Messenger No140

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The Messenger No. 140 – June 2010 S u m m a r i e s o f a l l E - E L T i n s t r u m e n t c o n c e p t s t u d i e s 2 0 y e a r s o f F O R S o p e r a t i o n s C R I R E S s e a r c h f o r e x o p l a n e t s S u p p o r t f o r C h i l e e a r t h q u a k e v i c t i m s 2 The Messenger 140 – June 2010 hardware and the provision of the detec- tor systems; in addition the consortium invested substantially in its infrastructure in support of the FORS pr
  The Messenger No. 140 – June 2010    S  u  m  m  a  r   i  e  s  o   f  a   l   l    E  -   E   L   T   i  n  s   t  r  u  m  e  n   t  c  o  n  c  e  p   t  s   t  u   d   i  e  s   2   0  y  e  a  r  s  o   f   F   O   R   S  o  p  e  r  a   t   i  o  n  s   C   R   I   R   E   S  s  e  a  r  c   h   f  o  r  e  x  o  p   l  a  n  e   t  s   S  u  p  p  o  r   t   f  o  r   C   h   i   l  e  e  a  r   t   h  q  u  a   k  e  v   i  c   t   i  m  s  2 The Messenger 140 – June 2010 hardware and the provision of the detec-tor systems; in addition the consortiuminvested substantially in its infrastructurein support of the FORS project and about130-person years of work until the com-pletion of the project. At the nal accept-ance of the FORS instruments by ESO,the FORS consortium even stayed underbudget. In return for its effort, the consor-tium was granted 66 nights of guaranteedobserving time to be spent on scienceprogrammes using the two FORS instru-ments in visitor mode. A highly motivated team of astronomersand engineers jumped to the task of designing the rst VLT science instrument,and the Preliminary Design Reviewtook place in April 1992. Originally it wasplanned to build two identical copiesof FORS and equip them with identicalsets of lters and grisms. During the FinalDesign Phase it was, however, decidedto purchase only one set of the expensivepolarisation optics; the Wollaston prismand the linear and circular retarder platemosaics being among the largest evermade, at least for astronomical applica-tions. The money saved was insteadinvested in high-resolution collimatoroptics, which avoided vignetting out tothe corners of the CCD and, in addition,also paid for the Mask Exchange Unit(MXU), later incorporated in FORS2. The Final Design Review was passed atthe end of 1994. In 1996 system integra-tion and tests started in an assembly hallrented at DLR (Deutsches Zentrum für Gero Rupprecht 1 Hermann Böhnhardt 2 Sabine Moehler 1 Palle Møller 1 Ivo Saviane 1 Bodo Ziegler 11 ESO 2 Max-Planck-Institut für Sonnensystem-forschung, Katlenburg-Lindau, Germany Celebrating the double jubilee of tenyears in operation of FORS1 andFORS2, this article summarises, froman insider’s point of view, the historyof the FORS instruments, arguably themost prolic on ESO’s Very Large Tele-scope. The FORS story began inthe early 1990s and FORS1 was the rst VLT user instrument to be commis-sioned. Both FORS instruments haveundergone considerable evolutionand quickly parted from the srcinalconcept of being identical twins. Theyhave both made major contributions toscientic research and have helpedshape VLT operations. In 2009, FORS1was retired, but FORS2 continues, fus-ing the best of both.  The rst of April 1999 marks the dayscience operations started on the VeryLarge Telescope (VLT), with Unit Tele-scope 1 (UT1, Antu) only. Two instrumentswere installed on Antu: ISAAC, (the Infra-red Spectrometer And Array Camera)designed and built by ESO, and FORS1,(the rst FOcal Reducer/low dispersionSpectrograph) created by VIC, the VLT Instrument Consortium, composed of theobservatories of Heidelberg, Göttingenand Munich (Germany). Exactly one yearlater, FORS2 took up duty on Paranal onUT2 Kueyen. Since then the FORS twins(Figure 1) have provided astronomers witha wealth of observational data of excel-lent quality. The successful teamwork of the two FORS instruments came to anend on 1 April 2009 with the retirement of FORS1 after exactly ten years of opera-tions. FORS2 reached the ten-year mark one year later, so on 1 April of this year,the two FORS instruments together hadracked up a total of 20 years of success-ful science operations.Some pre-history The srcins of what became FORS dateback a long way. The report of the Work-ing Group on Imaging and Low Resolu-tion Spectroscopy (VLT Report No. 52) inJuly 1986 recommended building “a set of general purpose focal reducers at oneNasmyth focus on each of the (four) array elements” (that is, the UTs). The onlydirect legacy of this far-reaching recom-mendation are the twin FORS instru-ments. In June 1989 ESO issued the VLT Instrumentation Plan asking for com-ments and expressions of interest fromEuropean institutes. In response to theESO call, a consortium was formed byImmo Appenzeller of the Landesstern-warte in Heidelberg as Principal Investi-gator, with Rolf Kudritzki of the Universityof Munich and Klaus Fricke of the Univer-sity of Göttingen as co-Investigators. This consortium intended to submit aproposal for the