Astronomy is an accessible and visual science,making it ideal for educational purposes. Overthe last few years the NASA1, ESA2, HubbleSpace Telescope and the ESO3 telescopes at theLa Silla and Paranal Observatories in Chile havepresented ever deeper and more spectacularviews of the Universe. However, Hubble and theESO telescopes have not just provided stunningnew images, they are also invaluable tools forastronomers. The telescopes have excellent spa-tial/angular resolution (image sharpness) andallow astronomers to peer further out into theUniverse than ever before and answer long-standing unsolved questions.

The analysis of such observations, while oftenhighly sophisticated in detail, is at times suffi-ciently simple in principle to give secondary-level students the opportunity to repeat it forthemselves.

This series of exercises has been produced bythe European partner in the Hubble project,ESA, which has access to 15% of the observingtime with Hubble, together with ESO.

The object of the series is to present varioussmall projects that will pass on some of the ex-citement and satisfaction in scientific discoveryto the students. Using elementary geometricaland physical considerations, the students will beable to derive answers that are comparable withthe results of much more sophisticated analysesdescribed in the scientific literature.

Here we give an overview of the motivation andideas behind Hubble and the ESO facilities, to-gether with a short description of the tele-scopes, their instruments and their mode of op-eration, in sufficient detail to clarify the typesof observations presented in the exercises.

The language for the ESA/ESO Astronomy Exer-cise Series is English. There are several reasonsfor this choice – it is the language used mostoften among scientists. Good knowledge andpractical experience in the use of this languageis a valuable asset for all students, particularlyfor somewhat technical texts like these.

In modern education it has been recognisedthat it is important to cross barriers betweendifferent subjects and to link them by using in-ter-disciplinary activities that develop andstrengthen several different types of skills.Thus, we recommend that the English text ofthese exercises may also serve as exercise in thepractical use of English. We have provided ver-sions in some of the other ESA/ESO memberstate languages (see the WWW links on http://www.astroex.org). However, if any effort is madeto translate the exercises into other languageswe would be glad to learn about this (see thelast page of the exercises for contact persons).

All the exercises are constructed with a back-ground text followed by a series of questions,measurements and calculations. The exercisescan be used either as texts in traditional class-room format or, as the exercises are quite self-explanatory, be given to smaller groups as apart of ‘project work’.

The exercises are intended to be independent ofeach other and a selection may be chosen to fitthe time available. However, we recommendthat the relevant parts of the Toolkits areworked through with the students prior to work-ing on the exercises, unless the content is al-ready familiar to them.

The ESA/ESO Astronomy Exercise Series

1National Aeronautics and Space Administration2European Space Agency3European Southern Observatory

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The Hubble Space Telescope

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Figure 1: Absorption of radiation by the Earth’s atmosphereAstronomical objects emit light over a wide range of wavelengths, but only certain wavelengths can penetrate the Earth’satmosphere. The rest are absorbed and scattered by the atmosphere. The diagram shows the transparency of the atmosphere as afunction of wavelength. It can be seen that ultraviolet light is almost totally absorbed or scattered and that a large fraction ofinfrared radiation is also absorbed.

The Hubble Space Telescope was deployed fromthe payload bay of the Space Shuttle ‘Discovery’on April 26th 1990, sixty-seven years after thepotential advantages of carrying out astronomi-cal observations in space, away from the Earth’satmosphere, were pointed out by the Germanrocket pioneer H. Oberth. The first serious pro-posals for a large astronomical space telescopewere received by NASA in the early sixties. Aftera series of feasibility studies, a joint NASA/ESAprogramme was finally approved and initiated in1977. With respect to resolution Hubble outper-forms all Earth-bound telescopes by a comforta-ble margin, even though, with a primary mirrordiameter of only 2.4 m, it is not a large tele-scope.

Images from telescopes on Earth are all affectedby distortions introduced as light passesthrough the turbulent layers of the Earth’s at-mosphere. Irrespective of the size of the tele-scope, this unavoidable ‘blurring’ effectivelylimits the resolution that typically can be ob-tained in ground-based astronomical images toabout one half second of arc (1 arcsecond =1/3600 degree). However, in space, light propa-gates freely (stars do not twinkle) and the lim-iting performance of any telescope is deter-mined solely by the inherent quality of the op-tics and the accuracy with which it can be keptpointed at the target during an exposure. Thus,the images taken by Hubble show five times thedetail of similar images taken from the ground.The resolution seen in images taken from theground is roughly equivalent to reading the

headlines of a newspaper from a distance of onekilometre, but with Hubble it is possible to readthe fine print as well!

It is primarily this dramatic, fivefold improve-ment in image quality that makes Hubble sospecial. Hubble not only enables astronomers tostudy familiar astronomical objects at a muchhigher level of discernible detail, but also todetect and study hitherto unknown objects se-veral times fainter than those observable fromthe ground. In this way Hubble has expandedthe volume of space that has been observed as-tronomically.

