The SALT Annual Report 2024 (also available as a high resolution pdf) includes science highlights, an introduction to the SALT partners, operations news and feedback on outreach & education.

The SALT Annual Report 2023 (also available as a high resolution pdf) includes science highlights, an introduction to the SALT partners, operations news and feedback on outreach & education.

The SALT Annual Report 2022 (also available as a high resolution pdf) includes science highlights, an introduction to the SALT partners, operations news and feedback on outreach & education.

The SALT Annual Report 2021 (also available as a high resolution pdf) includes science highlights, an introduction to the SALT partners, operations news and feedback on outreach & education.

The SALT Annual Report 2020 (also available as a high resolution pdf) includes science highlights, an introduction to the SALT partners, operations news and feedback on outreach & education.

At their November 2019 meeting in India, the SALT Board commissioned a study to explore the feasibility of pursuing the development of the SALT mini-tracker concept. The study report, along with the science case produced by Retha Pretorius and the initial 2018 SPIE paper are all included in the document: Mini-tracker Feasibility Study Report 2020.

The brochure outlines SALT’s strategic objectives ranging from enabling world-class science to driving human capital development.

What does multi-wavelength observations mean?

The illustration above shows our own galaxy in a variety of wavelengths. See the descriptions in the top right of each individual image to know in which wavelength range each was observed. It starts with a low energy (long wavelength) range and increase in energy as you move downwards through the images.

The optical picture should look more familiar, since that is how our eyes could see it in good conditions (if we were able to expose for a few seconds like cameras can). Our Milky Way has a very distinctive bulge in the near infrared because light travelling at those wavelengths can easily pass through the dense dust clouds that obscure it in other wavelengths. The spiral arms of our galaxy (where young stars are born) are beautifully visible in all the pictures, although they also suffer obscuration by their own dust in the optical wavelengths. And then there are bright features in some images that are not so prominent in others. See the bright spot that shows up in the x-ray and gamma ray wavelengths? That is a high-energy source. Accreting black holes are such sources.

Black holes were discovered when astronomers started studying the night sky using x-ray detectors. The first such detectors were launched high into the atmosphere in rockets or balloons, but later satellites were launched to study the universe in that particular wavelength range. Our atmosphere blocks x-rays (a good thing since prolonged exposure is harmful to humans) so to study them you have to get your detector above the atmosphere. There are several x-ray satellites orbiting the earth.

Accreting Black Holes

There are stellar mass black holes that are in binary pairs with companion stars. See the illustration for an artist’s impression of such a system where the yellow/orange star is the companion. Material flows from it’s surface toward the black hole (which is infinitely small and therefore invisible) and because they are orbiting around each other, the material swirls like water draining from a bathtub as it plunges down the gravitational well that the black hole creates in space. The in-spiralling material releases photons with increasing energy (coloured red, then blue and eventually white) as it moves ever closer to the black hole. We call this an accretion disc because material is being accreted into the black hole, thereby growing it in mass while the companion loses mass. When a large amount of mass is transferred in this way the system can become a microquasar, launching extremely energetic jets perpendicular to the accretion disc and increasing it’s brightness dramatically. These systems are also known as Black Hole binary systems.

One such system (BW Cir) was observed in optical and x-ray wavelengths. The x-ray data were obtained from the SWIFT, INTEGRAL and NuSTAR satellites. Simultaneous optical observations were done with SALT using the Berkely Visible Imaging Tube (BVIT), a visiting instrument currently occupying the auxiliary port on the telescope. The detector is capable of tagging every photon received to nanosecond accuracy, but requires a large collection area (i.e. a 10 meter class telescope like SALT) due to the low quantum efficiency of micro-channel plates (MCPs) in comparison to the more regularly used Charge Coupled Devices (CCDs). This allows us to bin photons together in 1 millisecond intervals, which is among the highest time-resolution achievable in the world.

After combining all the data and analysing it for correlations between the variations in brightness in the different wavelengths, PhD student Mayukh Pahari from the Inter-University Centre for Astronomy & Astrophysics and the University of Southampton (supervised by Poshak Gandhi) and various co-authors (including myself), published the results in:
http://adsabs.harvard.edu/abs/2017MNRAS.469..193P

A velocity map of the galaxy NGC 1325.

Spectroscopy is the most important tool for an astronomer.  It unlocks the hidden characteristics of objects too remote to be studied directly – things like the chemical composition of stars, the temperature, pressure and motion of interstellar gas clouds, the structure and mass of galaxies, and much more.  A spectrum is the measurement of the brightness of the light coming to us from an astronomical source, at each individual colour, or wavelength.  Think of a totally unromantic analysis of a rainbow, and you get the picture.  Astronomers use the term “resolution” to describe how finely the light is divided into its colours; a high-resolution spectrum can measure hundreds of thousands of colours!

