SAPHIRA detectors, which are HgCdTe linear avalanche photodiode arrays manufactured by Leonardo, enable high frame rate, high sensitivity, low noise, and low dark current imaging at near-infrared wavelengths. During all University of Hawaii Institute for Astronomy lab testing and observatory deployments of SAPHIRA detectors, there was approximately one meter of cables between the arrays and the readout controllers. The output drivers of the detectors struggled to stably send signals over this length to the readout controllers. As a result, voltage oscillations caused excess noise that prevented us from clocking much faster than 1 MHz. Additionally, during some deployments, such as at the SCExAO instrument at Subaru Telescope, radio-frequency interference from the telescope environment produced noise many times greater than what we experienced in the lab. In order to address these problems, collaborators at the Australia National University developed a cryogenic preamplifier system that holds the detector and buffers the signals from its outputs. During lab testing at 1 MHz clocking speeds, the preamplifiers reduced the read noise by 45% relative to data collected using the previous JK Henriksen detector mount. Additionally, the preamplifiers enabled us to increase the clocking frequency to 2 MHz, effectively doubling the frame rate to 760 Hz for a full (320x256 pixel) frame or 3.3 kHz for a 128x128 pixel subarray. Finally, the preamplifiers reduced the noise observed in the SCExAO environment by 65% (to essentially the same value observed in the lab) and eliminated the 32-pixel raised bars characteristic of radio-frequency interference that we previous observed there.
We present a summary of the cryogenic detector preamplifier development programme under way at the ANU. Cryogenic preamplifiers have been demonstrated for both near-infrared detectors (Teledyne H1RG and Leonardo SAPHIRA eAPD as part of development for the GMTIFS instrument) and optical CCDs (e2v CCD231-84 for use with the AAT/Veloce spectrograph). This approach to detector signal conditioning allows low-noise instrument amplifiers to be placed very close to an infra-red detector or optical CCD, isolating the readout path from external interference noise sources. Laboratory results demonstrate effective isolation of the readout path from external interference noise sources. Recent progress has focussed on the first on-sky deployment of four cryogenic preamp channels for the Veloce Rosso precision radial velocity spectrograph. We also outline future evolution of the current design, allowing higher speeds and further enhanced performance for the demanding applications required for the on instrument wavefront sensor on the Giant Magellan Integral Field Spectrograph (GMTIFS).
The Australian National University (ANU), we are undertaking to deploy a Lucky Imaging instrument on the 2.3 m telescope at Siding Springs using a Leonardo SAPHIRA near-infrared electron Avalanche Photo-Diode (eAPD) array, capable of high cadence imaging with frame rates of 10 - 5,000 Hz over the wavelength range of 0.8 μm to 2.5 μm. compact cryocooler capable of cooling the Leonardo SAPHRA APD and associated cryogenic electronics to temperatures below 100K with little to no vibration. An ideal candidate cryocooler is the Sunpower Cryotel GT with active vibration cancellation. The Cryotel GT is an orientation independent, Stirlng cycle cooler with water jacket heat rejection. This cooler will meet the system cooling requirements. The cryocooler has been integrated with the APD Lucky Imager cryostat through 3 rubber isolating mounts and bellows and tested while suspended from a stable frame. The tethers supporting the cryostat and cooler assembly are not attached to the cryostat and cooler. The exported vibration was measured simultaneously in all 3 axis on the external cryostat wall and internally on the cryostat getter attached directly to the cold tip of the cooler. The test results were collected while the cryocooler was cooling and at the stable set point, at various levels of cooling power and with thermal control enabled and disabled.
As space debris in lower Earth orbits are accumulating, techniques to lower the risk of space debris collisions must be developed. Within the context of the Space Environment Research Centre (SERC), the Australian National University (ANU) is developing an adaptive optics system for tracking and pushing space debris. The strategy is to pre-condition a laser launched from a 1.8 m telescope operated by Electro Optics Systems (EOS) on Mount Stromlo, Canberra and direct it at an object to perturb its orbit. Current progress towards implementing this experiment, which will ensure automated operation between the telescope and the adaptive optics system, will be presented.
Veloce is an ultra-stable fibre-fed R4 echelle spectrograph for the 3.9 m Anglo-Australian Telescope. The first channel to be commissioned, Veloce ‘Rosso’, utilises multiple low-cost design innovations to obtain Doppler velocities for sun-like and M-dwarf stars at <1 ms <sup>-1</sup> precision. The spectrograph has an asymmetric white-pupil format with a 100-mm beam diameter, delivering R>75,000 spectra over a 580-930 nm range for the Rosso channel. Simultaneous calibration is provided by a single-mode pulsed laser frequency comb in tandem with a traditional arc lamp. A bundle of 19 object fibres ensures full sampling of stellar targets from the AAT site. Veloce is housed in dual environmental enclosures that maintain positive air pressure at a stability of ±0.3 mbar, with a thermal stability of ±0.01 K on the optical bench. We present a technical overview and early performance data from Australia's next major spectroscopic machine.
Veloce is an ultra-stabilized Echelle spectrograph for precision radial velocity measurements of stars. In order to maximize the grating performance, the air temperature as well as the air pressure surrounding it must be maintained within tight tolerances. The control goal was set at +/-10 mK and +/-1 mbar for air temperature and pressure respectively. The strategy developed by the design team resulted in separate approaches for each of the two requirements. A constrained budget early in the concept phase quickly ruled out building a large vacuum vessel to achieve stable air pressure. Instead, a simplified approach making use of a slightly over pressurized enclosure containing the whole spectrograph was selected in conjunction with a commercially available pressure controller. The temperature stability of Veloce is maintained through a custom array of PID controlled heaters placed on the outer skin of the internal spectrograph enclosure. This enclosure is also fully lined with 19 mm thick insulating panels to minimize the thermal fluctuations. A second insulated enclosure, built around the internal one, adds a layer of conditioned air to further shield Veloce from the ambient thermal changes. Early success of the environment control system has already been demonstrated in the integration laboratory, achieving results that amply exceed the goals set forth. Results presented show the long term stability of operation under varying barometric conditions. This paper details the various challenges encountered during the implementation of the stated designs, with an emphasis on the control strategy and the mechanical constraints to implement the solutions.