medium-high resolutionspectrograph, now known as UVES.ESO, however, nally decided to build thisinstrument internally, so the consortiumchanged course at short notice andentered the competitive tender with a bidfor the focal reducer/low dispersion spec-trograph instead! The bid was successful,against one competing proposal, andafter lengthy contract negotiations involv-ing the project institutes, their host uni-versities and, by the end, three differentGerman federal states as well as severalfederal ministries, the contract with ESOwas signed in December 1991. The con-tract foresaw ESO paying for the FORS Telescopes and Instrumentation Twenty Years of FORS Science Operations on the VLT Figure 1. The FORStwins, in separate UTs.  3 The Messenger 140 – June 2010 Luft- und Raumfahrt) in Oberpfaffenhofen.It was here that an accident happenedduring a handling operation, and the fullyequipped collimator section of FORS1,in total about 600 kg of mechanics,optics and electronics, fell from the crane.Rumours spread quickly that someonehad been killed, which fortunately turnedout not to be true. Nobody had beenhurt, not even the optics, the only dam-age was to the metal housing and theoor of the DLR integration hall. Nowordering all major instrument parts twicepaid off: within three weeks the dam-aged part had been replaced by the cor-responding piece from FORS2 and areplacement ordered, which arrived bythe time it was needed for the assemblyof FORS2! After extensive tests, Prelimi-nary Acceptance of FORS1 in Europewas declared by ESO in July 1998 (seeFigure 2) and the instrument was shippedto Paranal for installation at the VLT.CommissioningUpon arrival of FORS1 and the integrationteam on site in August 1998, Paranal wasstill a full-blown construction site. Youcould only enter the Control Building wear-ing a hard hat — there were no properceilings, the rooms that later becameofces had no windows, there were nosanitary facilities available, etc.! UT1 wasstill undergoing commissioning — rstlight (see Tarenghi et al., 1998) had oc-curred just weeks before. In September,on-site assembly and testing in the Auxil-iary Telescope Hall (which was itself stillunder construction) was nished andFORS1 was attached to the Antu Casse-grain focus. In the hours preceding whatwas supposed to become the FORS1 rstlight on the night sky, one of us discov-ered, to everybody’s horror, that therewas water inside the instrument! FORS1was quickly but carefully dismountedagain, disassembled and painstakinglydried and cleaned using huge amountsof optical paper! The reason for this“ood” was a burst cooling hose insidethe telescope adapter/rotator. Afterreplacement of the hose, FORS1 was re-mounted and saw its true rst light duringthe night of 15 to 16 September 1998(Appenzeller et al., 1998).It turned out that the instrument was inoptimum focus when mounted, and thestars showed an image quality of 0.6 arc-seconds in the very rst image takenwith the instrument on sky! The rst expo-sures of an interesting celestial objectresulted in the famous image of spiralgalaxy NGC1232, which, one year later,the readers of  Sky and Telescope maga-zine voted to be among the ten mostinspiring astronomical images of the 20thcentury (Figure 3)! Figure 2. Provisional Acceptance in Europe of FORS1 at DLR Oberpfaffenhofen in June 1998.From left to right: front row Heinz Kotzlowski,Immo Appenzeller, Sandro D’Odorico, BernardMuschielok, Walter Seifert, Wolfgang Hummel.Middle row: Guy Monnet, Roberto Gilmozzi, ThomasSzeifert. Back row: Harald Nicklas, Rolf Kudritzki,Gerd Wieland, Gero Rupprecht, Reinhold Häfner,Wolfgang Meisl, Achim Hess. Tempus fugit  … Figure 3. Named oneof the most inspiringastronomical images of the 20th century: theFORS1 rst light imageof the galaxy NGC1232.  4 The Messenger 140 – June 2010 more versatile due to the MXU. Fromthe electro-mechanical point of view bothFORS instruments are almost identical,so it was possible, with only a smalleffort, to transfer the polarimetry optics(Wollaston prism and the two retarderplate mosaics) from FORS1 to FORS2, tocalibrate and commission this importantmode and to offer it from Period 83 on-wards with FORS2.FORS: popular, productive and highlycitedFrom the very rst period on the VLT,FORS1 was the most popular instrumentin terms of requested observing time,and it was only surpassed in popularityby UVES at the time FORS2 came onlineand the demand for FORS time couldbe divided between the two instruments(Figure 4). The demand for both FORSinstruments remained high until the dayof FORS1 decommissioning. This is alsoreected in their productivity. In Figure 5we plot the number of refereed publica-tions per year as a function of the timeafter the instrument came into operation.It is clearly seen that FORS1 and UVESwere the two most productive instrumentsfor the rst ve years of their careers,while FORS2 takes third position. BothFORS instruments together had pro-duced 1161 refereed papers by the endof 2009 with a total of 40783 citations. Only after FORS2 