Telescopes operating in space are also able tocollect light radiated by astronomical objectsover a much broader range of the electromag-netic spectrum than ground-based telescopes,which are restricted to wavelengths not ob-scured by the atmosphere (see Fig. 1).

This means that Hubble is not only capable ofviewing celestial objects in visible light, butalso in the ultraviolet and infrared. The ultravio-let spectral region is of particular importance toastronomers since it contains most of the so-called ‘atomic transitions’ of the common ele-ments. All chemical elements have their owncharacteristic signature in terms of being ableto absorb and emit light of particular wave-lengths and it is by identifying these signaturesin the spectra of celestial objects that theirchemical composition, temperature and physicalproperties can be determined.

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The ESO Very Large Telescope (VLT) is theworld’s largest optical-infrared telescope. Initia-tives aiming at the realisation of a large Euro-pean telescope started already in the late1970s. The basic design of the VLT was tho-roughly discussed among European astronomersin the early 1980s. Based on a detailed conceptand an associated financial plan for the con-struction and the subsequent operational phase,the ESO Council gave green light to the VLTproject in December 1987.

ESO, an intergovernmental research organisa-tion, was founded in 1962 by Belgium, France,Germany, the Netherlands and Sweden, “desirousof jointly creating an observatory equipped withpowerful instruments in the Southern Hemisphere

and accordingly promoting and organising co-op-eration in astronomical research”. Since thenDenmark, Italy, Portugal and Switzerland havejoined. The UK will join in 2002. More recentlyother countries have also expressed interest injoining ESO.

ESO operates two state-of-the-art observatories,Paranal and La Silla. Cerro Paranal, a 2635 mhigh mountain (24 deg 37 min S, 70 deg 24 minW), is located in the northern part of Chile, 12km from the Pacific coast, 130 km south of An-tofagasta, 1200 km north of Santiago de Chile,and 600 km north of La Silla. This site is loca-ted in one of the driest areas of the world, theAtacama Desert. As bad weather is the majorenemy of Astronomers, ESO performed extensiveclimatic studies before choosing Cerro Paranal asthe VLT site. Here there are up to 350 clearnights per year.

The VLT consists of four 8.2 m Unit Telescopes(UT); this measure indicates the diameter of thefour main mirrors. The secondary and tertiarymirrors are much smaller. As light passesthrough the Earth’s atmosphere, images are dis-torted: this is why the stars twinkle. The Adap-tive Optics System has been developed to cor-rect this unwanted effect, so that the imagesrecorded by the telescope become as sharp as ifthe VLT were in space.

The VLT is equipped with many different high-tech astronomical instruments. All four 8.2-mtelescopes were in operation by the end of2000. Many exciting scientific results have al-ready been obtained.

Three 1.8 m Auxiliary Telescopes (AT) are beingbuilt. It is possible to use each UT on its ownor, when the AT’s are finished, to combine allthe telescopes into the VLT Interferometer(VLTI). The VLTI will have the same imagesharpness as a telescope 200 metres in diame-ter. The first observations with the VLTI tookplace in 2001.

Figure 2: Map of ChileThe locations of ESO’s two observatories in Chile, LaSilla and Paranal, are indicated.

The ESO Very Large Telescope

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ANTU and FORS

Construction work at Cerro Paranal began in1991 and six years later, in 1997, the first ofthe four mirrors arrived at its final destination.After the installation, first light with ANTU(UT1) took place as scheduled, on May 25–26,1998. ANTU means “The Sun” in the MapucheLanguage. The other three giant telescopes had“first light” in March 1999, January 2000 andSetember 2000, respectively.

The VLT UTs have so-called alt-azimuth mounts.In such a mount the telescope tube movesaround a horizontal axis (the elevation axis).The two bearings, which support the tube, aremounted on a fork rotating around a verticalaxis (the azimuth axis) thus enabling it to pointover the entire sky.

On September 15, 1998 FORS1 (FOcal Reducerand Spectrograph), mounted on ANTU, saw firstlight and immediately began to obtain some ex-cellent astronomical images. FORS1 and the

other VLT instruments have opened a wealth ofnew opportunities for European Astronomy.

FORS1, with its twin (FORS2), is the product ofone of the most thorough and advanced techno-logical studies ever made of a ground-basedastronomical instrument. The FORS instrumentsare “multi-mode instruments” that may be usedin several different observation modes. Forexample, it is possible to take images with twodifferent image scales (magnifications) andspectra at different resolutions may be obtainedof individual or multiple objects. Thus, FORS canfirst detect the images of distant galaxies andimmediately make recordings of their spectra sothat the type and distances of the stars in thegalaxies can be found.

More information about the ESO VLT can befound at: http://www.eso.org.

Figure 3: A schematic view of the VLT Interferometer

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