MIRI Coronagraphic Performance (OLD)


The performance of the MIRI flight coronagraphs was tested and verified in the flight model (FM) imager prior to its delivery to be integrated with the full MIRI Optics Module (OM).

Introduction

The performance of the MIRI flight coronagraphs was verified in the flight model (FM) imager prior to its delivery to be integrated with the full MIRI Optics Module (OM) (Ronayette et al. 2010).  A broader discussion on JWST coronagraphy discusses terminology and implementation strategies.  

 


Test arrangement

The test arrangement consists of a warm source outside the cryostat with heavy filtering to block room-temperature thermal radiation.  A 30 μm diameter pinhole source (much smaller than the FWHM of the diffraction pattern) in the external apparatus was scanned in a raster pattern to measure the response of the coronagraphs.

Figure 1

Sample images at 11.40 μm: left without the coronagraph, right with it. The field-of-view is 8.36′′ (76 detector pixels) and the color scale indicates the contrast with respect to the PSF maximum. The level of the coronagraphic image is close to the background noise and the central residual is attributed to the size of the source, which is not perfectly point-like. The faint extended residual in the image to the right is due to local heating of the surroundings of the pinhole infrared source, which is not rejected by the coronagraph and places an artificial floor on its contrast performance.

 

The coronagraph shows good rejection of the central point spread function (PSF), by a factor larger than 100 on-axis (up to a few hundreds), although the level of rejection is proba- bly limited more by the experimental limitations than by the coronagraph. The scans also showed that the inner-working angle (IWA), defined as the angle with 50% of the maximum transmission, is at 1.3λ=D for the 4QPM units.

Figure 2

Radial transmission of the MIRI coronagraphs (4QPM in red, Lyot spot in blue).
Figure 3

Contrast achieved in the test of the flight 4QPM coronagraphs (red for 10.65 μm, magenta for 11.40 μm, blue for 15.50 μm) in the flight imager. The solid lines are for the unattenuated PSF and the dashed lines show the effect of the 4QPM.


Coronagraph Contrast Performance 

 Boccaletti et al. (2015) describe in detail two models for their analysis: Case A makes relatively optimistic assumptions about the wave front error, while Case B is more conservative.  These cases vary the waver front error (WFE) and offsetting accuracy.

 

Figure 4

Normalized noise-free contrasts obtained under Case A (smaller WFE, in red) and case B (larger WFE, in blue: only the reference star subtracted final result) in F1140C (left) and F1550C (right) filters on the PSF (solid) and the raw coronagraphic image (dashed). Estimated 5σ contrasts using reference star subtraction are also shown (dash-dotted).

 

 


Full System Performance 

 The second part of the simulation includes the detection noise.  The figure below displays four cases for various filters. Each subpanel shows the averaged contrast for the PSF (blue line) and the coronagraphic image (red solid line) as well as the 3-σ contrast for the reference star subtraction process (red dashed line) and the ideal noise level if only photon noise dominates (red dash-dotted line). Typical contrasts after postprocessing achieve 104 to 105 for separations larger than 0.5′′–1′′. Note that the radial transmissions of the MIRI coronagraphs are not accounted for in these contrast profiles. 

 

Figure 5
Estimated performance of MIRI in a few selected cases, assuming Case A (smaller WFE) and including the effects of noise. Colored symbols in Figure 10 stand for the intensity of Jovian planets orbiting G and M stars at 10 pc and for different assumed planetary temperatures. The radial transmission of the coronagraph for each modeled planet is highlighted with dotted lines.

The performance presented in this section is strongly related to the end-to-end optical system assumptions, such as the wavefront error and the whole optical train stability of the telescope. The simulated performance may be conservative, particularly with the development of operations and analysis techniques optimized for JWST.

The current model will have to be updated once the telescope is on orbit and commissioned. 


References

Boccaletti, A., et al. "The Mid-Infrared Instrument for the James Webb Space Telescope, V: Predicted Performance of the MIRI Coronagraphs." 2015, PASP, 127, 633.

Ronayette, S., et al. 2010, " Performance verification of the MIRI imager flight model at CEA," SPIE, 7731, 3.

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