the international ultraviolet explorer · and molecular hydrogen (e.g. caruthers 1970). in addition...

THE INTERNATIONAL ULTRAVIOLET EXPLORER:

ORIGINS AND LEGACY

ALLAN J. WILLIS

Department of Physics and AstronomyUniversity College LondonGower StreetLondon WC1E 6B, [email protected]

Abstract. The International Ultraviolet Explorer (IUE) satellite waslaunched on 26 January 1978, and operated as a Guest Observer spaceobservatory for nearly 19 years. IUE was, without doubt, one of the mostsuccessful astronomical facilities ever developed. Used by thousands of as-tronomers worldwide, it yielded over 100,000 UV spectra (in the wavelengthrange 1150-3250A), covering stars of all types, the interstellar medium inour own galaxy and other local galaxies, the galactic halo, normal and ac-tive galaxies and QSO’s, X-ray binaries, novae and supernovae, and in oursolar system comets, planets and their moons. Over 3500 refereed papershave been published based on IUE results, and a similar number of papersin conference proceedings. Over 600 PhD theses have been produced basedon IUE data. All IUE data are available in archives maintained by the threeagencies involved in the mission: NASA, ESA and the UK Science ResearchCouncil. In this paper I will discuss the origins of the IUE mission, the spe-cial design and operation which led to its spectacular success and the legacyit left for UV astronomy.

1. Introduction

The wavelength range from 3250-912A is referred to in astrophysics as thevacuum ultraviolet (UV) region. The upper bound reflects the approximatecut-off in the transmission of the Earth’s atmosphere due to absorption byOzone, whilst the lower bound corresponds to the ionisation energy of theneutral Hydrogen atom – below which interstellar H largely absorbs back-ground radiation from distant sources. With suitable reflecting coatings

Organizations, People and Strategies in Astronomy 2 (OPSA 2), 395-416 Ed. A. Heck, © 2013 Venngeist.

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Figure 1. Sir Robert Wilson (1927-2002). Wilson led the initial LAS and UVAS proposalsto ESRO for an Ultraviolet Space Observatory, which led to the development with NASAof the IUE mission. (Courtesy D. Rooks/UCL)

the optical elements for UV instrumentation are similar to those whichare generally used at visible wavelengths: i.e. normal-incidence telescopes,photometers and spectrographs, with either photographic or electronic de-tectors.

The UV region is one of the richest in spectral lines in the electromag-netic spectrum, encompassing a very large number of resonance lines ofmany common elements, covering many of their ionisation stages, e.g. HI,CI, CII, CIII, CIV, NI, NII, NIII, NV, OI, OII, OIII, OV, OVI, PIV, PV,SiI, SiII, SiIII, SiIV, SI, SII, SIII, SIV, FeI, FeII, MgI, MgII. These linesprovide crucial diagnostics for low-excitation plasmas covering a range oftemperatures from a few degrees up to about 1 million degrees, found inthe interstellar medium, stellar atmospheres, galaxies and AGN and plan-ets and comets. For these reasons, the UV was a natural spectral range fordevelopment in the early phases of space science.

For nearly 20 years one such UV mission formed a cornerstone of modernastrophysics – the International Ultraviolet Explorer (IUE) satellite. Inthis paper I will discuss the historical origin of IUE, its unique designand operation which led to its stupendous success and the legacy it has

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THE INTERNATIONAL ULTRAVIOLET EXPLORER 397

left. I was fortunate to become involved with the IUE mission some yearsbefore launch through my association at UCL with Robert Wilson – thenProject Director of the UK involvement in the satellite (Fig. 1). Although,of course, IUE owed its success to many hundreds of scientist and engineers,there can be little doubt the Wilson played an absolutely seminal role ingetting the mission developed – see below – and is generally regarded asthe “father of IUE”. My own involvement included pre-launch preparationsfor the commissioning phase, the UK High Priority Observing programmeconducted a few months after launch, as a Guest Observer (GO) with IUEover many years, as a member of the UK TAC and as Chairman of theEuropean IUE Time Allocation Committee.

2. Ultraviolet Astronomy in the pre-IUE Years

The development of UV astronomy during the 1960’s followed a similarpattern in both the USA and (to a smaller extent) in Europe, with groupsat Universities and Government establishments pioneering, ever-more so-phisticated experiments using rocket-borne instrumentation, followed bylonger-lived and more massive satellite payloads. In the US the Aerobeerockets could deliver payloads of up to 100Kg to altitudes of around 180km.Groups at the Goddard Space Flight Center (GSFC), Universities of Wis-consin, Princeton and Johns Hopkins, together with the Naval Researchlaboratory, produced a range of spectrometers to yield the first detailedUV spectra of early-type OB stars at sufficient spectral resolution to detectand measure individual spectral lines, particularly with the developmentof a 3-axis stabilisation system for the Aerobee rockets. These programmesled, inter alia, to the first measurements of UV P-Cygni profiles of ZetaPuppis and other OB stars, yielding estimates of wind velocities and mass-loss rates (Morton 1967), and analyses of interstellar lines in atomic speciesand molecular Hydrogen (e.g. Caruthers 1970). In addition the first mea-surements of the UV interstellar extinction curve, including the discoveryof the broad 2200A band, were secured.

