We present a concept low-resolution spectrograph design for the Southern African Large Telescope (SALT) based on the O’Donoghue-Clemens Spherical Transmission Grating Spectrometer (STGS) principle. The design delivers R ~ 700 between 380 nm and 760 nm with a peak total efficiency estimated at 45%. This instrument type remains largely invariant with telescope size and can fit into a very compact volume (650 mm × 650 mm × 400 mm). This can truly be a new technology which may transform future instrument design — especially when matched to extremely large telescopes.
Major improvements to the SALT prime focus guidance system have been implemented over the last two years in the form of a new compact, modular and removable system with high optical efficiency. A double-probe positioning system allows both translation and rotation guidance, while optional beam-splitting allows closed-loop focus feedback to stabilise the focal plane. The features of this new system, its mechanical and optical performance and its control system architecture are presented.
A new guide probe, using a Pyramid WaveFront Sensor (PWFS), has been built for the guider port of the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope (SALT). The PWFS splits the light from a guide star into a double image of the star at the guider image plane. These two images move with respect to each other with changes in focus. Analysis of the movements in such two-star images allows for simultaneous tracking and focus adjustments while guiding. This paper describes the image processing software developed for the new guidance system. The PWFS is a custom made prism with a 9 degree prism apex angle. Incident light is split as it passes through the prism resulting in two images identical in shape, size and orientation resolving at the guider image plane. A point source, like a star, thus appears as a double image of the star at the image plane. These two images are affected identically with seeing fluctuations allowing one or both images to be used to calculate guidance offsets independently of the focus measurement. The separation of the two images correlates to the focus divergence from the ideal which is used to correct the telescope focus. The prism can be moved off to one side, such that the incident light is not split. In this configuration the guide probe provides guidance offsets without the focus adjustments for maximum sensitivity with dim guide. The software was developed using existing guider images to ensure that the algorithm could handle the image artefacts that are commonly encountered. To reduce processing time and the effects of anomalies the surrounding image is masked out leaving only the star. A centroiding algorithm is then applied to the star to estimate its location in the image. For PWFS double-star images, Zemax was used to model the effect of shifting focus on the light passing through the prism. From this a simulated data set was produced and the software expanded to detect the two stars and measure the separation. The software is capable of measuring star translational movements and double star separation reliability and accurately.
The SALT Tracker was originally designed to carry a payload of approximately 1000 kg. The current loading exceeds 1300 kg and more instrumentation, for example, the Near-Infrared (NIR) arm of the Robert Stobie Spectrograph (RSS), is being designed for the telescope. In general, provision also had to be made to expand the envelope of the tracker payload carrying capacity for future growth as some of the systems on SALT are currently running with small safety margins. It was therefore decided to upgrade the SALT Tracker to be able to carry a payload of 1875 kg.
Before the project "Kick-Off" it became evident that neither SALT nor SAAO had the required standard of formal processes and procedures to execute a project of this nature. The Project Management, Mechanical Design and Review processes and procedures were adopted from the Aerospace Industry and tailored for our application. After training the project team in the application of these processes/procedures and gaining their commitment, the Tracker Upgrade Project was "Kicked-Off" in early May 2013.
The application of these aerospace-derived processes and procedures, as used during the Tracker Upgrade Project, were very successful as is shown in this paper where the authors also highlight some of the details of the implemented processes and procedures as well as specific challenges that needed to be met while executing a project of this nature and technical complexity.
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.
Liquid lens coupling provides excellent transmission efficiency when compared to multilayer coatings especially for applications where broadband transmission is required. However, long term reliability of liquid coupling is difficult to achieve. This is typically due to chemical compatibility issues affecting both the optical transmission and the integrity of the opto-mechanical support. As part of a recent service of the Robert Stobie Spectrograph on SALT we had the opportunity to study these problems further and in this paper we provide analysis of problems identified and some solutions to prevent them. We also present general guidelines which could aid future opto-mechanical designs for liquid coupling of lenses.
Following successful commissioning of the SALT Fiber Instrument Feed guidance system, the concept was developed further to re-design the guidance probe currently supporting observations with the Robert Stobie Spectrograph. Major advances of the new system include a compact, modular and line-replaceable design, high optical efficiency, a doubleprobe positioning system allowing both translation and rotation guidance corrections as well as closed-loop focus feedback.
The mechanical and optical designs, the control system architecture and performance aspects of the system are presented. The probe‘s integration with the greater telescope software control system is also discussed.
