Adaptive optics (AO) systems deliver high-resolution images that may be ideal for precisely measuring positions of stars (i.e., astrometry) if the system has stable and well-calibrated geometric optical distortions. A calibration unit equipped with a back-illuminated pinhole mask can be utilized to measure instrumental optical distortions. AO systems on the largest ground-based telescopes, such as the W. M. Keck Observatory and the Thirty Meter Telescope (TMT), require pinhole positions known to be ∼20 nm to achieve an astrometric precision of 0.001 of a resolution element. In pursuit of that goal, we characterize a photolithographic pinhole mask and explore the systematic errors that result from different experimental setups. We characterized the nonlinear geometric distortion of a simple imaging system using the mask, and we measured 857-nm root mean square of optical distortion with a final residual of 39 nm (equivalent to 20 μ for TMT). We use a sixth-order bivariate Legendre polynomial to model the optical distortion and allow the reference positions of the individual pinholes to vary. The nonlinear deviations in the pinhole pattern with respect to the manufacturing design of a square pattern are 47.2 nm ± 4.5 nm (random) ± 10.8 nm (systematic) over an area of 1788 mm2. These deviations reflect the additional error induced when assuming that the pinhole mask is manufactured perfectly square. We also find that ordered mask distortions are significantly more difficult to characterize than random mask distortions as the ordered distortions can alias into optical camera distortion. Future design simulations for astrometric calibration units should include ordered mask distortions. We conclude that photolithographic pinhole masks are >10 times better than the pinhole masks deployed in first-generation AO systems and are sufficient to meet the distortion calibration requirements for the upcoming 30-m-class telescopes.
Adaptive optics (AO) systems deliver high-resolution images that may be ideal for precisely measuring positions of stars (i.e. astrometry) if the system has stable and well-calibrated geometric distortions. Calibration units equipped with back-illuminated pinhole masks are often utilized to measure instrumental distortions. AO systems on the largest ground-based telescopes, such as the W. M. Keck Observatory and the Thirty Meter Telescope, or with large fields of view, such as the ‘imaka ground-layer adaptive optics experiment, require pinhole positions known to 20 nm to achieve astrometric precisions of 0.001 of a resolution element. We characterized the nonlinear geometric distortion of a simple imaging system using a photo-lithographic pinhole reference grid to be 1650 nm RMS with a final residual of 41 nm RMS (20.5 μas for TMT). Our system model uses fourth order polynomials to model the distortion and allows the reference positions of the pinholes to vary. The nonlinear deviations in the pinhole pattern with respect to the manufacturing design of a square pattern are estimated to be 29 nm over a 1200 mm2, which reflects
the additional error induced in a distortion measurement when assuming the pattern is perfectly manufactured.
By analyzing global covariance matrices from the imaka GLAO system at the UH 2.2m telescope, it is possible to reconstruct ground layer strength, the integrated turbulence strength as well as the vertical turbulence profile. These are compared to simultaneous profiles obtained by the Maunakea facility MASS/DIMM. A method has been developed to directly compute the phase structure function from the covariances of the slopes, obtained from the telemetry data. The phase structure function allows to test the validity of the Kolmogorov (or van Karman) model and the spatial frequency content of the turbulence: Dome and telescope tube seeing are expected to have an excess of high spatial frequencies, which is detrimental to the PSF by amplifying the halo, and which the AO system cannot correct. The telescope, the dome and their interaction with the ground layer produce a complex environment for the turbulence. We are therefore developing a small, portable optical turbulence sensor which we will be able to use to scan the dome and telescope tube to quantify the local presence of turbulence. This is the AIR-FLOW (Airborne Interferometric Recombiner - Fluctuations of Light at Optical Wavelengths) project. With imaka and AIR-FLOW we hope to generate a coherent and quantitative account of the turbulence type and strength present in the telescope beam and to accurately match this detailed phase information to the focal plane images. Such a level of detail is required to understand and eventually be able to control the local environment for optimized image quality. We foresee this expertise will be especially valuable for ELTs, where the halo around the PSF will act like an extra source of background.
We present on-sky results from the wide field ground-layer adaptive optics (GLAO) system on the University of Hawaii 2.2-meter telescope on Maunakea. We demonstrate improvements in image quality at visible wavelengths under a variety of seeing conditions. We discuss the gains for a variety of figures of merit including the full-width at half-maximum, the equivalent noise area, and the encircled energy diameter. These gains and figures of merit are discussed in the context of our GLAO science cases. In addition, we present the system image quality error budget, measurements of the dominant error terms, and their impact on the delivered focal plane images.
General relativity can be tested in the strong gravity regime by monitoring stars orbiting the supermassive black hole at the Galactic Center with adaptive optics. However, the limiting source of uncertainty is the spatial PSF variability due to atmospheric anisoplanatism and instrumental aberrations. The Galactic Center Group at UCLA has completed a project developing algorithms to predict PSF variability for Keck AO images. We have created a new software package (AIROPA), based on modified versions of StarFinder and Arroyo, that takes atmospheric turbulence profiles, instrumental aberration maps, and images as inputs and delivers improved photometry and astrometry on crowded fields. This software package will be made publicly available soon.
We present the integration status for 'imaka, the ground-layer adaptive optics (GLAO) system on the University of Hawaii 2.2-meter telescope on Maunakea, Hawaii. This wide-field GLAO pathfinder system exploits Maunakea's highly confined ground layer and weak free-atmosphere to push the corrected field of view to ∼1/3 of a degree, an areal field approaching an order of magnitude larger than any existing or planned GLAO system, with a FWHM ∼ 0.33" in the visible and near infrared. We discuss the unique design aspects of the instrument, the driving science cases and how they impact the system, and how we will demonstrate these cases on the sky.