirements presented in Table 1 scale as 1/ηp; i.e., get tougher to guarantee than in the monolithic case. For the sake of brevity, we have not included an updated wavefront error requirement table here.

Fig. 11

The sensitivity of an apodized vortex coronagraph to low-order aberrations on an off-axis, segmented telescope. Log irradiance owing to λ/1000  rms wavefront error in each mode, normalized to the peak value with the coronagraph masks removed. The dark zone has an angular diameter of 40λ/D. As in the case of a monolithic telescope, higher charge vortex coronagraphs passively suppress more low-order Zernike modes.

JATIS_4_1_015004_f011.png

The stellar leakage due to the angular size of stars is also equivalent to a vortex coronagraph without an apodizer. Figure 12 shows the leaked starlight as a function of stellar angular size. A charge 4 is sufficient to suppress stars 0.01  λ/D in diameter. Charges 6 or 8 may be used to maximize SNR (ηp/ηs) in the case of a larger star, such as Alpha Centauri A whose angular diameter is 8.5 mas or 0.5λ/D in the visible.

Fig. 12

The sensitivity of an apodized vortex coronagraph to stellar angular diameter on an off-axis, segmented telescope. Log stellar irradiance, normalized to the peak value with the coronagraph masks removed. The dark zone has a diameter of 40λ/D. The simulation is monochromatic but applies to all wavelengths. As in the case of a monolithic telescope, higher charge vortex coronagraphs leak less light from partially resolved stars.

JATIS_4_1_015004_f012.png

3.3.

Segment Cophasing Requirements

A major challenge for exoplanet imaging with a segmented telescope will be to keep the mirrors coaligned throughout observations. As shown in Fig. 13, small segment motions in piston and tip-tilt cause speckles to appear in the dark hole which may be difficult to calibrate and will likely contribute to both photon and spatial speckle noise. Figures 14(a) and 14(b) show the time average over many realization of the errors shown in Fig. 13 drawn from Gaussian distributions for both piston and tip-tilt with standard deviations of 100 pm and 0.005λ/D=71  μasrms. When the mirror segments have random piston errors only, the resulting distribution of light resembles the diffraction pattern of a single segment [Fig. 14(a)]. Random segment tip-tilt error tends to spread the leaked starlight to larger separations [Fig. 14(b)]. However, the leaked starlight is well approximated by a similar second-order power law in both cases [Fig. 14(c)], which yields a wavefront error requirement of 10  pmrms, similar in magnitude to unsuppressed low-order modes. In contrast, if the primary mirror segments undergo a coordinated movement that resembles a low-order Zernike polynomial Znm, the amount of leaked starlight would be significantly smaller if l>n+|m| and the tolerance to such a motion would be considerably relaxed.

Fig. 13

Left: (a) Example wavefront with 100 pm rms of random segment piston errors and (b) the corresponding stellar irradiance at λ=450  nm. Right: Same as left, but with an additional 0.005λ/D=71μ as RMS of random tip-tilt errors. These aberrations cause speckles to appear close to the star where planets are likely to reside.

JATIS_4_1_015004_f013.png

Fig. 14

(a)–(b) Time average over many realizations of leaked stellar irradiance at λ=450  nm due to (a) 100 pm rms of random segment piston and (b) with an additional 0.005λ/D=71μ as rms tip-tilt error. (c) Dependence of stellar irradiance at 3λ/D=43  mas on the rms wavefront error segment piston and tip-tilt. The simulation is monochromatic but applies to all wavelengths. Segment piston and tip-tilt must be controlled to 105 waves rms, or <10  pmrms, to ensure detection of Earth-like planets.

JATIS_4_1_015004_f014.png

3.4.

Fabrication of Grayscale Apodizing Masks

Achromatic grayscale apodizers have been fabricated using metallic microdots arranged in error-diffused patterns.29,30 Prototype masks produced specifically for the purpose of demonstrating apodized vortex coronagraphs are currently being tested at the High-Contrast Spectroscopy Testbed for Segmented Telescopes (HCST) at Caltech.31 These experiments seek to validate this approach for use on HabEx and prepare for future testing on vacuum testbeds.

