The Southern African Large Telescope (SALT) is a 10-m class fixed-elevation telescope with a primary mirror composed of 91 spherically figured one metre segments. A prime focus tracker assembly carries the spherical aberration corrector (SAC) and two of SALT’s instruments, SALTICAM (the acquisition and imaging camera) and the multi-purpose Robert Stobie spectrograph (RSS). Included in the tracker payload is a fibre-instrument feed, that positions ~45m long fibre cables coupled to the spectrographs in thermal enclosures beneath the telescope. These are the High-Resolution Spectrograph (HRS) and NIRWALS (Near InfraRed Washburn Astronomical Laboratories Spectrograph). The other major undertaking is a custom-built laser frequency comb and precision radial velocity data pipeline for the HRS, due in 2025. A novel RSS slit-mask IFU was recently commissioned, adding optical IFU spectroscopy to SALT’s capabilities. Work is also underway to develop a new red channel to turn the RSS into a dual-beam spectrograph. A study done in 2021 investigated the feasibility of building deployable robotic arms equipped with mini SACs to take advantage of SALT’s huge uncorrected field of view. Lastly, a pre-study is now underway to explore options for replacing the SAC and prime focus payload on the tracker to improve telescope performance and make provision for future instrument development.
We present a low-resolution spectrograph design for the Southern African Large Telescope (SALT) primarily aimed at efficient identification spectroscopy of transients. The design extends the existing Robert-Stobie Spectrograph (RSS) by adding a new simultaneous red channel for wide visible wavelength coverage (360 nm to 900 nm). The design delivers R ~800 in the blue channel using the existing RSS optics and R ~2000 at a peak end-to-end instrument efficiency of 44% via the new red channel. We describe the instrument’s requirements, optical design and expected performance. Synergies with existing RSS functionality are explored that will allow dual-beam multi-object and future integral field unit spectroscopy.
KEYWORDS: Sensors, Telescopes, Mirrors, Environmental sensing, Humidity, Simulation of CCA and DLA aggregates, Temperature metrology, Control systems, Transmitters, Actuators
The Southern African Large Telescope (SALT) is a 10-m class 91-segment fixed altitude telescope located at Sutherland, South Africa. The segment alignment is maintained by inductively coupled sensors mounted on Sitall brackets beneath the segments. An extensive period of testing in environmental chambers and on the telescope has been conducted to establish the stability of the sensors and their response to temperature and humidity variations in the telescope chamber. We present some of the test results, including a demonstration of the ability of the sensors to maintain the alignment of the primary mirror over a period of 6 days.
The development of an inductive edge sensor is in process for the control of the Southern African Large Telescope’s (SALT)1 segmented mirror primary. The original capacitive edge sensing system was not capable of maintaining the figure of the primary mirror due to excessive noise and a severe sensitivity to humidity despite exhaustive attempts at characterisation1. The prototype of the inductive edge sensor has progressed to a mature industrialised version that is in the process of being installed and commissioned on SALT. The performance of the sensor in response to temperature and RH is very good with a maximum error of 10nm typical after temperature compensation. The noise and control characteristics of the array have been simulated in order to establish the maximum cumulative error and error rate tolerable for the SALT specific case. It has been established through simulation that over the expected 5 day alignment cycle, a maximum cumulative error of 30nm can be tolerated.
The Southern African Large Telescope (SALT) recently (2008) abandoned attempts at using capacitive mirror edge
sensors, mainly due to poor performance at a relative humidity above ~60%, a not infrequent occurrence. Different
technologies are now being explored for alternative sensors on SALT. In this paper we describe the design and
development of a novel prototype optical edge sensor, based on the application of the interferential scanning principle,
as used in optical encoders. These prototype sensors were subsequently tested at SAAO and ESO, for potential
application on SALT and E-ELT.
Environmental tests, conducted in climatic control chambers, looked at temperature and relative humidity sensitivity,
long term stability and sensor noise. The temperature sensitivity for height and gap were, respectively, 10nm/°C and
44nm/°C, while for relative humidity they were 4nm/10% and 50nm/10%, respectively. These either met, or were close
to, the SALT specification. While there were significant lags in response, this was due to the sensor's relatively large
mass (~200 gm per sensor half), which was not optimized. This is likely to improve, should a revised design be
developed in future. Impressively the sensor noise was <0.015 nm RMS, over three orders of magnitude better than the
specification. Our conclusions are that optical edge sensing is a viable technique for use on segmented mirror telescopes.
KEYWORDS: Sensors, Mirrors, Environmental sensing, Space telescopes, Telescopes, Current controlled current source, Large telescopes, Nanotechnology, Sensor technology, Sensing systems
At the Southern African Large Telescope (SALT), in collaboration with FOGALE Nanotech, we have been testing the recently-developed new generation inductive edge sensors. The Fogale inductive sensor is one
technology being evaluated as a possible replacement for the now defunct capacitance-based edge sensing system.
We present the results of exhaustive environmental testing of two variants of the inductive sensor. In addition to the environmental testing including RH and temperature cycles, the sensor was tested for sensitivity to dust and metals. We also consider long-term sensor stability, as well as that of the electronics and of the glue used to bond the sensor to its supporting structure. A prototype design for an adjustable mount is presented which will allow for in-plane gap and shear variations present in the primary mirror configuration without adversely disturbing the figure of the individual mirror segments or the measurement accuracy.
KEYWORDS: Sensors, Humidity, Mirrors, Temperature metrology, Sensing systems, Simulation of CCA and DLA aggregates, Telescopes, Calibration, Environmental sensing, Nanotechnology
The SAMS (Segment Alignment Measurement System) is a
capacitance-based edge sensing solution for the active
alignment of the 10m SALT segmented primary mirror. Commissioning and calibrating the system has been an ongoing
task in an attempt to counteract the unfavourable response of the sensors to high humidity conditions and high dust
levels. Several solutions were implemented and tested including
real-time feedback systems and the application of
corrective functions.
In parallel with the continuing efforts to improve the performance of the capacitive sensors, we have also been testing a
prototype inductive sensor developed by Fogale Nanotech that is of a very similar flexible plate construction.
In this paper we present the results obtained and performance gains achieved thus far with the capacitive edge-sensing
system as well as a performance comparison of the Fogale inductive sensor to the capacitive edge sensor.
KEYWORDS: Sensors, Mirrors, Sensing systems, Humidity, Temperature metrology, Control systems, Simulation of CCA and DLA aggregates, Actuators, Environmental sensing, Large telescopes
The 10-m class Southern African Large Telescope (SALT) at Sutherland, South Africa, was inaugurated in November 2005, following completion of all its major sub-systems. It is the largest single optical telescope in the southern hemisphere. The SAMS (Segment Alignment Measurement System) is a unique capacitive edge sensing solution for the active alignment of the SALT primary mirror. Twelve thin film edge sensors are bonded directly onto the edges of each of the 91 segments, with heat-generating control electronics housed remotely in temperature-controlled enclosures. The SAMS is capable of measuring the tip/tilt and piston of each segment, as well as the change in global radius of curvature, a mode normally undetected by such a system. The primary objective was to build a system that offered an excellent cost-to-performance ratio without sacrificing measurement accuracy, a very necessary requirement because of the scale and number of sensors required for large segmented mirrors. This paper describes the results obtained during the commissioning and calibration of the completed system.
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