In Europe, and particularly in the UK, university-based teams were alsodeveloping UV spectrometers. The UK group at the Culham Laboratoryled by Robert Wilson, used Skylark rockets to launch 3-axis stabilised pay-loads from Woomera, South Australia, to secure high-resolution spectra ofthe Sun, to identify and study lines formed in the solar chromosphere tran-sition region and corona at both UV and soft-X-ray wavelengths (Burtonet al. 1967). These programmes included an echelle-spectrum of the solarlimb during a total eclipse in 1970 (Gabriel et al. 1971) and high-resolutionspectra of Zeta Puppis and the Wolf-Rayet star Gamma Velorum coveringthe wavelength range 900-3200A at 0.3A resolution. Since the duration of a

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typical rocket flight was limited to about 5 minutes or less, each flight couldonly yield data on one or at best 2-3 objects. Over time successive flightsbuilt up data for many of the bright OB stars, some late-type stars, andplanets. They had demonstrated the importance of the UV spectral regionin furthering astrophysics in the solar, stellar and interstellar fields, andpointed to the potential for further advances if fainter and more diverse ob-jects could be observed with larger instruments and longer exposure times,which required the use of stabilised satellite platforms.

This recognition led NASA to embark on a long-range programme of de-velopment of ultraviolet satellites, which was to culminate with the HubbleSpace Telescope. Starting in the mid-1960’s this programme centred aroundthe NASA Orbiting Astronomical Observatories (OAO-series). OAO-1 waslaunched in 1966, but due to a power failure was terminated after only3 days. Its spare parts were used to develop OAO-2, launched in 1968, witha spectrometer developed by the Goddard Space Flight Center yieldingmedium-resolution spectrophotometry, at about 10A resolution, coveringthe wavelength range 1100-3200A. This instrument was highly successful,obtaining data on hundreds of early-type stars and measurements of UVinterstellar extinction curves, as well as photometry of some galaxies (Codeet al. 1970; Davies et al. 1972).

OAO-3 (re-named Copernicus) was launched in 1972, with an 80cmtelescope feeding a high-resolution UV spectrograph provided by Prince-ton University (Rogerson et al. 1973). Scanning photomultiplier detectorswere used to record spectra at either 0.05A or 0.2A resolution, yielding linestrength and profile measurements of the plethora of atomic and molecularspecies in the wavelength range of 900-3200A. Rather long exposure timeswere needed to record the spectra, such that the higher resolution modewas restricted to observations of OB stars with V<∼2, and about 6th magfor the lower resolution mode. The mission was a huge success, providingdata on stellar mass-loss rates in the upper HR diagram, the effective tem-perature scale for OB stars, the discovery of OVI in the galactic halo, andthe distribution of molecular hydrogen and chemical composition of theatomic gas in the ISM. Copernicus was also what one might term the first“long-lived” space observatory, continuing operations until about 1980.

In Europe the fledgling European Space Research Organisation (ESRO)launched its first 3-axis stabilised satellite – TD-1 – in 1972, which car-ried two UV instruments. The Dutch/Utrecht S59 instrument compriseda low-resolution spectrometer covering 3 bands in the 2160-2870A range(Hoekstra et al. 1973). The second instrument – S2/68 – was developed bya joint UK/Belgium team, from Culham-UCL/ROE/Liege, and compriseda 27cm telescope feeding a scanning spectrophotometer. This covered thewavelength range 1350-2740A at about 35A spectral resolution, and per-

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formed an all-sky UV survey yielding data on stars of all types down toabout 9th magnitude (Boksenberg et al. 1973). The data from S2/68 wereput in the public domain through its Bright Star and Faint Star Catalogues(Thompson et al. 1978), and the high photometric integrity of the S2/68calibraton formed the basis of the calibrations for succeeding UV missions,including IUE and the HST. The ANS satellite (Astronomical NetherlandsSatellite) launched in 1974, provided broad-band UV photometry in 5 bandscovering 1540-3340A of a range of sources including white dwarfs, planetarynebulae, stars, clusters and galaxies.

3. The Origins of IUE

In October 1964 the Director General of ESRO (P. Auger) issued a letter tomember states inviting proposals for a Large Astronomical Satellite (LAS)against a deadline of December 1964. In response, three proposals weresubmitted from groups in Germany/Holland, France/Belgium/Switzerlandand the UK, and each was charged with developing detailed designs for afurther deadline of January 1966. After assessment the UK proposal wasaccepted for further development, and this was undertaken by a team fromCulham, UCL and Aldermaston, with Robert Wilson as project leader.The final report for the LAS was submitted to ESRO in September 1967(Wilson 1967).

