We present Mookodi (meaning “rainbow” in Sesotho), a multipurpose instrument with a low-resolution spectrograph mode and a multi-filter imaging mode for quick-reaction astronomical observations. The instrument, mounted on the 1-m Lesedi telescope at the South African Astronomical Observatory in Sutherland (South Africa), is based on the low-resolution spectrograph for the rapid acquisition of transients (SPRAT) instrument in operation on the 2-m Liverpool Telescope in La Palma (Canary Islands, Spain). Similar to SPRAT, Mookodi has a resolution R≈350 and an operating wavelength range in the visible (∼4000 to 8000 Å). The linear optical design, as in SPRAT, is made possible through the combination of a volume phase holographic transmission grating as the dispersive element and a prism pair (grism), which makes it possible to rapidly and seamlessly switch to an imaging mode by pneumatically removing the slit and grism from the beam and using the same detector as in spectrographic mode to image the sky. This imaging mode is used for auto-target acquisition, but the inclusion of filter slides in Mookodi’s design also provides the capability to perform imaging with a field-of-view ≈10′×10′ (∼0.6″/px) in the complete Sloan Digital Sky Survey filter set.
The Wide-Area Linear Optical Polarimeter (WALOP)-South instrument is an upcoming wide-field and high accuracy optical polarimeter to be used as a survey instrument for carrying out the Polar-Areas Stellar Imaging in Polarization High-Accuracy Experiment program. Designed to operate as a one-shot four-channel and four-camera imaging polarimeter, it will have a field of view of 35 × 35 arcminutes and will measure the Stokes parameters I, q, and u in a single exposure in the Sloan Digital Sky Survey-r broadband filter. The design goal for the instrument is to achieve an overall polarimetric measurement accuracy of 0.1% over the entire field of view. We present here the complete polarimetric modeling of the instrument, characterizing the amount and sources of instrumental polarization. To accurately retrieve the real Stokes parameters of a source from the measured values, we have developed a calibration method for the instrument. Using this calibration method and simulated data, we demonstrate how to correct for instrumental polarization and obtain 0.1% accuracy in degree of polarization, p. In addition, we tested and validated the calibration method by implementing it on a table-top WALOP like test-bed polarimeter in the laboratory.
Two unique wide-field and high-accuracy polarimeters named WALOP (Wide-Area Linear Optical Polarimeter)- North and WALOP-South are currently under development at the Inter-University Center for Astronomy and Astrophysics (IUCAA), India, to create a large area optical polarization map of the sky for the upcoming PASIPHAE sky survey. These instruments are designed to achieve a linear polarimetric measurement accuracy of 0.1% across a field of view (FoV) of 30×30 arcminutes. The WALOP-South instrument will be installed first on a 1 m telescope at the Sutherland Observatory, where the temperatures during the night can vary between 10 to -5°C. These temperature variations and the instrument’s pointing to various non-zenithal positions in the sky can introduce stress birefringence in the lenses, leading to time-varying instrumental polarization. This work estimates stress-induced birefringence due to thermal, and gravity stresses on WALOP-South lenses. Using the optomechanical model of the WALOP-South, we carried out Finite Element Analysis (FEA) simulations in SolidWorks software to estimate the stresses for various scenarios of temperature, telescope pointing airmass, and lens mount material (aluminum and titanium). Further, we use the stress tensor analysis to estimate the principal stresses and their directions and consequent birefringence and retardance introduced in the lenses. The stressinduced birefringence will change the optical path length for orthogonal polarization states of the beam passing through the lenses and introduce phase retardation. Overall, with the lens mount design of the instrument, we find that the retardation and consequent instrumental polarization will be within the instrumental accuracy requirements. Additionally, the stress birefringence is found to be higher for aluminum compared to titanium mounts. We further incorporated this retardance in the instrument Mueller matrix estimation to understand its effects on the polarization measurements.
We describe the software architecture of the Local Control Units (LCU) being deployed as part of the Intelligent Observatory project of the South African Astronomical Observatory. This is an integrated system for scheduling and controlling observations across several telescopes and instruments. As part of this, each telescope and its associated instruments fall under the control of an LCU. The LCU interfaces with the observatory-wide scheduler, executing observations as requested. It also monitors observing conditions and shuts down the telescope if necessary. The software is layered, modular and distributed, and allows remote and robotic control of the various instruments and telescopes.
