Space interferometers could, in principle, exploit the relatively stable space environment and ease of baseline reconfiguration to collect measurements beyond the limitations of ground-based interferometers. In particular, a two-element interferometer could provide excellent uv-plane coverage over a few tens of low Earth orbits. One of the challenges for free-flying interferometers is controlling the optical path distance with subwavelength accuracies despite the collectors flying up to hundreds of meters apart. We consider two approaches: an artificial in-orbit laser guide star (LGS) that provides a phase reference for the space interferometer and fringe tracking on the science target itself. The two approaches (LGS versus no LGS) would require different image processing techniques. In this work, we explore image processing with LGS phase residuals due to global positioning system (GPS) uncertainties. We use GPS uncertainties from the Gravity Recovery and Climate Experiment Follow-On mission to simulate image retrieval with a 300-m baseline laser-guided space interferometer. This is done by fitting the slowly varying phase errors of complex visibility measurements. We also consider a 40-m baseline interferometer with visibility(-modulus)-only measurements. In this case, we simulate the bias in visibility due to fringe tracking in the presence of parasitic forces acting on the spacecraft. We then use a modified version of the hybrid input–output phase retrieval algorithm for image reconstruction. We conclude that under our optimistic assumptions, both approaches could enable general imaging of a few large stars even with CubeSats, although an LGS would significantly improve the best resolution obtainable.
In the past few years, there has been a resurgence in studies of space-based optical/infrared interferometry, particularly with the vision to use the technique to discover and characterize temperate Earth-like exoplanets around solar analogs. One of the key technological leaps needed to make such a mission feasible is demonstrating that formation flying precision at the level needed for interferometry is possible. Here, we present Pyxis, a ground-based demonstrator for a future small satellite mission with the aim to demonstrate the precision metrology needed for space-based interferometry. We describe the science potential of such a ground-based instrument and detail the various subsystems: three six-axis robots, a multi-stage metrology system, an integrated optics beam combiner, and the control systems required for the necessary precision and stability. We conclude by looking toward the next stage of Pyxis: a collection of small satellites in Earth orbit.
Space interferometers could, in principle, exploit the relatively stable space environment and ease of baseline reconfiguration to collect measurements beyond the limitations of ground-based interferometers. In particular, a two-element interferometer could provide excellent uv-plane coverage over a few tens of low-Earth orbits. One of the challenges for free-flying interferometers is controlling the optical path distance with sub-wavelength accuracies despite the collectors flying up to hundreds of meters apart. This work considers two approaches: an artificial in-orbit laser guide star (LGS) that provides a phase reference for the space interferometer and fringe tracking on the science target itself. The two approaches (LGS vs. no LGS) would require different image processing techniques. In this work, we explore image processing with LGS phase residuals due to GPS uncertainties. We use GPS uncertainties from the GRACE-FO mission to simulate image retrieval with a 300 m baseline laser-guided space interferometer. This is done by fitting the slowly varying phase errors of complex visibility measurements. We also consider a 40 m baseline interferometer with visibility(-modulus)-only measurements. In this case, we simulate the bias in visibility due to fringe tracking in the presence of parasitic forces acting on the spacecraft. We then use a modified version of the Hybrid Input-Output phase retrieval algorithm for image reconstruction. We conclude that under our optimistic assumptions, both approaches could enable general imaging of a few large stars even with CubeSats, although an LGS would significantly improve the best resolution obtainable.
Dome seeing is an often overlooked avenue for seeing improvement for a telescope. Because most existing telescope domes have not been characterized for turbulence, there is an opportunity to improve the overall seeing by minimizing the dome contribution, thereby optimizing scientific productivity and operations. A dome turbulence sensor has recorded data in the Anglo-Australian Telescope (AAT) over the past year. The instrument consists of a collimated laser beam that propagates (and double passes) between the AAT’s primary mirror box and a flat mirror on the secondary strut. The angle-of-arrival fluctuations are used to derive a dome-seeing-proxy in arcsec. We found the dominant effects to be the temperature gradients and wind speed. Convection conditions are considerably more detrimental to the dome-seeing-proxy than thermal inversion conditions. Unlike other large telescopes, there is no discernible relationship between the dome-seeing-proxy and relative wind direction. Concerning telescope operations, it would be worth considering lowering the air-conditioning set point temperature to include a higher proportion of observations under thermal inversion. Nevertheless, this must be carefully weighed with the risk of condensation in the dome, a major concern for a site with frequent high relative humidity.
The Pyxis interferometer is a ground-based pathfinder for an optical space interferometer being built at Mt Stromlo Observatory. In this second paper, we discuss the system breakdown of the interferometer and its mechanical design. We outline the interferometer’s three robotic platforms, with two telescopic collectors and one central beam combiner. Each collector utilises a full aluminium, diamond-turned telescope, designed to remove thermal distortions for future space applications. We also explain the chosen control system for the interferometer and how it will be used to ensure the linear and angular control requirements, including sub-milli-radian angular control of each robot.
