Efficiency of the Lyot stop

 

 

Figure: a. Reference image obtained with a white light source inside the BOA adaptive optics system, using the $0.45\arcsec$ mask and without the Lyot stop. The Airy rings remain visible. Diffraction spikes from the spider arms can be seen as dotted lines, influenced by the ring structure. In such conditions a planetary companion would be invisible. In spite of the broad spectral bandwidth ($\Delta\lambda=650-850nm$) at $\lambda_0 = 635 nm$, the outer rings retain good contrast, owing to the Wynne corrector.

 

Figure: b. The same reference frame obtained with the mask and the $400\mu m$ Lyot stop. Here, the Airy pattern is markedly attenuated and the gain in sensitivity is about 1.7 magnitudes. Nevertheless, due to the inadapted Lyot stop, some bright features remain, like the rings around the mask and the four symmetrical speckles which could bring out wrong detections. These static defects can be partially removed with frame subtraction.

To calibrate the efficiency of the Lyot stop, we have acquired an internally-generated reference image using a single-mode fiber included in BOA. The Strehl ratio (SR) of this reference source is about $80\%$ and does not take into account the atmospheric turbulence or the static aberrations of the adaptive mirror. When the core of the Airy pattern is occulted by the mask, the edges of the pupil image become decorated with two bright fringes (Fig. 1). The complementary spatial filter in the pupil plane should suppress much of the diffracted light, except that caused by the wave bumpiness. The Lyot stop being a simple pinhole of $400\mu m$ instead of a telescope pupil image, some Airy-like rings remain visible in the final image (Fig. 3.b). Moreover, the spider spikes combined with the residual rings produce symmetrical side-lobes, especially bright within the 2 first rings. These artifacts remain on the compensated images despite the smoothing introduced by the atmospheric turbulence.
One can compute the rejection rate of the coronagraph as defined in [Malbet 1996] :

$$R={I_{w/o}\over I_w}$$ (1)

where I_w/o is the total intensity of the reference beam without coronagraph and I_w is its intensity with the coronagraph. To characterize the efficiency of the Lyot stop, one defines R_w the rejection rate with the Lyot stop and R_w/o without it. We can then estimate the gain in magnitude introduced by the Lyot stop with the following relation ([Malbet 1996]) :

$$\Delta m=2.52 Log(2R_w/R_{w/o})$$ (2)

A gain of 1.7 magnitude has been measured for the $0.45\arcsec$ mask and the $400\mu m$ Lyot stop. As this value is averaged over the entire field, it is therefore underestimated far from the axis and overestimated near the mask. An optimized Lyot stop, including secondary mirror and spider arms, should improve the gain by another 1.3 magnitude ([Malbet 1996]).