The Wide-Area Linear Optical Polarimeter (WALOP)-South instrument will be mounted on the 1-m South African Astronomical Observatory telescope in South Africa as part of the Polar-Areas Stellar Imaging Polarization High Accuracy Experiment (PASIPHAE) program to carry out a linear imaging polarization survey of the Galactic polar regions in the optical band. Designed to achieve polarimetric sensitivity of 0.05% across a 35 × 35 arc min field of view (FOV), it will be capable of measuring the Stokes parameters I, q, and u in a single exposure in the R broadband and narrowband filters between 0.5 to 0.7 μm. For each measurement, four images of the full field corresponding to linear polarization angles of 0 deg, 45 deg, 90 deg, and 135 deg in the instrument coordinate system will be created on four detectors from which the Stokes parameters can be found using differential photometry. In designing the optical system, major challenges included correcting for the dispersion introduced by large split angle Wollaston prisms used as analysers and other aberrations from the entire field to obtain imaging quality point spread function (PSF) at the detector. We present the optical design of the WALOP-South instrument which overcomes these challenges and delivers near seeing limited PSFs for the entire FOV.
We present an overview of the Intelligent Observatory (IO) and the architecture used at the South African Astronomical Observatory (SAAO) to develop instrument and telescope control and monitoring software. The IO aims to link and coordinate the usage of the SAAO telescopes and instruments for optimal efficiency. This will entail a Central Control System (CCS) selecting appropriate instruments and telescopes and controlling observations on these. This requires interoperable instrument and telescope control software. The SAAO software architecture is flexible, allows multiple user interfaces, and supports remote control and monitoring of both telescope and instrument through a web browser. Furthermore, the architecture allows an external agent (such as the IO CCS) simultaneous control of both instruments and telescopes.
One aspect of the SALT X-ray transient program is the identification of SALT observation time that overlaps with scheduled X-ray satellite observations. This is particularly relevant to the optical study of X-ray transients. Since SALT is a fixed elevation telescope, the target visibility is restricted to a circular annulus on the sky covering a total of 1 380 square degrees. In order to identify satellite targets that overlap with SALT visibility, a custom Python program was developed to scrape daily schedules of a number of satellites, and calculate the SALT annulus visibility period of the satellite targets to find overlapping observation time between the satellites and SALT. If a target observation time overlaps in visibility, the relevant information is published to a web page, as well as summarised in an email for dissemination by a mailing list. Transient alerts by email is an old established method, but it clogs inboxes and requires time during the day to evaluate for scheduling — followed by an independent process to request or submit a target for observation to a telescope. It also requires a human in the loop, which will become increasingly challenging as the frequency of alerts increase over time. To streamline the process, from evaluation to submission, SALT (and by extension the SAAO intelligent observatory) is in the midst of developing a prototype TOM for the X-ray transient observations. The aim of the prototype development is to identify and implement the components that will make SALT observations more easily undertaken for the transient community using TOMs1 for target management and observation. The current SALT X-ray transient email alert software, Xsats, contains all the components necessary to migrate to a TOM transient alert interface. Additionally, because the current email alerts have been running for a while, user needs and requirements are already folded into the code; thereby permitting a straight-up mapping onto new technology, using the existing system as independent verification. This paper presents the overall design describing the migration process, as well as application-based development that will be required.
WALOP (Wide-Area Linear Optical Polarimeter)-South, to be mounted on the 1m SAAO telescope in South Africa, is first of the two WALOP instruments currently under development for carrying out the PASIPHAE survey. Scheduled for commissioning in the year 2021, the WALOP instruments will be used to measure the linear polarization of around 106 stars in the SDSS-r broadband with 0.1 % polarimetric accuracy, covering 4000 square degrees in the Galactic polar regions. The combined capabilities of one-shot linear polarimetry, high polarimetric accuracy (< 0.1 %) and polarimetric sensitivity (< 0.05 %), and a large field of view (FOV) of
35 35 arcminutes make WALOP-South a unique astronomical instrument. In a single exposure, it is designed to measure the Stokes parameters I, q and u in the SDSS-r broadband and narrowband filters between 500-700 nm. During each measurement, four images of the full field corresponding to the polarization angles of 0°, 45°, 90° and 135° will be imaged on four detectors and carrying out differential photometry on these images will yield the Stokes parameters. Major challenges in designing WALOP-South instrument include- (a) in the optical design, correcting for the spectral dispersion introduced by large split angle Wollaston Prisms used as polarization analyzers as well as aberrations from the wide field, and (b) making an optomechanical design adherent to the tolerances required to obtain good imaging and polarimetric performance under all temperature conditions as well as telescope pointing positions. We present the optical and optomechanical design for WALOP-South which
overcomes these challenges.
