Polarization aberrations in next-generation giant segmented mirror telescopes (GSMTs)

Abstract

Context. Next-generation large segmented mirror telescopes are expected to perform direct imaging and characterization of Earthlike rocky planets, which requires contrast limits of 10 −7 to 10 −8 at wavelengths from I to J band. One critical aspect affecting the raw on-sky contrast are polarization aberrations (i.e., polarization-dependent phase and amplitude patterns in the pupil) arising from the reflection from the telescope's mirror surfaces and instrument optics. These polarization aberrations induce false signals for polarimetry that can be calibrated to a certain degree, but they can also fundamentally limit the achievable contrast of coronagraphic systems. Aims. We simulate the polarization aberrations and estimate their effect on the achievable contrast for three next-generation groundbased large segmented mirror telescopes. Methods. We performed ray-tracing in Zemax ® and computed the polarization aberrations and Jones pupil maps using the polarization ray-tracing algorithm. The impact of these aberrations on the contrast is estimated by propagating the Jones pupil maps through a set of idealized coronagraphs using hcipy, a physical optics-based simulation framework. Results. The optical modeling of the giant segmented mirror telescopes (GSMTs) shows that polarization aberrations create significant leakage through a coronagraphic system. The dominant aberration is retardance defocus, which originates from the steep angles on the primary and secondary mirrors. The retardance defocus limits the contrast to 10 −5 to 10 −4 at 1 λ/D at visible wavelengths, and 10 −5 to 10 −6 at infrared wavelengths. The simulations also show that the coating plays a major role in determining the strength of the aberrations. Conclusions. Polarization aberrations will need to be considered during the design of high-contrast imaging instruments for the next generation of extremely large telescopes. This can be achieved either through compensation optics, robust coronagraphs, specialized coatings, calibration, and data analysis approaches, or by incorporating polarimetry with high-contrast imaging to measure these effects.

Estimation of polarization aberrations and their effect on the coronagraphic performance for future space telescopes

“…The Al+ 18nm LiF coating does as well as Bare aluminum/Oxidized aluminum. It is well observed in the previous simulations in 7,16 that the smaller the difference in the amplitude and phase of the reflection coefficients, the smaller the magnitude of the polarization aberrations introduced in the optical system. So, Bare aluminum, Oxidized aluminum, and Al+18nm LiF are expected to have better coronagraphic performance than other coatings.…”

“…The full description of the PRT algorithm used in our simulations is explained in detail in 23 and. 16…”

“…The Modeling of polarization aberrations for the next-generation GSMTs by Anche et al (2023) 16,17 revealed that coating plays a significant role in the magnitude of these aberrations; The bare aluminum coating performs much better for the Giant Magellan Telescope than the Gemini-like coating, although the latter is optimized for higher reflectivity. Our simulations show that the peak raw contrast for GMT with Bare aluminum coating is in the order of 10 −4 and becomes an order worse ∼ 10 −3 for Silver and Gemini coating for a fourth-order perfect coronagraph.…”

“…The two telescopes considered are three-mirror anastigmats with a fourth mirror used to fold the beam. We also study the polarization aberration residuals at the focal plane of different coronagraph architectures: Perfect coronagraph of order 2, 4, and 6, 16 Vector Vortex coronagraph, 18 and Scalar Vortex Coronagraph 19 of charge 2,4 and 6. We aim to understand the combined effect of coating and coronagraphs to determine the best coating+coronagraph combination for future space telescopes.…”

FALCO simulations of high-contrast polarimetry with the Nancy Grace Roman Space Telescope coronagraph instrument

“…27,28 This happens with powered optics, like the primary and secondary mirror, or when converging beams are reflected off flat mirrors like tertiary mirrors of Nasmyth telescopes, 27,29 and has been characterized for a variety of telescopes. [30][31][32][33] The local changes in angle of incidence result in locally varying s-and p-polarization states for the Fresnel coefficients, resulting in locally changing reflection amplitudes and locally varying phases, introducing beam shifts. 28 The detrimental effect on the PSF of the phase aberrations is much stronger than that of the amplitude aberrations.…”