A spectrograph is the instrument that is attached to a telescope to measure spectra.  The most common form of spectrograph accepts light from only a single point or along a single line from the image that a telescope produces.  This is fine if you are interested in an object like a star, which is just a single point of light as seen by even the most powerful telescope.  But what if an astronomer wants to measure the spectrum everywhere in an extended object, like a glowing cloud of gas or an entire galaxy?  This is the realm of “imaging spectroscopy”, and it requires a specialized type of spectrograph.  SALT, the largest optical telescope in the Southern Hemisphere, is rare among the giant telescopes of the world in that its Robert Stobie Spectrograph has an imaging spectroscopy mode.  This is accomplished with a device called a Fabry-Pérot interferometer.  It can efficiently produce spectra covering SALT’s entire field of view.

The galaxy NGC 908. SALT’s Fabry-Pérot system has simultaneously measured the spectrum of all the stars and gas in this galaxy.

A Fabry-Pérot interferometer is essentially a tunable filter that produces a narrow-band image at one or a series of wavelengths that you specify.  SALT has three etalons (the optical part of the Fabry-Pérot) that can give spectral resolutions of 300, 600, 1500, or 9000 over the wavelength range of 430 to 900 nm covering the entire 8 arc-minute diameter field of view of the RSS.  A typical observing program takes a series of images covering a 2 to 4 nm spectral range of interest for the particular target.  Figure 1 below shows an example of FP data of the Ha emission of the galaxy NGC 1325, taken on 02 November 2011 in the medium resolution (R 1500) mode.  The upper left panel is taken at central wavelength 659.7 nm, the upper right at 660.1nm, and the lower left in nearby continuum at 663.1 nm; exposures are 90 seconds each.  You can see the effect of the galaxy’s rotation, Doppler shifting the emission of the HII regions.  The velocity map of the galaxy, generated by fitting all 23 images in the series, is shown in the lower right panel.  A sample spectrum of one of the HII regions is shown in Figure 2, with fits to the Ha and [NII] lines.  There are over 10,000 such spectra measured in the data cube, extending to a radial distance of more than 12 kpc (40,000 light-years) from the center of the galaxy.

Figure 1. Fabry-Pérot observations of the galaxy NGC 1325.

In addition to the emission from the galaxy, you can see fuzzy ring-shaped emission in the images in Figure 1.  These are night sky spectral features, arising from OH radicals in the Earth’s atmosphere.  Since their wavelengths are accurately known, they can be used to provide a precise wavelength calibration for the data.

The steps in reducing FP data are typically: a) clean the cosmic rays, b) correct for an internal ghost reflection, c) flatten the data using twilight sky flats, d) measure the brightness of the foreground stars and use this to correct for the varying collecting area of the telescope, e) measure the night sky rings, then subtract them from the data, and finally f) fit the spectra point by point, to extract maps of line strength, velocity, and line width.  Then of course, use these maps to pursue your scientific goals.  There are software programs available in a not-quite-ready-for-prime-time state to accomplish all these reduction tasks.

Figure 2. Spectrum of one HII region of NGC 1325

As of September 2017, the RSS FP system is working in its TF, LR, and HR modes (tunable filter, low resolution, and high resolution).  The MR (medium resolution) etalon has suffered environmental damage over the years and is currently being refurbished by the manufacturer in the UK – we hope to have it back on-line in 2018.  The two higher resolution modes, MR and HR, use two etalons in series to isolate individual spectral features properly.  Because of mechanical instabilities that produce a particularly nasty reflection between the two etalons, we have had to resort to using a circular polarizer to eliminate the problem.  This unfortunately reduces the throughput of the HR system by a factor of three, requiring longer exposures.  We have a fix planned that should restore full sensitivity that should be in place in 2018.  Fortunately, the TF and LR modes are unaffected by this problem and have their full throughput.

There is full documentation of the RSS FP system on the SALT web pages, and the PIPT supports the preparation of FP observing proposals.  Like most tools, the FP is really good for some tasks, but the wrong solution for others (cf. hammers and screwdrivers).  The FP aficionados on the SALT staff are happy to consult and offer advice about the use of the system – contact Ted Williams, Encarni Romero Colmenero, or Blaise Tapsoba.

Image: an RBG image of NGC 1365 created by Dr Marissa Kotze, taken using SALT.

Images with telescopes are taken in various filters and are then combined to create colour composites like the one above.

The Great Barred Spiral Galaxy (NGC 1365) lies in the constellation of Fornax (the Furnace) and is one of the largest known galaxies, located some 60 million light-years away. It extends roughly 200 000 light-years across and the galaxy nucleus contains a super-massive black hole of approximately 2 million solar masses.

This RGB image is a combination of 10 second exposures taken in red, green and blue filters on the Southern African Large Telescope (SALT) during November 2016. No image processing was done beyond the combination and scaling of the different colours (see raw images below). Such an image will show far more detail if longer exposures are made, but even in these extremely short exposures, the galactic nucleus is clearly visible together with dust lanes that extend along the bar to the spiral arms.