The VLTI PRIMA Facility

Transcription

1 The VLTI PRIMA Facility Gerard T. van Belle 1 Johannes Sahlmann Roberto Abuter 1 Matteo Accardo 1 Luigi Andolfato 1 Stephane Brillant 1 Jeroen de Jong 2 Frédéric Derie 1 Françoise Delplancke 1 Thanh Phan Duc 1 Christophe Dupuy 1 Bruno Gilli 1 Philippe Gitton 1 Pierre Haguenauer 1 Laurent Jocou 4 Andreas Jost 1 Nicola Di Lieto 1 Robert Frahm 1 Serge Ménardi 1 Sebastien Morel 1 Jean-Michel Moresmau 1 Ralf Palsa 1 Dan Popovic 1 Ester Pozna 1 Florence Puech 1 Samuel Lévêque 1 Andres Ramirez 1 Nicolas Schuhler 1 Fabio Somboli 1 Stefan Wehner 1 The ESPRI Consortium 5 1 ESO 2,@W/K@MBJ(MRSHSTSEXQDWSQ@SDQQDRSQHRBGD /GXRHJ,/$&@QBGHMF&DQL@MX.ARDQU@SNHQDCD&DM UD2@TUDQMX Switzerland 4 CNRS, Laboratoire d Astrophysique de K.ARDQU@SNHQDCD&QDMNAKD%Q@MBD 5 $2/1($WNOK@MDS2D@QBGVHSG/1(L@ "NMRNQSHTL.ARDQU@SNHQDCD&DM UD 2@TUDQMX2VHSYDQK@MC,@W/K@MBJ (MRSHSTSEXQ RSQNMNLHD,/( 'DHCDK - ADQF&DQL@MX@MCSGD+@MCDRRSDQM - V@QSD'DHCDKADQF&DQL@MX The Phase Referenced Imaging and Microarcsecond Astrometry (PRIMA) instrument was recently delivered to the summit of Cerro Paranal and installed as part of the Very Large Telescope Interferometer (VLTI) infrastructure. PRIMA is designed to (i) provide phase-referenced interferometric imaging at milliarcsecond scales, (ii) ena ble faint star science several magnitudes fainter than the current atmospheric limits of the VLTI, and (iii) provide astrometric measurements at the tens of micro-arcsecond level. PRIMA has suc- FHVVIXOO\VHHQ:UVWIULQJHVDQGLVFXU - rently (as of late 2008) undergoing initial commissioning tests. 3GD/1(,!HF!@MF@MCjQRSEQHMFDR The thirty-plus crates of optics, electronics, mechanics, and other assorted G@QCV@QD@MCRNESV@QDNESGD/G@RD1DE - DQDMBDC(L@FHMF@MC,HBQN@QBRDBNMC RSQNLDSQX/1(, HMRSQTLDMS@QQHUDC@S "DQQN/@Q@M@KHMLHC)TKXEQNL$2. &@QBGHMF@MCSGD/1(,!HF!@MFV@R TMCDQV@X/1(, ADSSDQJMNVM simply as the Big Bang spanned the ENKKNVHMFRDUDMVDDJR3D@LRNEDMFH assemble the complex instrument, carefully rationing out the limited resources of lab space, subsystem availability and RJXSHLD!@RHB@KKXVDVDQDITRSSQXHMF MNSSNRSDONMD@BGNSGDQRSNDR3VN VDDJR@ESDQSGDjQRSRGHOLDMS@ETQSGDQ BQ@SDR@QQHUDCEQNLNTQ/1(, O@QS - MDQR@SSGD&DMDU@.ARDQU@SNQX@KNMF VHSGRS@EEEQNL&DMDU@@MC,/( 'DHCDK - ADQF(MSGDDMCNUDQHMCHUHCT@KRO@Q - ticipated directly in a delicately choreographed dance of optical instrumentation HM@RO@BDMNK@QFDQSG@M$2.&@QBGHMFR KNAAX MTMCDQKXHMFRDMRDNEOTQONRD and esprit de - ing out the endgame of more than eight XD@QRNE/1(, CDUDKNOLDMS As of the beginning of September 2008, individual subsystems had been unboxed, HMRS@KKDC@MCS@JDMSGQNTFGSGDHQO@BDR but the larger question of system integra- SHNMV@RADFHMMHMFSNADBNMRHCDQDC(M SGDV@MHMFC@XRNESGD/1(,!HF!@MF SVNNESGD5+3(R@TWHKH@QXSDKDRBNODR thread light from a single star through the optical beam trains, delay lines, and into NMDNE/1(, REQHMFDRDMRNQTMHSR days ahead of the ambitious Big Bang schedule twin beams of starlight from '#RODBSQ@KSXOD* III; V Figure 2S@Q2DO@Q@SNQNTS@S 3CTQHMFSGD/1(,!HF!@MF 6 The Messenger 134 December 2008