We report on the extensively upgraded Cassegrain spectrograph on the South African Astronomical Observatory (SAAO) 1.9-m telescope. The introduction of new collimator and camera optics, a new detector and controller, a rear-of-slit viewing camera to facilitate acquisition, and a new instrument control and quick-look data-reduction software (to take advantage of the entire system now being governed by a programmable logic controller) has revolutionized this workhorse instrument on Africa’s second largest optical telescope. The improvement in throughput over the previous incarnation of the spectrograph is ∼50 % in the red, increasing to a factor of four at the blue end. A selection of 10 surface-relief diffraction gratings is available to users, offering a variety of wavelength ranges and resolutions, with resolving powers between ∼500 and 6500. SpUpNIC (Spectrograph Upgrade: Newly Improved Cassegrain) has been scheduled for ∼80 % of the time available on the 1.9-m since being installed on the telescope in late October 2015, providing the single-object spectroscopic capability to support the broad research interests of the SAAO’s local and international user community. We present an assortment of data obtained for various observing programs to demonstrate different aspects of the instrument’s enhanced performance following this comprehensive upgrade.
The South African astronomical community together with the international SALT community recently completed a process to detail a science strategy for SALT, the 10m international telescope that SAAO operates. After six years of science operations, the telescope is a very cost-effective large telescope science producer. The strategy was adopted by the SALT Board, and has already resulted in funding choices for the next stage of instrumentation. The SALT strategy intertwines with that of the SAAO and South African optical astronomy in general. This paper outlines the process followed, the main motivations and plans for the next stage, including risks and challenges. This paper in particular concentrates on the plans to making SAAO/SALT a major player in time domain astrophysics, one of three adopted strategic science focus areas. Plans include a novel design for a high-efficiency spectrograph serving transient follow-up, for which South Africa is well positioned; advanced
software aiming to make the whole mountain-top operate as a single transient machine; feasibility studies into revolutionizing SALT observations by utilizing the primary mirror's hundreds of square degree size uncorrected field-of-view. Other SPIE papers in this meeting describe these and other developments at SALT and SAAO in more detail
SpUpNIC (Spectrograph Upgrade: Newly Improved Cassegrain) is the extensively upgraded Cassegrain Spectrograph on the South African Astronomical Observatory's 74-inch (1.9-m) telescope. The inverse-Cassegrain collimator mirrors and woefully inefficient Maksutov-Cassegrain camera optics have been replaced, along with the CCD and SDSU controller. All moving mechanisms are now governed by a programmable logic controller, allowing remote configuration of the instrument via an intuitive new graphical user interface. The new collimator produces a larger beam to match the optically faster Folded-Schmidt camera design and nine surface-relief diffraction gratings offer various wavelength ranges and resolutions across the optical domain. The new camera optics (a fused silica Schmidt plate, a slotted fold flat and a spherically figured primary mirror, both Zerodur, and a fused silica field-flattener lens forming the cryostat window) reduce the camera’s central obscuration to increase the instrument throughput. The physically larger and more sensitive CCD extends the available wavelength range; weak arc lines are now detectable down to 325 nm and the red end extends beyond one micron. A rear-of-slit viewing camera has streamlined the observing process by enabling accurate target placement on the slit and facilitating telescope focus optimisation. An interactive quick-look data reduction tool further enhances the user-friendliness of SpUpNI
The Robert Stobie Spectrograph is currently the main workhorse spectroscopic instrument on the Southern African Large Telescope (SALT), which has been undergoing regular scientific operations since 2011. The visible beam of the RSS was designed to perform polarimetry in all of its modes, imaging and grating spectroscopy (with Multi Object Spectroscopy capability) from 3200 to 9000 Å. The polarimetric field of view is 4×8 arcmin. Initial early commissioning of the polarimetric modes was stalled in 2011 because a coupling fluid leak developed in the polarizing beamsplitter after less than a year of operation. As a result, it was decided to redesign the beamsplitter to use a different optical couplant. This was complicated by the unusual thermal expansion properties of the calcite optic, and by the necessity of aligning the individual elements in the beamsplitter mosaic (RSS is the first instrument to use a mosaic beamsplitter). Laboratory work selected a new couplant: a gel, Nye 451. Testing was completed with satisfactory results on a "sacrificial" calcite prism with the same geometry as an actual mosaic element. A successful assembly was performed and the beamsplitter was re-installed in SALT in mid-2015. We describe results from the renewed commissioning efforts to characterize polarimetry from SALT and include some early performance verification science.
humidity, air pressure, wind speed and wind direction) into a database. Built upon this database, we have developed a remarkably simple approach to derive a functional weather predictor. The aim is provide up to the minute local weather predictions in order to e.g. prepare dome environment conditions ready for night time operations or plan, prioritize and update weather dependent observing queues.
