Large Segmented Mirror Telescope Research Articles

Terrace mode of the Keck telescope primary mirror.

It has been well-established that for the extremely large segmented-mirror telescopes (ELTs) currently under construction, segment clocking (in-plane rotation) will result in piston errors between neighboring segments. By contrast, the Keck telescopes utilize a fundamentally different edge sensor geometry, which should in principle render them insensitive to this effect. However, we show that as the result of a systematic internal misalignment of the Keck edge sensors, they in fact suffer from clocking-dependent effects that are remarkably similar to those anticipated for segmented-mirror ELTs. The Keck telescopes thus provide a convenient testbed for studying segment clocking and its associated effects. Analysis of Keck phasing data shows that the segment clocking effects are not random, but systematic and lead to a global segment misalignment mode of striking symmetry in which the primary mirror assumes a terraced structure. This terrace mode has been known for some time at Keck but has only recently been understood to be a direct consequence of segment clocking. A quantitative measurement of terrace mode can be used to diagnose and calibrate segment clocking effects at Keck and for future ELTs.

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

Context. Next-generation large segmented mirror telescopes are expected to perform direct imaging and characterization of Earth-like 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 ground-based 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.

Approach based on χ2 to phasing segmented-mirror telescopes using multiple wavelengths: data reduction, wavelength selection, capture range.

The design of extremely large segmented-mirror telescopes (ELTs) calls for their segments to be phased, and a number of different interferometric techniques have been proposed in the literature for this purpose. To achieve the required large capture ranges for ELTs (typically a few microns), most of these techniques use measurements at multiple wavelengths, similar to what is done in standard distance-measuring laser interferometry. In this paper, we present a χ2-based framework not tied to a specific phasing technique to obtain intersegment edge heights from multiwavelength data. We show how this scheme can be used to select an optimal set of wavelengths that maximizes the capture range while achieving the very low error rates required for ELT phasing. We take into account the wavelength uncertainty associated with the finite-width interference filters used in on-sky phasing. Finally, we present data from the Keck 2 telescope that illustrate the method for three-wavelength phasing.

Primary mirror segmentation for large optical telescopes: an inverse map projection approach.

Large segmented-mirror telescopes are powerful tools used in modern astronomy. Segmentation of the primary mirror is important for support design, mirror fabrication, and construction cost reduction. A curvature study showed that a primary mirror with a solid angle of less than π/6 can be well divided by projecting identical hexagonal tilings onto a curved surface. Three approaches, based on map projection, are presented. The conformal method, preserving the angle locally, provides optimal regularity. The equal-area algorithm generates a partition having zero area variation. The equidistant projection minimizes the circumscribed diameter of the segments. These approaches are unified by a newly proposed compromise approach, which provides the necessary adjustment range for individual preferences. The performances of the three approaches are numerically verified using the Thirty Meter Telescope model. This study verifies the effectiveness of the scaling rule and sheds light on the mechanism of primary mirror segmentation.

Gravitational Lensing and Dynamics (GLaD): combined analysis to unveil properties of high-redshift galaxies

The dynamical modelling of integral field unit (IFU) stellar kinematics is a powerful tool to unveil the dynamical structure and mass build-up of galaxies in the local Universe, while gravitational lensing is nature’s cosmic telescope to explore the properties of galaxies beyond the local Universe. We present a new approach, which unifies dynamical modelling of galaxies with the magnification power of strong gravitational lensing, to reconstruct the structural and dynamical properties of high-redshift galaxies. By means of axisymmetric Jeans modelling, we create a dynamical model of the source galaxy, assuming a surface brightness and surface mass density profile. We then predict how the source’s surface brightness and kinematics would look when lensed by the foreground mass distribution and compare with the mock observed arcs of strong gravitational lensing systems. For demonstration purposes, we created and also analysed mock data of the strong lensing system RX J1131−1231. By modelling both the lens and source, we recover the dynamical mass within the effective radius of strongly lensed high-redshift sources within 5% uncertainty, and we improve the constraints on the lens mass parameters by up to 50%. This machinery is particularly well-suited for future observations from large segmented-mirror telescopes, such as the James Webb Space Telescope, which will yield high sensitivity and angular-resolution IFU data for studies on distant and faint galaxies.

Active control of adaptive optics system in a large segmented mirror telescope

For a large adaptive optics system such as a large segmented mirror telescope (SMT), it is often difficult, although not impossible, to directly apply common multi-input multi-output (MIMO) controller design methods due to the computational burden imposed by the large dimension of the system model. In this article, a practical controller design method is proposed which significantly reduces the system dimension for a system where the dimension required to represent the dynamics of the plant is much smaller than the dimension of the full plant model. The proposed method decouples the dynamic and static parts of the plant model by a modal decomposition technique to separately design a controller for each part. Two controllers are then combined using the so-called sensitivity decoupling method so that the resulting feedback loop becomes the superposition of the two individual feedback loops of the dynamic and static parts. A MIMO controller was designed by the proposed method using the H ∞ loop-shaping technique for an SMT model to be compared with other controllers proposed in the literature. Frequency-domain analysis and time-domain simulation results show the superior performance of the proposed controller.