The Advanced Technology Solar Telescope (ATST) requires active control of quasi-static telescope aberrations
in order to meet image quality standards set by its science requirements. Wavefront control is managed by
the Telescope Control System, with many telescope subsystems playing key roles. We present the design of
the ATST quasi-static wavefront and alignment control architecture and the algorithms used to control its four
active mirrors. Two control algorithms are presented, one that minimizes force on M1 actuators and another that
employs a neutral-pointing constraint on M2 to reduce pointing error. We also present simulations that generate
typical daily active mirror trajectories which correct optical misalignments due to changing gravitational and
The Advanced Technology Solar Telescope (ATST) is a 4m off-axis telescope with a Gregorian front end. At the
time of its construction it will be the world's largest solar astronomical telescope. During scientific operations
the ATST mirrors and structure will be deformed due to thermal and gravitational loading. The ATST team
has developed a quasi-static alignment scheme that utilizes the wavefront sensing signals from at least one and
as many as three wavefront sensors in the telescope science field of view, and active figure control of the primary
mirror and rigid body control of the secondary mirror to achieve least-squares optical control of the telescope.
This paper presents the quasi-static alignment model for the ATST, and three different active alignment schemes
that are the damped least-squares control, force optimized control that defines a least-squares aligned state of
the telescope subject to minimum primary actuator force, and pivot-point control of the secondary mirror. All
three strategies achieve the desired minimum RMS wavefront error, but demonstrate different optimized states
of the telescope.
The high order adaptive optics (HOAO) system is the centerpiece of the ATST wavefront correction system. The ATST
wavefront correction system is required to achieve a Strehl of
S = 0.6 or better at visible wavelength. The system design
closely follows the successful HOAO implementation at the Dunn Solar Telescope and is based on the correlating
Shack-Hartmann wavefront sensor. In addition to HOAO the ATST will utilize wavefront sensors to implement active
optics (aO) and Quasi Static Alignment (QSA) of the telescope optics, which includes several off-axis elements.
Provisions for implementation of Multi-conjugate adaptive optics have been made with the design of the optical path that
feeds the instrumentation at the coudé station. We will give an overview of the design of individual subsystems of the
ATST wavefront correction system and describe some of the unique features of the ATST wavefront correction system,
such as the need for thermally controlled corrective elements.
This paper presents three characteristics in the simulated active alignment strategy of the James Webb Space
Telescope. The first includes the analysis and comparison of a baseline active alignment strategy with a damped
least squares strategy. This baseline utilizes prior knowledge by means of direct human operator interaction
to engage sets of telescope compensators to target specific aberration signatures. The baseline is compared to
a damped least-squares strategy that utilizes simultaneous engagement of all telescope compensators without
explicit human operator interaction to achieve a least-squares telescope compensation. Second, we discuss how
the active alignment of the JWST is encapsulated in a linear optical model developed at the Space Telescope
Science Institute. This linear optical model provides a framework for an efficient and robust description of the
optical control properties of the JWST and clearly articulates the necessity for having a multi-instrument multifield
wavefront sensing strategy to overcome control system non independence and the effects of non-common
path errors in the main wavefront sensing camera. Finally, we present analytical results that explicitly map the
telescope wavefront responses to the telescope control modes, and we present Monte-Carlo optical performance
simulation results that demonstrate the efficacy of the damped least-squares active alignment and the priorknowledge
active alignment schemes.
An important part of a large solar telescope is the ability to correct, in real time, optical alignment errors caused by gravitational bending of the telescope structure and wavefront errors caused by atmospheric seeing. The National Solar Observatory is currently designing the 4 meter Advanced Technology Solar Telescope (ATST). The ATST wavefront correction system, described in this paper, will incorporate a number of interacting wavefront control systems to provide diffraction limited imaging performance. We will describe these systems and summarize the interaction between the various sub-systems and present results of performance modeling.
The Advanced Technology Solar Telescope (ATST) is a complex off-axis Gregorian design to be used for solar astronomy. In order the counteract the effects of mirror and telescope structure flexure, the ATST requires an active optics alignment strategy. This paper presents an active optics alignment strategy that uses three wavefront sensors distributed in the ATST field-of-view to form a least-squares alignment solution with respect to RMS wavefront error. The least squares solution is realized by means of a damped least squares linear reconstructor. The results of optical modelling simulations are presented for the ATST degrees-of-freedom subject to random perturbations. Typical results include residual RMS wavefront errors less than 20 nm. The results quoted include up to 25 nm RMS wavefront sensor signal noise, random figure errors on the mirrors up to 500 nm amplitude, random decenter range up to 500 μm, and random tilts up to 10e - 03 degrees (36 arc-secs) range.
Mirror and dome seeing are critical effects influencing the optical performance of large ground based telescopes. Computational Fluid Dynamics (CFD) and optical models that simulate mirror seeing in the Thirty Meter Telescope (TMT) are presented. The optical model used to quantify the effects of seeing utilizes the spatially varying refractive index resulting from the expected theoretical flow field, while the developed CFD post-processing model estimates the corresponding CN2 distribution. The seeing effects corresponding to a flat plate are calculated. Plots of seeing versus different temperature differences, velocities and plate lengths are generated. The discussion presented contains comparisons of the results from the two models with published empirical relations.
Ground-based telescopes operate in a turbulent atmosphere that affects the optical path across the aperture by changing both the mirror positions (wind seeing) and the air refraction index in the light path (atmospheric seeing). In wide field observations, when adaptive optics is not feasible, active optics are the only means of minimizing the effects of wind buffeting. An integrated, dynamic model of wind buffeting, telescope structure, and optical performance was devleoped to investigate wind energy propagation into primary mirror modes and secondary mirror rigid body motion.Although the rsults showed that the current level of wind modeling was not appropriate to decisively settle the need for optical feedback loops in active optics, the simulations strongly indicated the capability of a limited bandwidth edge sensor loop to maintain the continuity of the primary mirror inside the preliminary error budget. It was also found that the largest contributor to the wind seeing is image jitter, i.e. OPD tip/tilt.
Diffraction-limited performance of 30-m class telescopes requires the integration of structural, optical and control systems to sense and counteract real time disturbances to the telescope. Accurate simulation of an integrated telescope model is essential for optical performance estimation and design validation. Our approach to integrated time domain modeling of large telescopes is to interface commercially available structural,optical and control modeling software packages. The model architecture, data structures, and the interfacing tools of the simulation environment are presented. Preliminary simulation results of a 30-m class telescope subject to
wind load and a ground layer phase screen are presented.
The modulation of the irradiance in the exit pupil of an optical data storage scanning system is described by analyzing the behavior of scan-dependent interference fringes. These fringes are grouped into three independent irradiance components. The variation of the exit pupil irradiance pattern as a function of groove depth is discussed.
Differential phase detection (DPD) is a method of generating a tracking error signal from rotation of the irradiance pattern formed in the pupil of an optical data storage system. The amount of rotation is related to the distance between the scanning spot and the center of the track being scanned. Media parameters, such as reflectivities of the marks and land and the groove depth affect the DPD signal. The representation of mark and land reflectivity vectors on a complex plane diagram proves to be an elegant technique for designers to optimize a given medium for DPD.
Our group has investigated a number of finite conjugate aberration compensation systems in conjunction high numerical aperture objectives for multiple-layer optical data storage. The results of our investigation are that a Burch-type objective lens in conjunction with a Galilean telescope is a compact, simple and effective optical system for spherical aberration compensation.