Astronomical instrumentation

A vision of space

INPE researchers build a telescope to capture x-rays and cosmic gamma rays

MIGUEL BOYAYANWill everything work? This question has been haunting the researcher Thyrso Villela Neto for at least two years, when the team that he coordinates finished building the telescope Masco. The name is in reference to the technique of coded masks, used to obtain images of x-rays or gamma radiation. After eight years of intense work, which involved a series of drawbacks and learning situations, the Masco is stored in the Integration and Testing Laboratory (LIT) of the National Institute for Space Research (INPE), in São José dos Campos, awaiting the ideal climatic conditions to send it into the stratosphere to an altitude of forty kilometers. In order to arrive and to complete its mission – to observe the center of the galaxy -, it will journey there, always fixed to a balloon that will be 150 meters in diameter, 20 meters longer than the maximum length of an official football field.

The satisfaction of the INPE group on viewing the completed instrument is enormous. But perhaps there will be even greater anxiety and doubts about what might happen with the telescope during the next stage of the project. There will be two crucial moments: the launch and the return. The risks involved in each one of them are not to be undervalued. And the uncertainties transform themselves into agony. Will it go up? Were the temperature and pressure tests sufficient? Will the telescope be able to turn itself to the celestial objects and register their images with precision? And on the return, how will it go?

If everything goes well on the launch, the Brazilian telescope, some 7 meters in height and weighing 2 tons, will take two hours to reach the earth’s stratosphere. After that, it will fly for close to fifteen hours tied to the enormous balloon, purchased in the United States, which will be inflated with hydrogen. Up top, its velocity could reach 100 km/h. On its return to earth, parachutes will be used. In order to determine a safe landing place, the team are going to follow the balloon by plane, at a much lower altitude.

To understand the universe
When it reachesits desired altitude, the Masco will be programmed to point its camera to the center of our galaxy, the Milky way, capturingx-rays and gamma radiation – electromagnetic radiation with smaller wave lengths when compared with visible radiation, emitted by various celestial objects. “We would like to contribute to an improved understanding of the universe, because this is the general objective of astrophysics”, says Villela.

The obtaining of pictures of the cosmos through x-rays and gamma radiation is centered on the coded mask, a large disk with cells that open and close, placed on the extremity of the telescope. Made of lead, it weighs one hundred kilograms and is sustained by a structure of polystyrene and carbon fibers. The holes existing in it have been made based on mathematical calculus in order to avoid the formation of nebulous images. The x-rays and gamma rays hit at an angle on the mark so that afterwards they will be transformed into images using a sodium iodide detector doped (enriched) with thallium NaI (TI). Hence it will be possible to discover from where the information came and what was the object that gave it off.

Energy emission
The researcher reminds us that the Masco technology seems somewhat like that of older instruments such as the North American GRIP (Gamma Ray Imaging Payload) which, at the beginning of the 90s, made several images of the galactic center and the supernova SN1987A (possibly the final stage of a star). And it is for this reason, he assures, that the scientific potential of the Brazilian instrument is considerably high. The images produced will help towards a better understanding of how the emission of energy in the universe happens and the way in which the stars, quasars, pulsars, active nuclei of galaxies, remains of supernovas, and even possibly black holes, function.

The need to send up a telescope in a balloon is due to the fact that observation of x-rays and gamma radiation coming from astrophysics sources cannot be carried out on the ground. The atmosphere absorbs practically all of the radiation coming from space in these forms of energy. Hence the challenge of putting together an instrument that could, fastened to a balloon and automatically pointed to a determined point in space, go to great altitudes in order to capture more precisely information about cosmic sources.

The launch of the Masco, which should occur in October, has not got a second launch date. Probably it will occur in February. The problem lies with the climatic changes brought about by the phenomenon El Niño, with its turnabouts in the weather, which brings uncertainty to the group. He one certain thing is that the operation will use as its base the landing field at Nova Ponte, a town located the state of Minas Gerais, some 186 kilometers from the city of Belo Horizonte, and where the air traffic is of low intensity.

In order to guarantee the precision of the images captured by the telescope, which will be taken in by a balloon in constant motion, it was necessary to develop a system for Masco to be always directing itself towards certain celestial objects and registering them with absolute faithfulness, in spite of the swaying brought about by the wind. In order to confront this challenge, the researchers developed the Directional and Altitude Reference System (SARA) with totally Brazilian technology.