4.

Conclusions and Outlook

Vortex coronagraphs provide a viable pathway toward imaging Earth-like exoplanets with the HabEx decadal mission concept with a fully off-axis telescope architecture. We have provided an overview of the performance of vortex coronagraphs and wavefront stability requirements. The off-axis design of the HabEx telescope allows for the best possible performance in terms of throughput, inner working angle, and robustness to aberrations. With a vortex coronagraph, the low-order wavefront error requirements for imaging Earth-like planets with HabEx are comparable with those to be demonstrated by the WFIRST-CGI for imaging Jupiters.32 A segmented primary mirror does not fundamentally change the performance characteristics of a vortex coronagraph. However, mirror segments introduce challenging segment cophasing requirements and the need for apodization. In addition to the grayscale pupil mask presented here, alternate apodization approaches are available that shape the pupil amplitude using deformable mirrors.33 A trade study is needed to identify the performance trades between these apodization solutions. In all cases, we find that the throughput of vortex coronagraphs and robust to wavefront errors degrades significantly on centrally obscured telescopes. Several studies are underway to improve performance for such architectures.34

Appendices

Appendix A:

Zernike Polynomials

The Zernike polynomials35 may be written as

(18)

Znm(r/a,θ)=Rn|m|(r/a){cos(mθ)m0sin(|m|θ)m<0,ra,
where Rnm(r/a) is the radial Zernike polynomial given by

(19)

Rnm(r/a)=k=0nm2(1)k(nk)!k!(n+m2k)!(nm2k)!(r/a)n2k,r/a1,
where nm is even. The indices n and m are the integers, respectively, known as the degree and azimuthal order. The first few radial polynomials are R00=1, R11=r/a, R20=2(r/a)21, R22=(r/a)2, R31=3(r/a)32(r/a), and R33=(r/a)3.

Appendix B:

Weber–Schafheitlin Integrals

The pupil functions generated by vortex coronagraphs are a subset of solutions of the discontinuous integral of Weber and Schafheitlin,36 which in its conventional form is written

(20)

Wν,μ,λ(t;α,β)=0Jν(αt)Jμ(βt)tλdt,
where ν, μ, λ are the integers and α and β are the constants. The integral is convergent provided ν+μλ0 and λ0. If 0<α<β

(21)

Wν,μ,λ(t;α,β)=ανΓ(ν+μλ+12)2λβνλ+1Γ(ν+μ+λ+12)Γ(ν+1)×F12(ν+μλ+12,νμλ+12;ν+1;α2β2),
where Γ() is the gamma function and F12() is a hypergeometric function.37 On the other hand, if 0<β<α

(22)

Wν,μ,λ(t;α,β)=βνΓ(ν+μλ+12)2λανλ+1Γ(νμ+λ+12)Γ(ν+1)×F12(ν+μλ+12,ν+μλ+12;μ+1;β2α2).

Integrals with the form of Eq. (20), namely a product of Bessel functions, appear in the output function integral in cases, where the input function is circular or may be described by a Zernike polynomial in amplitude.38

Appendix C:

First-Order Exit Pupil Modes

Here, we provide the analytical solutions to Eq. (10) for l0 and m0.

C1.

Piston Z00

(23)

g000(r,θ)={1ra0r>a,

(24)

g002(r,θ)={0ra(ar)2ei2θr>a,

(25)

g004(r,θ)={0ra[3(ar)4+2(ar)2]ei4θr>a,

(26)

g006(r,θ)={0ra[10(ar)612(ar)4+3(ar)2]ei6θr>a,

(27)

g008(r,θ)={0ra[35(ar)860(ar)6+30(ar)44(ar)2]ei8θr>a.

C2.

Tip-Tilt Z11

(28)

g110(r,θ)={racos(θ)ra0r>a,

(29)

g112(r,θ)=12{raeiθra(ar)3ei3θr>a,

(30)

g114(r,θ)=12{0ra[4(ar)53(ar)3]ei5θ+(ar)3ei3θr>a,

(31)

g116(r,θ)=12{0ra[15(ar)720(ar)5+6(ar)3]ei7θ+[4(ar)53(ar)3]ei5θr>a,

(32)

g118(r,θ)=12{0ra[56(ar)9105(ar)7+60(ar)510(ar)3]ei9θ+[15(ar)720(ar)5+6(ar)3]ei7θr>a.