The Southern African Large Telescope has till recently operated without active closed loop control of its Primary Mirror. The reason for this was that there were no suitable edge sensor system available on the market. Recently a system became available and SALT form Fogale Nanotech. The system consist of a sensor, cables and control electronics. The system was still under development and SALT was responsible for the integration of the sensors before deployment on the Telescope. Several issues still had to be addressed. One of these issues was the integration of the sensors at an appropriate production rate. The sensors was supplied as flexible pc boards with different types making up the transmitters and receivers. These flexible boards were bonded to ClearCeram Z L-Brackets before the appropriate connectors were installed. This paper describes the process used to integrate and test the sensors.
SALT is a 10-m class optical telescope located in Sutherland, South Africa. We present an update on all observatory performance metrics since the start of full science operations in late 2011, as well as key statistics describing the science efficiency and output of SALT, including the completion fractions of observations per priority class, and analysis of the more than 140 refereed papers to date. After addressing technical challenges and streamlining operations, these first years of full operations at SALT have seen good and consistently increasing rates of completion of high priority observations and, in particular, very cost-effective production of science publications.
The efficient operation of a telescope requires awareness of its performance on a daily and long-term basis. This paper outlines the Fault Tracker, WebSAMMI and the Dashboard used by the Southern African Large Telescope (SALT) to achieve this aim. Faults are mostly logged automatically, but the Fault Tracker allows users to add and edit faults. The SALT Astronomer and SALT Operator record weather conditions and telescope usage with WebSAMMI. Various efficiency metrics are shown for different time periods on the Dashboard. A kiosk mode for displaying on a public screen is included. Possible applications for other telescopes are discussed.
Lessons learnt during the design, integration and commissioning of the Southern African Large Telescope’s Fiber
Instrument Feed and its guidance probe are presented along with initial on-sky performance results.
Advances over the original design include enhanced target acquisition efficiency by including an imaging camera
directly on the fiber-positioning stage, use of linear optical encoders on all motion stages, thermally compensated
designs, a guidance probe optical design with much improved throughput as well as a new non-contact metrology
process to accurately model the guider.
The system has since been successfully used in commissioning the SALT High-Resolution Spectrograph.
The Southern African Large Telescope (SALT) High Resolution Spectrograph (HRS) is a fibre-fed R4 échelle
spectrograph employing a white pupil design with red and blue channels for wavelength coverage from 370–890nm.
The instrument has four modes, each with object and sky fibres: Low (R~15000), Medium (R~40000) and High
Resolution (R~65000), as well as a High Stability mode for enhanced radial velocity precision at R~65000. The High
Stability mode contains a fibre double-scrambler and offers optional simultaneous Th-Ar arc injection, or the inclusion
of an iodine cell in the beam. The LR mode has unsliced 500μm fibres and makes provision for nod-and-shuffle for
improved background subtraction. The MR mode also uses 500μm fibres, while the HR and HS fibres are 350μm. The
latter three modes employ modified Bowen-Walraven image-slicers to subdivide each fibre into three slices. All but the
High Stability bench is sealed within a vacuum tank, which itself is enclosed in an interlocking Styrostone enclosure, to
insulate the spectrograph against temperature and atmospheric pressure variations. The Fibre Instrument Feed (FIF)
couples the four pairs of fibres to the telescope focal plane and allows the selection of the appropriate fibre pair for a
given mode, and adjustment of the fibre separation to optimally position the sky fibre. The HRS employs a
photomultiplier tube for an exposure meter and has a dedicated auto-guider attached to the FIF. We report here on the
commissioning results and overall instrument performance since achieving first light on 28 September 2013.
Images obtained with the Southern African Large Telescope (SALT) during its commissioning phase in 2006 showed degradation due to a large focus gradient, astigmatism, and higher order optical aberrations. An extensive forensic investigation exonerated the primary mirror and the science instruments before pointing to the mechanical interface between the telescope and the spherical aberration corrector, the complex optical subassembly which corrects the spherical aberration introduced by the 11-m primary mirror. Having diagnosed the problem, a detailed repair plan was formulated and implemented when the corrector was removed from the telescope in April 2009. The problematic interface was replaced, and the four aspheric mirrors were optically tested and re-aligned. Individual mirror surface figures were confirmed to meet specification, and a full system test after the re-alignment yielded a root mean square wavefront error of 0.15 waves. The corrector was reinstalled in August 2010 and aligned with respect to the payload and primary mirror. Subsequent on-sky tests revealed spurious signals being sent to the tracker by the auto-collimator, the instrument that maintains the alignment of the corrector with respect to the primary mirror. After rectifying this minor issue, the telescope yielded uniform 1.1 arcsec star images over the full 10-arcmin field of view.