was upgraded didFORS1 drop in popularity, as many usersmoved to FORS2 instead. The publica-tion rate is, to some extent, a result of how much time was allocated. Lookinginstead at the publication efciency,i.e. publication rate per allocated night,ISAAC and FORS2 currently produceroughly 0.75 refereed papers per observ-ing night, UVES produces 0.9, whileFORS1, during its last few periods, pro-duced about 1.0 paper per night andwas the most publication-efcient VLT instrument so far. If we look at the number of citations of  VLT papers, it is symptomatic that atleast one of the two FORS instrumentswas involved in eight of the ten mostcited VLT papers. The most-cited papersare those of large collaborations that in-clude all major telescopes — the Hubble A further (and nal!) water incident duringthe second commissioning run, againdue to a hose in the telescope adapter,earned FORS1 the nickname “Yellow Sub-marine” among the mountain crew. Thishad no effect, however, on the excellentrelationship between the FORS consor-tium team and the colleagues on Paranalwho gave superb support to the com-missioning activities. One important com-missioning task involved the vericationon the telescope of the specied imagemotion due to telescope tilt and instru-ment rotation. To accomplish the task, themechanics specialist from the instrumentteam spent several nights laboriously driv-ing FORS through endless measure-ment cycles that conrmed the nite ele-ment prediction and rst assessmentsat a telescope simulator in Europe: imagemotion is so small, below a quarter of a CCD pixel in two hours for the standardcollimator, that FORS is known to bethe only focal reducer type instrument toallow the image distortion by weak lens-ing around remote galaxy clusters tobe measured. After two weeks of Science Verication in January 1999, FORS1was nally handed over to ESO for opera-tion at the VLT. As a last step, now per-formed by ESO, the instrument was“Paranalised”, i.e. its operation and con-trol scheme were tuned to fully matchthe observatory operation environmentand science operations concept. Duringthe VLT inauguration ceremony in March1999, a few weeks before science oper-ations began, FORS1 delivered some of the sharpest ground-based images evertaken without special techniques, suchas adaptive optics or speckle imaging:0.25 arcseconds full width at half maxi-mum in I -band — a very promising startindeed!One side effect of being the rst instru-ment on the VLT was the need to recordall night activities, so the FORS teaminvented the structure of the night logs,which is still basically the same today,although the night astronomer can nowrely on software tools to ll them, whichthe team did not have at their disposalback in 1999.On 1 April 1999 Massimo Tarenghi, therst director of the VLT, declared that fromnow on Paranal was no longer a con-struction site, but a working observatoryand FORS1, together with ISAAC, openedthe science operations period at the VLT. In September 1999 its twin, FORS2arrived on Paranal and entered regularservice on UT2 Kueyen in April 2000(without the embarrassment that FORS1had to go through). Although this hadnot been foreseen, both FORS instru-ments changed telescopes several timesin the following years to optimise the useof the UTs as telescopes came onlineone after the other. Every Cassegrainfocus of the VLT has by now successfullyhosted at least one FORS at some time,a testimony to the precision with whichthe UTs were built to be identical, and tothe robustness of the FORS instrumentsat the VLT!Upgrades and the FORS mergerEarlier than srcinally expected, new CCDswith a better red sensitivity became avail-able, so work on the replacement of one of the srcinal TEK 2k × 2k detectorsstarted when FORS2 was barely on thetelescope. The Garching Optical Detector Team provided a CCD mosaic with twoMIT 2k × 4k (15 μm pixel) chips completewith a new FIERA controller, and the sys-tem was successfully commissioned inOctober 2001. It has been in continuousscience operations on FORS2 since April2002, only recently interrupted some-times when the blue–sensitive FORS1CCD is mounted. Upgrading FORS1 witha blue-sensitive CCD mosaic took muchlonger: a mosaic of blue sensitive e2vCCDs with the same format as the redsystem on FORS2 was ready for commis-sioning only in early 2007. Since theretirement of FORS1 in 2009 it has beenoffered in visitor mode on FORS2.In addition to the replacement of thedetectors, ESO improved the efciencyof both FORS instruments by acquiringa number of highly efcient VolumePhased Holographic Grisms (VPHG) aswell as various high throughput broad-and medium-band lters. The need to free a VLT Cassegrain focusfor X-shooter (Vernet et al., 2009), therst of the second generation VLT instru-ments, sealed the fate of FORS1.Thedecision was to keep FORS2 operationalsince it is younger and is operationally Telescopes and InstrumentationRupprecht G. et al., Twenty Years of FORS Science Operations on the VLT 
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