The design centred around an 80cm Cassegrain telescope feeding aPaschen-Runge spectrometer operating in the wavelength range of 900-3200A, providing spectral resolution of 0.1-0.2A, allowing observations ofstars down to a limiting magnitude of V<9. A subsidiary X-ray telescopearray was included at the prime focus, operating in the spectral range2-100A. A key element of the LAS strategy in the UK proposal was itsrecognition that “the wide range of astrophysical studies made possible bythe main (UV) instrument ... emphasises the suitability of this payload asan observatory enabling wide participation by European astronomers withvaried interests”. In other words, at the outset, LAS was to be a true guestobserver facility. The total cost of the LAS was estimated at 38.3M SwissFrancs (probably about 250M US$ in today’s prices). This cost was greaterthan ESRO had expected and it decided to abandon the LAS project.

Undaunted, Wilson and his team embarked on a major re-design of thesystem. As noted above, his group at Culham had used echelle spectro-graphs in their rocket programme and this experience was used to pro-duce a new proposal called the Ultraviolet Astronomical Satellite (UVAS).The UVAS plan was based on a 45cm Cassegrain telescope feeding anechelle spectrometer giving a resolving power of about 104 over the wave-length range 1150-3200A, using an SEC Vidicon as the detector. The use of

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an echelle grating provided the high spectral resolution AFTER the lightpassed through the spectrograph entrance aperture (which could now beseveral arcsec) allowing about a factor of ten reduction in the pointing, ther-mal a nd mechanical tolerances compared to the original LAS requirements.This greatly reduced the cost of the system. UVAS was to be launched bya US Thor-Delta rocket into a low-earth orbit, with an interactive oper-ations system with the satellite and instrument responding to commandwhen visible from a ground-station.

The final UVAS proposal was submitted to ESRO in November 1968(Wilson 1968), but again was not accepted for further development. Afterchecking the propriety rights for the UVAS study with the UK authorities,Wilson decided to send the UVAS design report to NASA for considerationby copying it to Leo Goldberg (then Chairman of the NASA AstronomyMissions Board). After positive assessments by Goldberg and his Board,NASA HQ and GSFC, Wilson was invited to Goddard to discuss the pos-sibility of a joint NASA-UK project at meetings held in May 1969 (seeBoggess & Wilson 1987). The Goddard team (led by L. Meredith) recog-nised that with the planned enhancements to the Delta rocket, it wouldbe possible to launch the mission into a geosynchronous orbit and thusprovide continual ground-station contact. This in turn would allow a real-time interactive control and operation with the guest observer identifyingsources in the telescope field of view (using ground-based finding charts)going much fainter than could otherwise be accomplished. In addition a“low-resolution” spectral mode was included by the option, on command,of flipping a plane mirror in front of the echelle grating, producing a low-dispersion spectrum from the cross disperser (used to separate the indi-vidual echelle orders in the high-dispersion mode). The sensitivity of thesystem would thus be around V∼10 at high resolution and about V∼15 atlow resolution. The true guest observer philosophy of UVAS was maintainedand the observatory nature of what was to become IUE was established.

Subsequent discussions between NASA, the UK authorities and ESROtook place and agreements to proceed with a 3-Agency project were in placein 1970 and finalised in 1971 after Phase A studies for the SAS-D project(as it was now called in the US, Krueger et al. 1971) had been carriedout. NASA would provide the spacecraft, telescope and ground-station atGSFC; the UK would provide the onboard Vidicon detectors and software;ESRO would provide a European Ground Station (subsequently located atVillafranca del Castillo, Spain) and solar panels. The division of observingtime would be 2/3 US, and 1/6th each to UK and European astronomers.

The memoranda of understanding between the three agencies, agreed in1972, further enshrined the international use of what had now formally be-come the International Ultraviolet Explorer (IUE) mission formally stating:

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Figure 2. An impression of the IUE satellite in its geosynchronous orbit and thetwo ground stations at GSFC, Maryland USA, and VILSPA in Spain. (CourtesyNASA/ESA/SERC)

(a) “SRC, ESRO and NASA plan that, to the greatest extent possible, theavailable viewing time shall be made available to guest observers from theastronomy community, regardless of their nationality and on the basis ofthe scientific merit of proposals made to the three agencies”, and (b) “useof the data will be reserved for the observer for a six month period begin-ning from receipt of data in a form suitable for analysis. Subsequent to theperiod of exclusive observer usage, the reduced data will be deposited withthe UK Radio and Space Research Station (RSRS), with the US NationalSpace Science Data Center (NSSDC), and with the Data Library of theEuropean Space Research Operations Center (ESOC)”. This latter aspectformed the basis of what was to become the IUE Data Archives, in thethree agencies, which allowed for the widest possible dissemination of IUEdata to astronomers throughout the world.

