Direct Imaging and wavefront control- Coronagraphs

Question 1

What is the purpose of a coronagraph?

Answer 1

A coronagraph is an optical device designed to suppress (or strongly attenuate) the on-axis coherent starlight while allowing the off-axis planet (or circumstellar disk) light to transmit through.

Question 2

If you knew the PSF exactly (say from a perfect optical model and wavefront sensor data), would it be good enough to subtract it, or do you need a coronagraph regardless?

Answer 2

The light from the central source greatly reduces the signal-to-noise achievable even with perfect PSF subtraction.

Question 3

What are the metrics that are important in a coronagraph?

Answer 3

Raw Contrast: ratio of local surface brightness to peak PSF surface brightness

Inner Working Angle: How close can a faint source be imaged from the star, i.e the point at which the source throughput is 50% of max throughput

Stellar Normalised Intensity: To measure the efficiency of the minimisation of star’s intensity. Normalized intensity values usually range from 1 in the center of the focal plane in the absence of a coronagraph to lower than 1e−10 or better for very good coronagraphic systems.

Throughput: The ratio of the open telescope area remaining after high contrast is achieved. In other words, a measurement of the amount of light from the the exoplanet that reaches the focal plane

Bandwidth: The wavelengths at which high contrast is achieved.

Sensitivity: The degree to which contrast is degraded in the presence of aberrations

Question 4

Lyot coronagraph

Answer 4

An intermediate focal plane is created in the telescope’s optical system, and a dark spot is placed in the center. Alternatively, a mirror with a hole could be placed at this focal plane, and the light from the star goes through the hole, and everything else proceeds on to the detector. Drawbacks are it is a fixed sized, often needs to be oversized, and extra diffraction effects need to be carefully controlled

Question 5

Vortex coronagraph

Answer 5

Vortex coronagraph works by adding a ramping phase in a spiral around the center of the pupil-plane. This creates a perfect nulling for any light on-axis. Amplitude vortex’s use optics that directly impose this phase spiral, but only work for one wavelength of light. Vector vortex’s instead rotate the angle of polarization around the pupil plane that has the same effect, but is wavelength agnostic

Question 6

Four Quadrant Phase Mask

Answer 6

Focal plane masks induces a π-phase shift on two quadrant in a diagonal with respect to two others. Contrary to the classical Lyot coronagraph, the stellar light is not blocked but only phase-shifted in the focal plane. The phase-shift is such that all the starlight is diffracted outside the geometric pupil and stopped by the Lyot stop in the focal plane (creating a self-destructive interference). No starlight reaches the detector.

Question 7

What causes speckles in the coronagraphic image

Answer 7

NCPAs
- Phase errors on the optical surface , i.e. delays or advances of part of the wavefront with respect to a flat wavefront
- Amplitude errors, i.e. local transmission differences over the beam. They can be caused by small holes, dust or coating defects on the optics.