PIAA coronagraph architecture

There are several possible coronagraph architectures using PIAA optics. Figure 1 shows both the minimum PIAA coronagraph conceptual architecture, and a full architecture. The minimum architecture simply produces an image after the PIAA optics. The central source's light is highly concentrated in a single narrow diffraction spot, allowing much fainter off-axis sources to be imaged. This simple architecture, which is described in Guyon 2003, is however limited in both performance and manufacturability :
Figure 1: Two example of PIAA coronagraph system architectures. Both architectures use a set of aspheric optics (top left) to perform lossless apodization of the telescope beam. The minimum PIAA coronagraph system (top right) consists of a PIAA apodization unit introduced before the instrument's focal plane detector array. A full PIAA coronagraph system (bottom) can also include an apodizer, a focal plane mask, a pupil stop and an inverse PIAA optics unit.
These issues can be solved or mitigated with a more complete PIAA system design, as shown in the bottom of Figure 1. A complete PIAA system includes the following components in addition to the PIAA aspheric optics:
Figure 2: Five possible PIAA coronagraph architectures, approximately ordered from top to bottom in increasing performance and manufacturing ease.
Figure 2 shows five possible PIAA system configuration implementing none (a), a subset (b-d) or all (e) of the components listed above. Figure 3 shows how these architectures compare in both performance and technical difficulty (mostly manufacturing). The manufacturing challenges associated with each of the five architectures are listed in the table shown on Figure 4.
Figure 3: Approximate location of the five architectures shown in Figure 2 on a 2-D plane with performance (x axis) and degree of technical difficulty (y axis) coordinates.
Figure 4: For each of the four architectures, manufacturing challenges are different, as shown in this table.

Wavefront control in a PIAA system: importance of deformable mirror location

The outer working angle (OWA) is defined by the furthest (from the optical axis) speckles that the DM(s) can cancel in the science focal plane. In non-PIAA coronagraph and with actuators organized on a regular square grid, the OWA is therefore (N/2)x(λ/D), where N is the number of actuators along the diameter of the pupil. The OWA then appears in high contrast adaptive optics (AO) systems as the size of the "high contrast" box in the focal plane. In a PIAA system this straightforward relationship becomes more complicated, due to the pupil remapping. The location of the deformable mirror in a PIAA system can therefore have a large impact on the outer working angle (OWA) of the system. Because of the remapping, the local radial slope of the wavefront finds itself magnified by the local value of the apodization function (Martinache et al. 2006). If one notes A(r) the radial apodization profile after PIAA remapping (in square root of the intensity), the post-PIAA wavefront radial slope of an off-axis source (angular separation α, azimuth θ0) therefore writes as:
δφ(r,θ)/δr = α A(r) cos(θ-θ0). (equ 1)
Figure 5 illustrates the impact of the remapping on the wavefront slope: in the inner part of the beam, where A(r) is maximum, the slope is amplified by a factor βa > 1, while near the outer edge of the beam, it is reduced by a factor βr < 1. Typical values for these factors are βa = 3 and βr = 0.3. Because in the post-PIAA beam, most of the light is concentrated toward the center of the beam. It is therefore this part of the wavefront, and the factor βa that will define the location of the off-axis image pseudo-core.
Figure 5: Slope amplification and reduction factors in a PIAA system. The remapping introduced by a PIAA system amplifies the wavefront slope at the center of the apodized beam (where most of the light is located) and reduces the wavefront slope at the edges of the beam.
These factors can quantitavely be used to estimate the OWA for several PIAA+DM system configurations (Figure 6), in which the DM(s) must correct for wavefront errors both before and after the remapping :
Figure 6: Four possible architectures for a PIAA coronagraph with wavefront control. For each configuration, the outer working angle of the wavefront control system and the field of view imposed by remapping are given. The PIAA slope amplification factor βa = 3 and slope reduction factor βr = 0.3 are considered here. Configurations shown in gray (configurations 2 and 4) should be avoided (see text for details). The unaberrated FOV values are given assuming N=32 actuators across the pupil diameter.
In both configurations 1 and 2, the unaberrated field of view is limited to r ~ 5 λ/D by the field aberrations introduced by the PIAA optics. This effect is described in Figures 4 to 7 in Guyon et al. 05. There is therefore little advantage to increasing the wavefront control OWA much beyond this radius, although it can be beneficial to do so in polychromatic light to avoid chromatic speckles within the OWA due to non-linear frequency folding of speckles just outside the OWA (Giveon 2005). We note that the OWA and the unaberrated field of view are equal for N ~ 30 and N ~ 100 in configurations 1 and 2 respectively. We now explore configurations including inverse PIAA optics to provide a wide unaberrated field of view (> 100 λ/D in radius as shown in Guyon et al 2005, Figure 11). In these configurations, the field of view for high contrast observations is limited by the OWA of the wavefront control system. In order to optimize the use of a given number of actuators, the DM(s) should therefore be placed after the PIAA optics in a PIAA system without inverse PIAA optics (configuration 1), or before the PIAA optics if inverse PIAA optics are included (configuration 3). The configuration adopted in our laboratory demonstration is configuration 1 (DM after PIAA optics, no inverse PIAA optics).