4. IUE Design, Development and Operations

The basic system parameters of IUE are summarised in Table 1. A detaileddescription of the IUE spacecraft and scientific instrument have been givenby Boggess et al. (1978) and Boggess & Wilson (1987). As noted above,much of the basic design of the UVAS instrument was incorporated intoIUE, notably the 45cm telescope, echelle spectrographs and SEC Vidiconcamera detectors.

However, there were several important changes and enhancements. Prob-ably the most important, from an operations standpoint, was the changefrom a low orbit to a geosynchronous one (Fig. 2). This had tremendousadvantages for the scientific operation of the mission. The telescope bafflingsystem allowed observations to be made in full sunlight, anywhere in the

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Figure 3. A schematic of the IUE spacecraft with the solar panels deployed, the apogeeboost motor, telescope and baffle, VHF and S-band antennas and Sun sensors. (CourtesyNASA/ESA/SERC)

sky more than 43 degrees from the Sun; with the earth only subtendingan angle of about 17 degrees, the unconstrained area of sky available atany one time was much greater than from low orbits. This allowed a ca-pability to conduct long, uninterrupted exposures, allowing observations offaint sources as well as monitoring variable sources. The IUE orbit was ar-ranged so that the satellite was in continuous real-time control from the USground station at GSFC, and for at least 10hrs per day from the Europeanground station located at Villafranca del Castillo, near Madrid in Spain(VILSPA) – see Fig. 2. Unlike any previous mission (and indeed any since)the scientific observations of IUE were carried out by the Guest Observersthemselves at one of these ground stations – as they would normally do ina ground-based observatory.

The satellite 3-axis stabilisation and pointing/slewing package com-prised 6 gyroscopes, two star-trackers (Fine Error Sensors, FES), Sun sen-sors, a set of momentum-exchange reaction wheels for attitude control,and hydrazine thrusters for momentum dumping. Power supplied by theon-board Solar panels was supplemented during eclipses by two on-boardNi-Cd batteries. Communications between the two ground stations andspacecraft used VHF transmitters/receivers and S-band antennae (used fortelemetry), with two, redundant ON-Board Computers (OBC) processingcommands and controlling the operations of the scientific payload.

Fig. 3 shows the layout of the IUE spacecraft and Fig. 4 a schematicof one of the spectrographs, FES and cameras. An important element ofthe design was to incorporate considerable redundancy in the systems.

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Thus there were two FES’s, and two cameras each for the short-wavelengthranges (Short Wavelength Prime – SWP, and Short Wavelength Redundant– SWR), and long-wavelength ranges (Long Wavelength Prime – LWP andRedundant – LWR) cameras, covering respectively the wavelength ranges1150-1950A and 1850-3250A. The short-wavelength limit of 1150A was theresult of the MgF2 reflective coatings selected for all the system opticalcomponents, to maximise throughput above that limit. Fig. 5 provides aschematic view of the IUE cameras employed, which consisted of a front-end UV-Visible converter, a Secondary Electron Conduction (SEC) imagesection and the backend SEC readout section. This provided a 2-D 768×768pixel image of the spectral data (see Figs. 6 & 7).

The echelle gratings provided a spectral resolution of about 0.1A in theSWP range and 0.2A in the LWR range, with an overall system sensitivityallowing spectra to be obtained of blue objects down to about 12th magni-tude in exposures of about 2hrs. By flipping a plane mirror in front of theechelle grating, spectra at around 6A resolution could be secured using thecross disperser alone, again covering either the short- or long-wavelengthrange. In this case high quality, low-resolution spectra could be secured inan hour exposure of blue objects down to a magnitude of about V∼16. Itquickly became apparent that the spectral resolution was similar whetherthe small (3′′) or large (10′′×20′′) aperture was used, and since the latterretained the full photometric integrity of the data it was most often used.Additionally, the signals from the FES were found to be stable and cali-brated to provide an optical photometric measurement of the target source,which was routinely supplied to the observer. This proved most useful inthe study of variable sources providing simultaneous coverage of UV andoptical changes.

At the outset, each agency (NASA, ESA and the UK) solicited observingproposals from their respective communities, which were peer reviewed byseparate Time Allocation Committees, on an annual basis. NASA operatedtwo 8-hour observing shifts per day from GSFC, and ESA/UK the third8-hour shift from VILSPA. Successful proposers would then be scheduledfor their allocated number of shifts during an optimal time of year andwould go to the relevant ground station to conduct their programme withthe assistance of resident astronomers and spacecraft operators.