Images obtained with the Southern African Large Telescope (SALT) during its commissioning phase showed
degradation due to a large focus gradient and a variety of other optical aberrations. An extensive forensic investigation
eventually traced the problem to the mechanical interface between the telescope and the secondary optics that form the
Spherical Aberration Corrector (SAC). The SAC was brought down from the telescope in 2009 April, the problematic
interface was replaced and the four corrector mirrors were optically tested and re-aligned. The surface figures of the SAC
mirrors were confirmed to be within specification and a full system test following the re-alignment process yielded a
RMS wavefront error of just 0.15 waves. The SAC was re-installed on the tracker in 2010 August and aligned with
respect to the payload and primary mirror. Subsequent on-sky tests produced alarming results which were due to
spurious signals being sent to the tracker by the auto-collimator, the instrument responsible for controlling the attitude of
the SAC with respect to the primary mirror. Once this minor issue was resolved, we obtained uniform 1.1 arcsecond star
images over the full 10 arcminute field of view of the telescope.
This paper describes the cleaning of M5, one of the four mirrors that make up the Southern African Large Telescope's
Spherical Aberration Corrector. As the top upward-facing mirror in a relatively exposed environment, M5 had
accumulated a considerable amount of dust and dirt during the six years it had been on the telescope. With the corrector
on the ground for re-alignment and testing, we had the opportunity to remove, wash and replace the mirror. Various
cleaning techniques were investigated, including an unsuccessful trial application of First Contact surface cleaning
polymer film - fortunately only to a small region outside the mirror's clear aperture. Ultimately, "drag-wiping" with
wads of cotton wool soaked in a 10g/l sodium lauryl sulphate solution proved highly effective in restoring the reflectivity
of M5's optical surface. Following this success, we repeated the procedure for M3, the other upward-facing mirror in the
corrector. The results for M3 were equally spectacular.
The construction of the Southern African Large Telescope (SALT) was largely completed by the end of 2005. At the
beginning of 2006, it was realized that the telescope's image quality suffered from optical aberrations, chiefly a focus
gradient across the focal plane, but also accompanied by astigmatism and higher order aberrations. In the previous
conference in this series, a paper was presented describing the optical system engineering investigation which had been
conducted to diagnose the problem. This investigation exonerated the primary mirror as the cause, as well as the science
instruments, and was isolated to the interface between the telescope and a major optical sub-system, the spherical
aberration corrector (SAC). This is a complex sub-system of four aspheric mirrors which corrects the spherical
aberration of the 11-m primary mirror. In the last two years, a solution to this problem was developed which involved
removing the SAC from the telescope, installing a modification of the SAC/telescope interface, re-aligning and testing
the four SAC mirrors and re-installation on the telescope. This paper describes the plan, discusses the details and shows
progress to date and the current status.
SALT developed an automated CO2 Mirror Cleaning System (MCS) for its 11 meter diameter segmented primary
mirror. In this paper we report on the mechanical design of the system, the safety considerations taken into account given
that the mechanism has to be lowered over the primary mirror for every cleaning cycle, the computerized control system
and the CO2 installation which feeds the cleaning wand with liquid CO2. The paper also addresses the complexities
experienced in providing high pressure liquid CO2 for the effective operation of the cleaning wand as well as the safety
precautions implemented to ensure the safety of staff members at all times. The performance results are also presented
although the system is still being optimised in a trade off between cleaning efficiency, CO2 consumption, the duration of
a cleaning cycle and the cleaning frequency.
Construction of the Southern African Large Telescope (SALT) was largely completed by the end of 2005 and since then
it has been in intensive commissioning. This has now almost been completed except for the telescope's image quality
which shows optical aberrations, chiefly a focus gradient across the focal plane, along with astigmatism and other less
significant aberrations. This paper describes the optical systems engineering investigation that has been conducted since
early 2006 to diagnose the problem. A rigorous approach has been followed which has entailed breaking down the
system into the major sub-systems and subjecting them to testing on an individual basis. Significant progress has been
achieved with many components of the optical system shown to be operating correctly. The fault has been isolated to a
major optical sub-system. We present the results obtained so far, and discuss what remains to be done.