Figure 1 . Design drawing of the
Natural Guide Star
Next Generation Sensor ( NGS2 ) unit ( shown in green ) occupying one corner of the Canopus
AO optical bench .
Figure 2 . The NGS2 unit , nearly fully integrated in the ANU laboratory . At the bottom of the image one can see two large fold mirrors that channel the light into the re-imaging optics . The large unit at the top left houses the electron multiplying CCD detector .
Image credit : courtesy of the Australian
National University cal pickup probes patrolling the field to find and track the guide stars , the future system will use an electron multiplying charge-coupled device ( CCD ) detector that can image the whole field , allowing straightforward identification of guide stars .
Multiple regions of interest centered on the guide stars can also be configured and read out at very high speed — up to 800 hertz with very low read noise . This new sensor converts the existing delicate opto-mechanical arrangements of the guide probes to a system that will essentially be software configurable and more robust . Its higher efficiency is expected to improve the detection limit for natural guide stars by some 2.5 magnitudes , which will result in a dramatic improvement in sky coverage .
In 2014 , we secured funding from the Australian Research Council for a proposal led by the Australian National University ( ANU ). This — together with additional funding from Gemini , the Australian Astronomical Observatory , and the Swinburne University of Technology — opened the possibility to design and build this Natural Guide Star Next Generation Sensor , or NGS2 , in short ( see Figures 1 and 2 ).
We expect the new NGS2 subsystem will become an integral part of the Canopus adaptive optics system , replacing the existing NGS unit with the minimum necessary modifications . In a nutshell , NGS2 is composed of an optical system that re-images the focal plane onto a high-speed electron multiplying CCD detector . In full-frame readout , the guide stars can be easily identified . For tip-tilt sensing only , small areas around the stars are read out at high speed . A dedicated central processing unit will determine the centroids and pass the necessary information to the adaptive optics ( AO ) real-time control system .
The space constraint for the new NGS2 system , which needs to fit onto the existing AO optical bench , is very demanding ; the alignment tolerances and system integration are challenging features , as well . Figure 1 shows the design drawing of how NGS2 will just fit into a corner of the AO optical bench . Furthermore , the detector generates heat that has to be actively removed , so as not to affect the rest of the AO system . And finally , one functionality of the existing system — the focus sensing of the natural guide stars — cannot be incorporated in the NGS2 design . Therefore , one of the existing peripheral wavefront sensors will take over that function .
Very good progress has been made to date in the detector upgrade . Essentially the opto-mechanical system and the detector system have been completed and are going through the final stages of integration and alignment at the ANU laboratories . Figure 2 shows a recent picture of the NGS2 system on the bench .
The system is in a very advanced stage of development and currently undergoing the final stages of alignment . Figure 2 also shows a recent picture of the built NGS2 module on the bench , ready for testing . Formal Acceptance Testing was successfully carried out in early December .
Still , much work remains to be done . In particular , a significant amount of software development is pending while resources are very tight . Also the integration of NGS2 into Canopus will be a delicate activity and re-
46 GeminiFocus January 2017 | 2016 Year in Review