The GO would arrive armed with finding charts for his/her target fields,estimated exposure times and observation strategy for the programme. Thesatellite would be commanded to slew to the first target, take an FES imageof the sky and telemeter this to the ground console for the observer toidentify the field and desired target, which would then be directed to thedesired spectrograph aperture and camera (SWP or LWR) and the exposurecommenced. At the end of the exposure the camera would be read out and

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Figure 4. A schematic of the IUE scientific instrument. The locations of the echelle andcross-disperser gratings, collimator, camera mirror and duplicate SEC Vidicon cameradetectors are shown, as are the duplicate Fine error Sensors imaging the telescope 16′

field of view. (Courtesy NASA/ESA/SERC)

spectrograph image transmitted to the ground for immediate inspectionby the GO. In this way the GO could gauge immediately whether or notthe data were of the desired quality and could either command anotherexposure, different camera or move to the next target. At the end of eachshift the acquired data would be pipelined processed and the observer wouldreceive magnetic tapes with their raw and processed data for immediateanalysis back at their home institution.

It was recognised by each of the three agencies that IUE had the poten-tial to make significant advances in a wide range of astrophysical fields, andeach TAC comprised specialist panels to consider applications in six main

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Figure 5. A schematic of one of the four SEC Vidicon Detectors used to record eachspectral image, showing the front-end UV-to-visible converter, the secondary electronconduction section and back-end readout section. The cameras proved remarkably stableshowing only a few percent sensitivity reduction after 18.5 years operation. (CourtesyNASA/ESA/SERC)

areas: Interstellar medium; Hot stars; Cool stars, Active galaxies; Normalgalaxies; and the Solar System. Each area had a preliminary total shiftallocation to ensure that the mission would yield a balanced programme –a pattern that would become the norm for subsequent space observatories.

The TACs operated independently, and often approved programmes in-cluded sources duplicated in US, UK and European allocations. At firstsight, this might have been considered inefficient, but in fact it turnedout to have profound scientific advantages, especially when it was discov-ered that sources showed significant spectral variability. In addition, thehigh orbit and real-time control of the mission meant that IUE could re-act swiftly to unexpected events, designated Targets of Opportunity andeach TAC/agency agreed that such TOO would be incorporated as overridepriorities in the observing schedules. Examples included Novae, Supernovaeand Comets, but in addition Guest Observers could bid for additional TOOtargets on scientific merit. Throughout its lifetime the oversubscription forobserving time remained high at the 4-5 level.

IUE was launched from Cape Canaveral (now Cape Kennedy) on a Delta2914 rocket on 26 January 1978 into a transfer orbit and then, using the on-board Apogee Boost motor, into a 24-hour Geosynchronous orbit, locatedover the South Atlantic. The mission hardware was designed for a minimumlifetime of three years.

It is a testament to those who designed and built the mission that in factit lasted for 18.5 years, and during that time the overall system sensitivityshowed a degradation of only about 15%. The extended lifetime of IUE wasalso made possible by the superb ingenuity of the operations and engineer-

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Figure 6. An example 2-D image from a high-resolution echelle spectrum, with a 768x768pixel format. Pipeline processing of such data included corrections for geometrical dis-tortion, photometric correction and scanning each order and inter-order background,to yield a flux-calibrated spectrum covering 1150-1950A at about 0.1A resolution. Thelong-wavelength spectrograph produced similar images and spectra covering 1850-3250A.(Courtesy NASA/ESA/SERC)

ing staff, in coping with inevitable system failures and consumables usage.Careful optimisation of slews and systems preparations minimised the useof the on-board hydrazine, whilst the engineering teams developed novels

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Figure 7. An example of a low-resolution IUE spectrum, from the cross-disperser grating alone, in the short-wavelengthregion covering 1150-1950A. Two spectra are shown on one image produced by locating the object in the 3′′ and 20′′

×20′′

apertures. Similar low-resolution spectra were secured using the long-wavelength spectrograph covering 1850-3250A. (CourtesyNASA/ESA/SERC)

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TABLE 1. Design Parameters of the IUE Mission

Launch: Delta 2914 rocket on 26 January 1978

Orbit: Geosynchronous period 23.927hrs

Perigee: 25669km

Apogee: 45887km

Inclination: 28.6 degree

Ground Stations and visibility:

Goddard Space Flight Center (USA) 24hrs per day

Villafranca del Castillo (Spain) 16hrs per day

Scientific Instrument:

45-cm f/15 Richey-Cretien Cassegrain telescope

Telescope Image quality 3′′

Two echelle spectrographs + cross dispersers

Wavelength ranges: 1150-1950A, 1850-3250A

High resolution mode: ∼0.1A

Low resolution mode: ∼6A

Two spectrograph entrance apertures: 3′′ and 10′′×20′′

SEC Vidicon Detectors: 768×768 pixels

Fine Error sensor for 16′ Field of View for target identification and acquisition

ways of pointing and slewing algorithms to accommodate gyro failures. Thetremendous impact that IUE had on modern astrophysics is summarised inthe next section. Its great success was recognised after its first ten years ofoperation when the IUE Project was selected for the 1988 US PresidentialAward for Design Excellence, the first, and as far as I am aware the onlytime that an astronomical facility has won such an accolade.

