Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Non-common path aberration compensation and a dark hole loop with a pyramid adaptive optics system: Application to SAXO+

C. Goulas, R. Galicher, F. Vidal, J. Mazoyer, F. Ferreira, A. Sevin, A. Potier, A. Boccaletti, E. Gendron, C. Béchet, M. Tallon, M. Langlois, C. Kulcsár, H-F. Raynaud, N. Galland, L. Schreiber, I. Bernardino Dinis, F. Wildi, G. Chauvin

Abstract

In ground-based high-contrast instruments, non-common path aberrations (NCPAs) limit detection performance, as they are unseen by the adaptive optics (AO) wavefront sensor but impact the astrophysical image, creating quasi-static speckles. SAXO+, the upgrade of the SAXO (SPHERE AO system) includes a second loop of AO downstream of the SAXO loop that is equipped with a near-infrared pyramid wavefront sensor whose nonlinearities, usually described with modal optical gains, might be challenging for removing quasi-static speckles. We investigated two methods of quasi-static speckle removal : NCPA compensation and a dark hole loop, behind a pyramid AO system, measuring the interest of compensating for the pyramid optical gains. We performed end-to-end numerical simulations under various astrophysical conditions. We offset the pyramid wavefront sensor operating point to apply both the speckle suppression methods, with or without optical gain calibration. We evaluated the performance by measuring the residual starlight in the coronagraph image. A by-product of our study is an on-sky calibration method of measuring the pyramid optical gains. NCPA compensation reduces the residual starlight in the coronagraph image by a factor of 20 for seeing between 0.7" and 1" for a bright star and a factor of 2 at 0.7" for a faint star. Optical gains compensation enhances the performance at poor seeing and small pyramid modulation radius with a bright star, but shows a useless or even negative impact due to estimation inaccuracies at faint targets. On the other hand, the dark hole loop reduces the residual starlight by a factor of 200. The optical gain calibration enhances the dark hole performance behind a single pyramid AO system but is useless behind the SAXO+ system. Our parametric study gives baseline values for the efficient control of the dark hole loop for the SAXO+ system.

Non-common path aberration compensation and a dark hole loop with a pyramid adaptive optics system: Application to SAXO+

Abstract

In ground-based high-contrast instruments, non-common path aberrations (NCPAs) limit detection performance, as they are unseen by the adaptive optics (AO) wavefront sensor but impact the astrophysical image, creating quasi-static speckles. SAXO+, the upgrade of the SAXO (SPHERE AO system) includes a second loop of AO downstream of the SAXO loop that is equipped with a near-infrared pyramid wavefront sensor whose nonlinearities, usually described with modal optical gains, might be challenging for removing quasi-static speckles. We investigated two methods of quasi-static speckle removal : NCPA compensation and a dark hole loop, behind a pyramid AO system, measuring the interest of compensating for the pyramid optical gains. We performed end-to-end numerical simulations under various astrophysical conditions. We offset the pyramid wavefront sensor operating point to apply both the speckle suppression methods, with or without optical gain calibration. We evaluated the performance by measuring the residual starlight in the coronagraph image. A by-product of our study is an on-sky calibration method of measuring the pyramid optical gains. NCPA compensation reduces the residual starlight in the coronagraph image by a factor of 20 for seeing between 0.7" and 1" for a bright star and a factor of 2 at 0.7" for a faint star. Optical gains compensation enhances the performance at poor seeing and small pyramid modulation radius with a bright star, but shows a useless or even negative impact due to estimation inaccuracies at faint targets. On the other hand, the dark hole loop reduces the residual starlight by a factor of 200. The optical gain calibration enhances the dark hole performance behind a single pyramid AO system but is useless behind the SAXO+ system. Our parametric study gives baseline values for the efficient control of the dark hole loop for the SAXO+ system.
Paper Structure (14 sections, 4 equations, 17 figures, 2 tables)

This paper contains 14 sections, 4 equations, 17 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Speckle removal techniques
  3. NCPA compensation or dark hole loop
  4. The pyramid optical gains issue
  5. Simulation assumptions
  6. Pyramid optical gains estimation
  7. Overview of the method
  8. Parametric optimization
  9. NCPA compensation
  10. Dark hole loop
  11. Probe with the pyramid wavefront sensor
  12. Dark hole loop along a single-stage AO with a PWFS
  13. Dark hole loop with the two-stage SAXO+ system
  14. Conclusions

Figures (17)

  • Figure 1: SAXO+ current design. The first stage maximum frequency is 1.38 kHz and second stage maximum frequency is 3 kHz.
  • Figure 2: Optical gain versus the mode order, for several seeings. Top : Modal gains measurement (gray) and fit functions (red, blue and green). Bottom : Difference between the fit and the measurement. Science case : Bright, $\tau_0 = 2$ ms.
  • Figure 3: Measured optical gain versus the mode order, for several frequencies from 20 Hz in blue to 400 Hz in purple. Science case: Bright, seeing = 0.8", $\tau_0 = 3$ ms.
  • Figure 4: Standard deviation of the fit residuals versus the modulation amplitude and the modulation exposure. Science case: Bright, $\tau_0 = 2$ ms, seeing = 1".
  • Figure 5: Coronagraph images obtained after a 10 seconds exposure. Left : With NCPA, not compensated for. Middle : Compensation of NCPA without calibrating optical gains. Right : Compensation of NCPA, calibrating optical gains. Seeing = 0.85", $\tau_0 =$ 2 ms.
  • ...and 12 more figures