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 which can patrol the whole field . But these probes have only a very small field-of-view which , due to flexure and variable field distortions , sometimes makes it time consuming to acquire the stars .
Thanks to current state-of-the-art detector technology that has become available , these limitations can now be tackled . A key change is that rather than having three mechanical 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 optomechanical 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 .
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 af-
16 GeminiFocus January 2017