5. The IUE Archive and Data Distribution

As noted above, the three agencies agreed in the MOUs that all IUE datashould be made available to the widest astronomical communities world-wide, after the 6-month period of propriety rights to data secured by theoriginal guest observer. To achieve this goal the IUE project developed pio-neering techniques to provide an archive of all IUE data and procedures for

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delivering data as a “service” to the community in response to requests forspecific observations. This concept was in stark contrast to the situationwhich was commonplace for data obtained from ground-based observatoriesup to that time. There observers would take their data (usually in photo-graphic plate form) back to their home institutions and no “copies” wereheld at the observatories themselves. Thus such data, generally, remainedsoley the property of the initial observer and not made widely available.The observatory would hold files of observing logs specifying which objecthad been observed, when and in what form (i.e. spectral or photometry),but another astronomer essentially had to contact the initial observer toask whether they could access the original data. A similar pattern of dataaccess, or lack of it, effectively applied to earlier space missions. All thiswas to dramatically change with IUE.

The development of the IUE archives has been described by Harris &Sonneborn (1987) and Giaretta et al. (1987), and further comments of howthe archive evolved have also been summarised by Benvenuti (2002). Toinform the wide community of what data had been obtained the projectmaintained an IUE Merged Observing Log (combining the data secured atthe two ground stations, VILSPA and at GSFC) updated every two monthsand, initially, made available on microfiche (to be supplanted later on viainteractive facilities on the WWW).

The three agencies agreed that explicit identifications would be requiredfor all targets in the logs, with HD numbers being used for stars and NGCnumbers for non-stellar sources. This mandatory harmonisation procedureprovided a much-needed standardization of log records, avoidance of anyunnecessary duplication of observations and subsequent smooth exploita-tion of the archives. The Log provided information on the specific astronom-ical target, the IUE instrument configuration and data calibration. Theseincluded the Object identification, coordinates, object class, camera em-ployed, exposure time, aperture used etc, and any special observing proce-dures such as blind-offsets, multiple exposures in the LORES mode usingthe large aperture. Often the FES image associated with a specific spectralimage was also archived. For the spectrum, the camera image number, theraw, geometric-corrected image and the calibrated image was also archived,as well as the calibrated, extracted spectrum. Photowrites of the three typesof image were also archived and made available to allow the user to quicklyaccess data quality.

A key aspect of the operation of the IUE archives run by the threeagencies was to make the resource easily and speedily accessible to therequesting astronomer, and to provide a service which was essentially costfree to them. In the early phases of the mission the requester would scanthe Logs, identify the source(s) he/she was interested in a send in to the

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relevant agency a paper request of the image numbers required. The datacenter would then undertake a tape-copying job of all the relevant dataand send a tape of the requested data to the requestor via normal mailservice. Typically, the requestor would receive their desired data within afew weeks.

As time progressed, this whole process would be supplanted by e-mailedrequests and data transmission over the Internet. This speedy transmissionof IUE data to the wider community represented a massive advance overprevious facilities and set a benchmark which was to be followed by subse-quent missions, including the HST. As the IUE mission progressed in time,it was recognised that improvements to the calibration and data extrac-tion procedures should be incorporated in the archival data. To that end,special teams in the three agencies developed standardised techniques forre-processing all IUE data to allow, inter alia, homogeneity of data takenat different epochs to maximise the scientific integrity of the data. Thisreprocessing led to the Uniform Low Dispersion Archive (ULDA) and, forhigh resolution spectra, the IUE Newly extracted Spectra (INES) archive.

The ease of access and speedy availability of IUE data to the widercommunity, ensured that the usage, exploitation and scientific return fromthe mission was greatly enhanced. It was clear from the outset, and inher-ent in the initial planning of the three agencies, that IUE data could andwould be of scientific importance above and beyond the goals of the initialobserving proposal. This was essentially a result of the full spectral rangecoverage of each exposure and especially so in the case of the high resolu-tion echelle data. A few examples will illustrate this. In the study of stellarwinds from massive OB and WR stars, hundreds of stars were observedin the Galaxy and Magellanic Clouds, to study their P Cygni profiles inspecies like SiIV, CIV, NV and other stellar features. Each stellar spectrumalso had superimposed a plethora of interstellar absorption lines producedin the intervening interstellar gas. Thus these data were of great interest toastronomers studying the ISM in the disk of our galaxy, the galactic haloand the ISM in the Magellanic Clouds themselves.

Similarly, HIRES data initially secured for the purpose of studying theISM was of great interest to the hot star fraternity. By making the IUE dataaccessible to the broader community, after the initial propriety period, awider range of scientific outcomes could be achieved. A second example,which relates to the longevity of the mission, and the fact that the threeagencies operated independent guest observing allocations, meant that re-peat observations of individual sources were obtained which led to the dis-covery of significant time-variability in their UV spectra, often hithertounknown. This led to significant changes to our understanding of the struc-ture and variability in the winds and atmospheres of hot and cool stars,

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and in AGN (see below). By making the IUE data to readily accessible viathe archives ensured an enhanced and optimal scientific return. During thecourse of the mission the number of scientific papers published based onuse of archival data became comparable to those from the ongoing guestobserver data.