This system involves various resources, among them two solar detectors. The first, an analogue detector is also known as a tracker and is formed by a mobileoptical head that acts as atype of sunflower, always looking towards the sun and indicating its position. The other, a digital device, receives solar rays and manages to inform the on-board computers exactly where the star is located. A digital compass measures the displacement of the telescope in relation to the earth’s magnetic field.

The Masco also has a stable sensor that, starting from an image containing at least three stars, is capable of comparing this information with the reference coordinates which are carried in the computers, determining the direction of the telescope. Three other important sensors are the acceleration meter, the gyroscope and the Global Positioning System (GPS), a positioning system that makes use of satellites. The first registers the accelerations of the equipment. The gyroscope detects the changes of the telescope’s position without external references and the GPS informs on the geographical position.

“The sensors perceive the changes of position and pass on this information to the controlling software – also developed by us -, which calculates what must be corrected”, explains Villela. Afterwards, the order of correction on the direction is relayed to the reaction disc – a mechanical device that assists the equipment to remain stable – and to the electric motors that have the mission of correcting elevation in relation to the horizon and the azimuthal angle (the angle of direction of the stars in relation to ground level) of the telescope.

Sun aligned
So that unexpected surprises do not occur, the INPE equipment carries out a rigorous and tiresome routine. Villela has filed into his computer a type of check-list, which tallies up both the possible problems that the telescope might be faced with and the safety measures adopted to avoid them.

At the altitude of the telescope’s flight path, atmospheric pressure is approximately 3 millibars, a number well below that registered at sea level of 1,013 millibars, and therefore the Masco will be in an extremely rarified atmospheric position. During the night periods of the flight there will be one side of the telescope pointed towards the sun, whilst the other will be much colder. This temperature difference could reach as high as 20° Celsius. The air, almost non-existent, makes the computer ventilators useless. If the equipment overheats, it might enter break down. The engine that make up part of the telescope cannot have grease. Without oxygen, it may harden and the instrument may stop working. The computer’s hard disks have to be protected and covered so that they can work under low atmospheric pressure and so that they can resist the impact of landing on Masco’s return to ground level. And these are only some of the items that make up Villela’s check list.

“After the launch, if some system goes down, there will be no return. It will be like a death sentence”, he compares. “And the cruelest part is that we have to anticipate the possible errors that might occur.” The problem, he insisted, is to lose almost what is complete control of the situation, since the telescope is programmed to have a life of its own. This is a dynamic situation very different from a bench experiment in a laboratory. If something does not go well, it is possible to interfere and correct the procedures.

Up until now the routine is as forecast. Over the last three years, all of Masco’s systems, including the solar sensors of the gyroscope, were tested, both individually and together. But not everything was plain sailing. During all of this period moments of despair occurred. In 1997, shortly after the launch of the project, the team discovered that the Brazilian company that was to provide the type of aluminum specified had stopped manufacturing. At that point it was decided to import it from the United States.

While they were awaiting a truck which would bring aluminum bars of six meters each, they were surprised with the arrival of a Fiat Uno. The parts were contained in a shoe box. The company responsible for sending the imported order made a serious mistake when writing down the measurements. The solution found was to make use of national aluminum, of another type and of lower resistance, by applying a structural reinforcement.

A better detector
During the construction of Masco other important facts occurred. A partnership with the Energy and Nuclear Research Institute (IPEN) allowed for the approval and improve of the technology for the scintillating plastic detectors (they emit light when they receive radiation) capable of locating and quantifying the radioactivity of the human body. They were used for the construction of the protective cover of the telescope. With one meter in height, this safety cover makes the instrument chamber read only the x-rays and the gamma radiation emitted by the object that it is being analyzing, avoiding photon interference (for example, the light coming from the stars) originating in other regions of space. Returning to the initial application, now it is possible to build greater detectors than those previous, capable of covering all of the human body.

Currently, the group is hoping for success with the launch and its return to earth, when the data and the images produced will be analyzed, a task that could take as long as six months. Counting the days and the hours until the launch, Villela imagines how the eve will be. With a serious countenance, he said: “I have dedicated a considerable part of my life to the project. If it does not go well… But science involves risks. If it wasn’t so… I chose this option. I have to learn to live with this”, he says, attempting to convince himself, before shortly adding: “However, if I could only go up with the balloon, then I could control the telescope…”

Construction of an Imaging Telescope for X-rays and Gamma Radiation
Completion of the Construction of the Masco Telescope
Regular line of research project assistance
Thyrso Villela Neto – INPE
R$ 141,264.41 and US$ 21,667.00