DAO [VOLT] Victoria Open Loop Testbed

The goal of the Victoria Open Loop Testbed (VOLT) is to demonstrate open loop control in the lab and on-sky (at the DAO 1.2m telescope) using a simple on-axis natural guide star AO testbed.

Project Leader: David Andersen

Project Team: Mike Fischer, Rodolphe Conan, Murray Fletcher, Jim Stilburn, Jean-Pierre Véran, Kris Caputa, Dmitry Monin, John Pazder, Kaushala Bandara and Aaron Hilton; NRC-HIA Computer Services and Machine Shop

Project Overview

The next generation Adaptive Optics instruments will employ Multi-Object Adaptive Optics (MOAO). Such MOAO systems offer great promise to produce locally diffraction-limited imaging over a very wide field of view and potentially produce large multiplex gains. MOAO systems however, require open loop control, and open loop control presents the greatest unknown and therefore the greatest risk in the construction of such a system. Before a MOAO instrument can seriously be considered for construction, the risk of open loop control will need to be retired through projects like VOLT.

VOLT optical layout. The red lines show the principal ray as it enters from the telescope (bottom right), and is directed to the open loop wavefront sensor (WFS A), the deformable mirror (an ALPAO DM-52), and the closed loop WFS (B). On a separate optical path (orange), WFS C acts as a figure sensor.

In May, 2008, VOLT successfully ran in open loop resulting in image sharpening that shrank the full-width half maximum (FWHM) of the stellar point spread function (PSF) from 2.5 arcseconds to 0.5 arcseconds and resulted in a factor of 5 increase in the peak flux (Figure 2).

Figure 2: VOLT stellar image observed without adaptive optics (left) and with adaptive optics (right) from September 2009. The seeing-limited image in the top right has the same stretch as the AO-corrected image to its right. The image on the bottom left is scaled to the same peak flux. 

Through VOLT the ATRG-V has been gaining experience with open loop systems. In particular we are developing alignment and calibration schemes and learning more about operating high dynamic range wavefront sensors. Furthermore, VOLT employs the ALPAO DM-52 deformable mirror, which may play a role in future (MO)AO systems. As part of the VOLT project we have measured the ALPAO DM-52 hysteresis and non-linearity, and are assessing its dynamic performance.  We have also measured the rejection transfer function (RTF) both in the lab and on-sky (Figure 3).

Figure 3: The Open Loop RTF measured from an artificial point source (blue) and a bright star (red). The control bandwidth of open loop systems is higher than for closed loop systems because unity gain is used. However, any open loop error results in an observed floor of the RTF at low temporal frequencies which limits the overall performance. Developing open loop calibration and alignment techniques will be critical to the future success of MOAO systems.

Related Information

Institutes:

• NRC Herzberg Institute of Astrophysics