Subsequent space observatories, especially the HST, were able to learnfrom and capitalise on the experience of the development of the IUE archiveto provide a comparable service and maximise the scientific return fromtheir own data.

6. The IUE Legacy

During its long lifetime IUE proved to be one of the most successful as-tronomical observatories ever developed. It was used by thousands of as-tronomers from all over the world, generating over 100,000 ultraviolet spec-troscopic observations of a very wide range of objects. Over 3500 refereedpapers have appeared based on IUE data, and the proceedings of 10 in-ternational conferences dedicated to IUE results have also been published.It is estimated that some 600 PhD theses have been produced based onIUE data. All IUE data, in both raw image and processed spectral formare available in Data Archives, easily accessible from the three agencies.These IUE data archives are still being “mined” at the time of writing.The main legacy of the IUE mission has been its impact on astrophysics. Irefer the reader to the volume “Exploring the Universe with the IUE Satel-lite” (Kondo et al. 1987) in order to gain a fuller appreciation of the hugerange of scientific advances that the mission engendered. A few examplesare given below to illustrate the special mission characteristics and oper-ations that led to its success, notably its sensitivity, orbit and real-timecontrol, and longevity.

The increase in sensitivity over previous missions can be illustrated bythe following: the Copernicus satellite yielded the full UV spectrum of theV=2 O4f star Zeta Puppis in an integration time of about 24hrs – the IUEspectrum of the same star was achieved in a few seconds. This huge advancein sensitivity extended observations of the UV spectra of luminous OB andrelated WR stars from the Copernicus limit of 6th magnitude to objects asfaint as 16th magnitude in the Galaxy and Magellanic Clouds. This allowedthe study of mass loss and stellar winds in different galaxy and metallic-ity environments, the discovery of mass loss and winds from subluminousOB subdwarf stars and central stars of planetary nebulae, testing the the-ory of radiation-pressure-driven winds over a ten-decade range of stellarluminosities. Repeat observations of individual OB (and WR) stars showedvariability in their UV P-Cygni profiles and the ubiquitous occurrence of

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Discrete Absorption Components (DAC), whilst the ability with IUE tomonitor individual stars for many days uninterrupted led to the discoverythat these DACs varied in strength and velocity on timescales associatedwith stellar rotation.

The concept that hot star winds were steady and spherically symmetricwas refuted and led to the realisation that their winds are highly struc-tured and variable, the physical origin of which is still poorly understoodbut probably linked to radiatively-induced instabilities and non-radial pul-sations. This has had a profound effect on models of stellar evolution whenmass loss has to be included. The longevity of IUE further revolutionisedmassive star astrophysics when it was discovered that some OB stars alteredtheir spectral appearance dramatically, changing from a high-temperatureO or WR appearance to a low-temperature Luminous Blue Variable ap-pearance in only a few years – for the first time it became possible toinvestigate the advanced phases of massive stellar evolution in “real time”.The increase in sensitivity provided the first UV spectral data for a hugerange of different stellar types, including Pre-Main Sequence stars, Symbi-otic stars, interacting binaries, Be stars, White Dwarfs, Planetary Nebulae,the Chromospheres and Coronae of Late-Type, cool stars and their massoutflows.

The IUE sensitivity greatly extended studies of the Interstellar Medium,in both gas and dust phases by probing sightlines out to large distancesusing stars in the Magellanic Clouds and other galaxies as backgroundsources. This lead, inter alia, to the mapping and extent of a hot phase inCIV, SiIV and NV, in the Galactic Halo and tests of the Galactic Fountainmodel for the ejection of hot gas from supernovae and subsequent coolingand infall – of importance in the interpretation of QSO absorption lines.In addition, studies of the chemistry and dynamics of the interstellar gasin diffuse and dark clouds became possible throughout the Galaxy andMagellanic Clouds, and the nature of the UV interstellar extinction couldbe determined in different metallicity environments.

The geosynchronous IUE orbit allowed multiple observations of individ-ual sources to continue uninterrupted for days at a time, and this greatlyfacilitated multi-wavelength observing campaigns, using ground-based tele-scopes at different longitudes (e.g. in Australia, South Africa, Canaries andthe USA) to achieve 24-hour coverage, as well as X-ray satellites. One ex-ample of this was a two-week campaign in 1979 known as the X-ray BinaryFortnight, when US, UK and European scientists agreed to pool their timein a coordinated study of four prime X-ray binary sources: Cyg X-1, ScoX-1, Her X-1 and HD153519 (Dupree et al. 1980, Gursky et al. 1980, Treveset al. 1980, Willis et al. 1980). For each source, variations in the IUE UVspectra were combined with simultaneous data from optical spectroscopy

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and photometry and X-ray data, to lead to great advances in our under-standing of the accretion phemonenon onto compact stars (White Dwarfsand neutron stars) in binary systems.

