The Mount Stromlo LGS facility includes two laser systems: a fiber-based sum-frequency laser designed and built by EOS Space Systems in Australia, and a Semiconductor Guidestar Laser designed and built by Aret´e Associates in the USA under contract with the Australian National University. The Beam Transfer Optics (BTO) enable either simultaneous or separate propagation of the two lasers to create a single LGS on the sky. This paper provides an overview of the Mount Stromlo LGS facility design, integration and testing of the two sodium guidestar lasers in the laboratory and on the EOS 1.8m telescope.
With the rise of extremely large telescopes such as ELT, TMT, and GMT, the precision of mirror segment alignment will become critical to maintaining the resolution of the full aperture. A Shack-Hartmann wavefront sensor can be used to determine the tip/tilt of individual segments, but since it measures the displacement of a focal spot from the center, it is blind to any piston step (θ) between segments. However, if a lenslet is centered over the gap between two mirror segments, the image results in an interference pattern from those segments. The interference pattern is highly dependent on the piston step between the two segments and is mathematically expressed by a modified sinc function. This leaves three main parameters that can be used to identify θ; the curve shape, the peak intensity, and the primary peak position. Curve fitting and correlation algorithms have previously been used to recover θ from the curve shape. However, they run significantly slower than a centroiding algorithm used for the position measurement of spots in the Shack-Hartmann wavefront sensor. For a system that is stable to segment piston, this is not an issue. But for a system like the GMT, where piston will need to be corrected almost continuously, an algorithm that is of a similar speed to centroiding could enable piston sensing to be integrated into a traditional Shack-Hartmann wavefront sensor (SHWFS). Also, the number of pixels needed to sample the interference spot has been much greater than the number for regular spot sampling in a SHWFS. This paper presents the derivation, simulation results, and an optical bench demonstration of a fast alternative piston-sensing algorithm; called the pixel difference algorithm, that is capable of utilizing a limited number of pixels across the interference spot (nominally 7). The pixel difference algorithm is based on the primary peak intensity parameter and is significantly faster than the curve fitting algorithm; by approximately a factor of 22 both in the simulation and in the optical bench demonstration. The simulation showed this speed comes with a trade-off; the method is more susceptible to noise and has approximately twice the error of the curve fitting algorithm at signal-to-noise ratios (SNR)<10. However, at high SNR<22, the pixel difference method has comparable, or better, accuracy than the curve fitting algorithm. The primary peak position parameter was also investigated but was found to have the same challenges as the pixel difference method, and so was not pursued in this paper. The pixel difference method presents an opportunity to use a Shack-Hartmann wavefront sensor for mirror segment alignment in an environment where θ needs to be corrected on a fast timescale.
One of the main challenges for the new generation of extremely large telescopes (ELT) such as the Giant Magellan Telescope (GMT) is apparent in their ability to phase the segments in their primary mirror. Due to the lack of viability of manufacturing enormous mirrors, these primary mirrors are composed of smaller segments, and therefore they must be phased. Prior to the full construction of GMT, there has been proposal to develop a small-scale laboratory testbed to reproduce elements of GMT’s design, major disturbances, and control systems. This would serve to reduce the risk in cost and time prior to commissioning.
The team at the Australian National University’s (ANU) Research School of Astronomy and Astrophysics (RSAA) have developed a design concept for such a miniature version, coined Pocket-GMT. Pocket-GMT is designed to simulate GMT’s segmented primary mirror as well as introduce aberrations and distortions similar to what GMT will experience. This would present an opportunity to optimize the functionality of GMT’s control software and wavefront sensors, and to demonstrate phasing within the laboratory prior to full-scale telescope implementation. Pocket-GMT would also be compatible with later GMT instrument prototypes, thus ensuring its usefulness going into the future.
This paper presents a preliminary analysis of the first results we have obtained from the adaptive optics systems built for EOS 1.8 m telescope at Mount Stromlo. This presentation focuses on the single-camera stereo-SCIDAR for monitoring the atmospheric seeing. We briefly summarize the system, describe its on-sky performance during commissioning, compare results to numerical simulations and evaluate the remaining challenges going into the future.
Space debris in low Earth orbit (LEO) below 1500 km is becoming an increasing threat to spacecrafts. To manage the threat, we are developing systems to improve the ground-based tracking and imaging of space debris and satellites. We also intend to demonstrate that it is possible to launch a high-power laser that modifies the orbits of the debris. However, atmospheric turbulence makes it necessary to use adaptive optics with such systems. When engaging with objects in LEO, the objects are available only a limited amount of time. During the observation window, the object has to be acquired and performance of all adaptive optics feedback loops optimised. We have implemented a high-level adaptive optics supervision tool to automatise time-consuming tasks related to calibration and performance monitoring. This paper describes in detail the current features of our software.
As space debris in lower Earth orbits are accumulating, techniques to lower the risk of space debris collisions must be developed. Within the context of the Space Environment Research Centre (SERC), the Australian National University (ANU) is developing an adaptive optics system for tracking and pushing space debris. The strategy is to pre-condition a laser launched from a 1.8 m telescope operated by Electro Optics Systems (EOS) on Mount Stromlo, Canberra and direct it at an object to perturb its orbit. Current progress towards implementing this experiment, which will ensure automated operation between the telescope and the adaptive optics system, will be presented.
We present the status of the site-characterisation campaign at Mount Stromlo Observatory. The main goal of the project is to aid the development and operation of new adaptive optics (AO) systems for space debris tracking and pushing as well as satellite imaging. The main method we use for the characterisation is based on the SCIntillation Detection And Ranging (SCIDAR) technique. We have designed a unique version of the SCIDAR instrument: a stereo-SCIDAR system that uses a roof prism to separate beams from a double-star system to obtain two isolated pupil images on a single detector. The instrument is installed on the 1.8 m telescope of Electro-Optic Systems (EOS), sharing facilities with the adaptive optics systems we are currently building. The SCIDAR instrument will be operated intermittently, weather and availability permitting, until sufficient amount of data has been collected to characterise the site. This paper reports the current status of the project: we have recently started the commissioning phase and obtained first measurements with the instrument.