In order to predict the weather for the next 24 hours, we take the current live weather readings and search the entire archive for similar conditions. Predictions are made against an averaged, subsequent 24 hours of the closest matches for the current readings. We use an Evolutionary Algorithm to optimize our formula through weighted parameters.
The accuracy of the predictor is routinely tested and tuned against the full, updated archive to account for seasonal trends and total, climate shifts. The live (updated every 5 minutes) SALT weather predictor can be viewed here: http://www.saao.ac.za/~sbp/suthweather_predict.html
PySALT is the python/PyRAF-based data reduction and analysis pipeline for the Southern African Large Telescope
(SALT), a modern 10m class telescope with a large user community consisting of 13 partner institutions. The two first
generation instruments on SALT are SALTICAM, a wide-field imager, and the Robert Stobie Spectrograph (RSS). Along
with traditional imaging and spectroscopy modes, these instruments provide a wide range of observing modes, including
Fabry-Perot imaging, polarimetric observations, and high-speed observations. Due to the large user community, resources
available, and unique observational modes of SALT, the development of reduction and analysis software is key to
maximizing the scientific return of the telescope. PySALT is developed in the Python/PyRAF environment and takes
advantage of a large library of open-source astronomical software. The goals in the development of PySALT are: (1)
Provide science quality reductions for the major operational modes of SALT, (2) Create analysis tools for the unique
modes of SALT, and (3) Create a framework for the archiving and distribution of SALT data. The data reduction software
currently provides support for the reduction and analysis of regular imaging, high-speed imaging, and long slit
spectroscopy with planned support for multi-object spectroscopy, high-speed spectroscopy, Fabry-Perot imaging, and
polarimetric data sets. We will describe the development and current status of PySALT and highlight its benefits through
early scientific results from SALT.
While time resolved astronomical observations are not new, the extension of such studies to sub-second time resolution
is and has resulted in the opening of a new observational frontier, High Time Resolution Astronomy (HTRA). HTRA
studies are well suited to objects like compact binary stars (CVs and X-ray binaries) and pulsars, while asteroseismology
of pulsating stars, occultations, transits and the study of transients, will all benefit from such HTRA studies.
HTRA has been a SALT science driver from the outset and the first-light instruments, namely the UV-VIS imager,
SALTICAM, and the multi-purpose Robert Stobie Spectrograph (RSS), both have high time resolution modes. These are
described, together with some observational examples. We also discuss the commissioning observations with the photon
counting Berkeley Visible Image Tube camera (BVIT) on SALT. Finally we describe the software tools, developed in
Python, to reduce SALT time resolved observations.
The large (~10 m) aperture of the Southern African Large Telescope (SALT) coupled with the unique capabilities
of the Robert Stobie Spectrograph (RSS), promises unparalleled prospects for polarimetric observations on an
8 - 10 m class telescope. RSS is a highly versatile first-generation instrument of the SALT. Results from
some of the first commissioning observations with the RSS are presented. A method for reducing SALT RSS
spectropolarimetry data is proposed and verified on observations of unpolarised and polarised standard stars. The
results provide estimates of telescope and instrumental polarisation as well as a calibration of the instrument's
polarimetric position angle offset.
We report on the completion of a new 2 channel, HIgh speed Photo-POlarimeter (HIPPO) for the 1.9m optical telescope of the South African Astronomical Observatory. The instrument makes use of rapidly counter-rotating (10Hz), super-achromatic half- and quarter-waveplates, a fixed Glan-Thompson beamsplitter and two photo-multiplier tubes that record the modulated O and E beams. Each modulated beam permits an independent measurement of the polarisation and therefore simultaneous 2 filter observations. All Stokes parameters are recorded every 0.1sec and photometry every 1 millisecond. Post-binning of data is possible in order to improve the signal. This is ideal for measuring e.g. the rapid variability of the optical polarisation from magnetic Cataclysmic Variable stars. First light was obtained in February 2008.
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