A second example was the multi-wavelength campaign studying vari-ations in Active Galactic Nuclei, especially the Seyfert galaxy NGC4151(Penston et al. 1979). As well as improving our understanding of the ioni-sation and structure of the narrow- and broad-lined regions in these AGN,the different timescales and velocities measured in the variations seen inthe FeII, SiIV, CIV and NV emission lines in NGC4151 allowed the massof the central black hole in that object to be measured accurately for thefirst time.

The high IUE orbit and real-time control also greatly facilitated theability to respond quickly to the occurrence of targets of Opportunitylike classical Novae and Supernovae, and to monitor the development ofthe source UV spectrum after outburst. As an example, Nova Cygni 1978reached V=6.2 on 12 September 1978, and observations with IUE starteda few hours later, and continued monitoring with 194 spectra secured untilnear the end of 1980 when it had declined to V=14. The analysis of thenebular phase of this nova showed that enhanced CNO abundances, expan-sion velocities, ejected mass and kinetic energy were all consistent with theproposed theoretical explanation for a nova as a CNO outburst onto a 1.0solar mass white dwarf from material accretion from a late-type companion(Stickland et al. 1979).

In subsequent years many more novae were observed in a similar fash-ion. Many Supernovae were also observed with IUE soon after their initialoutbursts, including SN 1987 A in the LMC. IUE monitoring of this sourceled to the discovery a enhanced NIII emission some 1.7 years after the ini-tial explosion, the light-time-travel to a precursor ejected ring of materialconfirming the evolutionary nature of the precursor star and also allowingan accurate measurement of the distance to the LMC.

Although primarily intended as a mission to investigate stars, the ISMand galaxies, IUE was also used to study objects in the solar system, andprovided pioneering spectroscopy of planetary atmospheres and Aurorae,planetary satellites and Comets (the latter as targets of opportunity). As anexample, one of the earliest discoveries was the variable Sulphur emissionslines in the spectra of Jupiter which were shown to arise from volcanicemissions from its moon Io (see Moos & Encrenaz 1987).

As the mission progressed, its sensitivity limit was pushed to furtherextremes from that originally envisaged, in some cases employing single ex-posure times as long as 36hrs. A good example here involved the pioneer-ing UV study of the Double Quasar 0957+581 A,B undertaken by Wilson,Gondhalekar, Burke and Dupree. At some 19th magnitude this source was

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one of the faintest objects successfully observed in the low-resolution mode.IUE was used to monitor the UV variations in the A/B lensed componentsof the Quasar over several years. Variations in the UV brightness ratio A/Bwas found to be the same as found at radio wavelengths, confirming a basicprediction of a gravitational lens – NO chromatic aberration, whilst themeasured time delay between variations in components A and B provideda direct measurement of the Hubble constant of H = 67 ±8 km/s/MPC−1,in very good agreement with later determinations from the Hubble Spacetelescope (Gondhalekar & Wilson 1982).

IUE is the only space facility to have been operated like a “normal”ground-based observatory with guest astronomers going to the ground sta-tions at GSFC and VILSPA to conduct their own science programmes and“see” their data in real-time. This was possible through the choice of ageosynchronous orbit with constant ground-station contact. The observerwas thus able to ensure the optimum quality of his/her data, by adjust-ing observing times accordingly, and the on-source operational efficiencywas extremely high at about 80-90 percent. The system also allowed forconsiderable flexibility in observing targets and modes, unlike the case of alow-orbit mission where observations are generally pre-programmed days orweeks in advance. Guest observers also had a unique opportunity to under-stand the intricacies of the satellite and instruments through direct interac-tions and contact with the spacecraft engineers and ground-station supportstaff and resident astronomers. This and the near-annual IUE conferencesgreatly facilitated international science collaborations and new initiatives.

The extraordinary lifetime of the IUE mission at 18.5 years did, perhaps,have one unfortunate legacy. Despite numerous attempts and proposals toESA for a follow-up UV mission during the period 1979-1995, none wereselected in the competitive mission selection exercises. It was, perhaps,generally considered that with HST up, and IUE still operational, therewas less of a need for a further facility, despite the great improvements insensitivity and wavelength coverage that were being proposed.

Whilst NASA did go ahead with the Far Ultraviolet Satellite Explorer(FUSE) mission, launched in 1999 (lasting to 2006), there has been andremains no plans in the ESA programme for a UV spectroscopic observatoryto follow the great success of IUE, which is a pity. Has the UV field, at leastin Europe, been the “victim of its own success” ?

Acknowledgements

I am grateful to Karen Levay (IUE archive team at StSci) for providingcopies of the IUE photowrites in Figs. 6 & 7, and to David Stickland for acareful reading of this paper.

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