Optical interferometry from space is arguably the most exciting prospect for high angular resolution astrophysics; including the analysis of exoplanet atmospheres. This was highlighted in the recent ESA Voyage 2050 plan, which pointed out the exciting potential of this technology, but also indicated the critical need for technological demonstrators. Here we present the Pyxis interferometer; a ground-based pathfinder for a CubeSat space interferometer, currently being built at Mt Stromlo Observatory. We outline its technological and scientific potential as the only visible wavelength interferometer in the Southern Hemisphere, and the optical systems designed to provide CubeSat compatible metrology for formation flying.
Two major contributors to the overall seeing that degrades astronomical images are turbulence from the atmosphere and turbulence within the telescope dome structure. Dome seeing generally contributes less than 1 arcsec to the overall seeing. However, most existing telescope domes have not been characterized for dome seeing; there is an opportunity to significantly improve the overall seeing by optimizing the dome seeing. An instrument that measures a proxy to dome seeing was installed at the Anglo-Australian Telescope (AAT) at Siding Spring Observatory in Australia. The instrument is based on a similar ’dome seeing monitor’ built and tested by Bustos and Tokovinin for the 4 m Blanco telescope in 2018. The instrument consists of a collimated laser beam that propagates from the AAT’s primary mirror box, reflects off a flat mirror on the secondary strut, back down to the primary mirror box, and is imaged by a camera. The angle-of-arrival fluctuations are used to derive the seeing proxy in arcsec. Meteorology is recorded in parallel to the dome seeing proxy, including inside, outside, and mirror temperature, humidity, pressure, wind speed and direction, and telescope azimuth and elevation. These meteorology variables were tested for correlation to the dome seeing proxy. There are 77 nights worth of data, spanning from August 2021 to May 2022. The highly correlated variables were the outdoor/indoor and indoor/mirror temperature difference, the wind speed and humidity. Poorly correlated variables include the wind-to-dome slit angle, the sky/ambient temperature difference and elevation. Thermal convection conditions were found to significantly affect the dome-seeing-proxy compared to thermal inversion conditions.
Beam combiners are important components of an optical/infrared astrophysical interferometer, with many variants as to how to optimally combine two or more beams of light to fringe-track and obtain the complex fringe visibility. One such method is the use of an integrated optics chip that can instantaneously provide the measurement of visibility without temporal or spatial modulation of the optical path. Current asymmetric planar designs are complex, resulting in a throughput penalty, and so here, we present developments into a three-dimensional triangular tricoupler that can provide the required interferometric information with a simple design and only three outputs. Such a beam combiner is planned to be integrated into the upcoming Pyxis interferometer, where it can serve as a high-throughput beam combiner with a low size footprint. Results into the characterization of such a coupler are presented, highlighting a throughput of 85 ± 7 % and a flux splitting ratio between 33:33:33 and 52:31:17 over a 20% bandpass. We also show the response of the chip to changes in the optical path, obtaining an instantaneous complex visibility and group delay estimate at each input delay.
Space interferometry is the inevitable endpoint of high angular resolution astrophysics, and a key technology that can be leveraged to analyse exoplanet formation and atmospheres with exceptional detail. Here, we present a feasibility study into a small scale formation flying interferometric array, flying in Low Earth Orbit, that will aim to prove the technical concepts involved with space interferometry while still making unique astrophysical measurements. We will detail the design of the mission, as well as present orbital simulations that show that the array should be stable enough to perform interferometry with <50 m/s/year Δv and one thruster per spacecraft. We also conduct observability simulations to identify what parts of the sky are visible for a given orbital configuration. We conclude with optimism that this design is achievable, but a more detailed control simulation factoring in a metrology system is the next step to demonstrate full mission feasibility.
A space interferometer could reach a sensitivity and angular resolution which is unattainable on Earth due to the distortion and absorption of the atmosphere. It would enable many unique science cases, including the direct imaging and characterisation of temperate terrestrial exoplanets. This ambitious vision relies on the formation flying of individual spacecrafts, and the demonstration of precision metrology measuring positions in to better than 1mm in at least 2 dimensions, and velocities in the range of nm/s. These significant technical challenges are one of the main reasons progress in space interferometry has been seriously hampered in the two last decades. To overcome this obstacle, we propose a novel metrology concept operating in two steps. The coarse positioning of the array elements is achieved through commercially demonstrated components, such as GPS, wide angle cameras and time-of-flight sensors. For the critical fine metrology, multiple longitudinal mode Fabry-Perot lasers in a central spacecraft are split and retro-reflected off each telescope bearing spacecraft. The reflected beams are then coherently combined in the central spacecraft and the resulting fringes are spectrally dispersed. In this manner, the phase difference is measured at the different Fabry-Perot wavelengths, allowing the unambiguous differential position measurements over a couple of mm capture range. We present the concept together with a prototype system in the laboratory.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.