C3.

Defocus Z20

(33)

g200(r,θ)={2(ra)21ra0r>a,

(34)

g202(r,θ)={(ra)2ei2θra0r>a,

(35)

g204(r,θ)={0ra(ar)4ei4θr>a,

(36)

g206(r,θ)={0ra[5(ar)64(ar)4]ei6θr>a,

(37)

g208(r,θ)={0ra[21(ar)830(ar)6+10(ar)4]ei8θr>a.

C4.

Astigmatism Z22

(38)

g220(r,θ)={(ra)2cos(2θ)ra0r>a,

(39)

g222(r,θ)=12{2(ra)21ra(ar)4ei4θr>a,

(40)

g224(r,θ)=12{(ra)2ei2θra[5(ar)64(ar)4]ei6θr>a,

(41)

g226(r,θ)={0ra[212(ar)815(ar)6+5(ar)4]ei8θ+12(ar)4ei4θr>a,

(42)

g228(r,θ)={0ra[42(ar)1084(ar)8+1052(ar)610(ar)4]ei10θ+[52(ar)62(ar)4]ei6θr>a.

C5.

Coma Z31

(43)

g310(r,θ)={[3(ra)32ra]cos(θ)ra0r>a,

(44)

g312(r,θ)={12(ra)3ei3θ+[32(ra)3ra]eiθra0r>a,

(45)

g314(r,θ)=12{(ra)3ei3θra(ar)5ei5θr>a,

(46)

g316(r,θ)={0ra[3(ar)752(ar)5]ei7θ+12(ar)5ei5θr>a,

(47)

g318(r,θ)={0ra[14(ar)921(ar)7+152(ar)5]ei9θ+[3(ar)752(ar)5]ei7θr>a.

C6.

Spherical Z40

(48)

g400(r,θ)={6(ra)46(ra)2+1ra0r>a,

(49)

g402(r,θ)={[4(ra)43(ra)2]ei2θra0r>a,

(50)

g404(r,θ)={(ra)4ei4θra0r>a,

(51)

g406(r,θ)={0ra(ar)6ei6θr>a,

(52)

g408(r,θ){0ra[7(ar)86(ar)6]ei8θr>a.

C7.

Trefoil Z33

(53)

g330(r,θ)={(ra)3cos(3θ)ra0r>a,

(54)

g332(r,θ)=12{[3(ra)32ra]eiθra(ar)5ei5θr>a,

(55)

g334(r,θ)=12{[3(ra)32ra]eiθra[6(ar)35(ar)5]ei7θr>a,

(56)

g336(r,θ)=12{(ra)3ei3θra[28(ar)942(ar)7+15(ar)5]ei9θr>a,

(57)

g338(r,θ)={0ra[60(ar)11126(ar)9+84(ar)7352(ar)5]ei11θ+12(ar)5ei5θr>a.

Acknowledgments

We thank the HabEx Coronagraph Technology Working Group (CTWG) for useful discussions. G. Ruane is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1602444. This work was supported by the Exoplanet Exploration Program (ExEP), Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA.

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© The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
Garreth Ruane, Garreth Ruane, Dimitri Mawet, Dimitri Mawet, Bertrand Mennesson, Bertrand Mennesson, Jeffrey B. Jewell, Jeffrey B. Jewell, Stuart B. Shaklan, Stuart B. Shaklan, } "Vortex coronagraphs for the Habitable Exoplanet Imaging Mission concept: theoretical performance and telescope requirements," Journal of Astronomical Telescopes, Instruments, and Systems 4(1), 015004 (28 March 2018). https://doi.org/10.1117/1.JATIS.4.1.015004 . Submission: Received: 14 November 2017; Accepted: 9 March 2018
Received: 14 November 2017; Accepted: 9 March 2018; Published: 28 March 2018
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