This paper provides a status update on the Natural Guidestar (NGS) Adaptive Optics (AO)
system being built for Castor, the meter class telescope at the Starfire Optical Range. We present a radiometric case study for a range of variable parameters such as source brightness, number of Shack-Hartmann sub-apertures, AO and Track loop frame rate and bandwidth. We gauge system performance by contrast and adapt the error budget
to allow detection of a dim object near a bright star. We present wave band splits between the different AO components such as track sensor, wavefront sensor, scoring camera, and science camera. We show the different configurations that allow to switch between dim object and bright object tracking. The opto-mechanical design of the AO system is
also presented.
Past research in adaptive optics (AO) has demonstrated the link between apparent beacon extent and wavefront gradient estimation sensitivity, or optical gain, of a classical Shack-Hartmann (SH) subaperture when using quad-cell detector regions. Pixel diffusion and residual wavefront error broaden the effective subaperture point spread functions as the atmospheric seeing varies in time. Although the AO community has generally shifted toward resolved subapertures to combat these interlinked issues, the quad-cell subaperture design offers efficient light usage for dim beacons, integrating less pixel noise while also reducing sensor readout latency. Particularly for telescopes in poor seeing conditions, in order to reduce beacon magnitude requirements, a quad-cell SH design, coupled with the proposed algorithm, can be an enabling solution. We present research conducted at the Starfire Optical Range over the past 8 years in implementing a robust approach that measures the real-time sensitivity on the site’s natural guidestar and laser beacon AO systems at the 3.5- and 1.5-m telescopes. Emphasis is given toward the practical aspects that must be considered beyond the pure theory, which has been presented in several prior works. A high-signal-to-noise strategy has been implemented that estimates the aperture-averaged subaperture sensitivity (related to beacon size) by exploiting the null space of the least-squares wavefront reconstructor. Careful consideration has gone into the implementation of this estimation method to avoid unintended effects, particularly at low-light levels. Unfortunately, this solution does not in itself address aperture-variant effects, such as sodium beacon elongation for extremely large telescopes.
The performance of closed-loop tilt-control and adaptive-optics systems suffers when conditions change. Examples of changing conditions are angular extent of the object, signal-to-noise ratio, and characteristics of the disturbance. A simple learning algorithm motivated by neural network theory is developed to change the closed-loop gain in real-time to adapt quickly to changing conditions. This technique finds the correct loop gain within seconds with no operator intervention, which saves several minutes for each observation. Simulation and experimental results show improvement for both tilt-control and adaptive-optics systems.
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.