2 the telltale wiggle of interfering light was seen as the delay lines swept through the position of equal path length for each SGHRBNQDDKDLDMSNESGD/1(, RXRSDL was not only sensing fringes on far dim- - ODMCDMSKXSGD/1(, system was also successfully operated the interferometry laboratory to two ATs 6G@SHR/1(, /1(, 5+3(HMRSQTLDMSR@KSGNTFGHSRBNLOKDWHSX and spot in the queue as the last of the - /1(, B@MADENTMCHM#DKOK@MBJDDS@K /1(, HMSDQEDQNLDSDQRHMNMD(SHR@AKDSNBNK lect starlight in each of two telescopes from not just one source but two, and simul taneously send these pairs of star- - DMC3GD LRS@QKHFGSEQNL SGDjQRSRNTQBDHRQDBNLAHMDCHMSGD5+3( laboratory to form interference fringes, while fringes are collected simultaneously NMSGDRDBNMCRNTQBD #DSDBSNQ"NTMSR #4./#LHBQNM 3HLD R Figure 2.%HQRSEQHMFDRENQ/1(, NAS@HMDCNM%QHMFD 2DMRNQ4MHS NM2DOSDLADQNARDQUHMFSGD RS@Q'#VHSGSVN 3RRDO@Q@SDCAXL@S 5+3(RS@SHNMR&@MC& '# L* ' L 13.E ERDS 'NVDUDQ/1(, HR@KRNLTBGLNQDSG@M two interferometers that just happen to BNG@AHSSGDR@LDK@A SSGDSDKDRBNODR RS@QRDO@Q@SNQ232TMHSRS@JDSGDjDKC observed by each telescope and split HSRDMCHMFNMDRNTQBDCNVMNMD/1(, beam path and a second source down SGDO@Q@KKDKNMD RDBNMC@QXHMSDQEDQN - metric delay line, the differential delay line ##+ATHKSAXSGD$2/1(BNMRNQSHTL accounts for small pathlength differences between the two sources due to their RKHFGSKXCHEEDQDMSONRHSHNMRNMSGDRJX@MC allows fringes to be obtained simulta- MDNTRKXEQNLANSGRNTQBDR$@BGAD@L combiner contains a laser metrology F@TFD/1(,$3SG@SHRHMIDBSDC@SSGD ONHMSNEBNLAHM@SHNMSQ@UDKRA@BJV@QCR through the system to the STSs, and is QDSQNQDkDBSDCA@BJEQNLSG@SONHMSLNMH - toring the pairs of telescope feeds and./#"rs@sd 3HLD R tying them together at the nanometre KDUDK3GDRD@CCHSHNM@KRTARXRSDLRS@JD /1(, NTSNESGDQD@KLNERHLOKX@BGHDU - ing contemporaneous fringes and empowers it with three unique capabilities RDD%HFTQD 3GDjQRSNESGDRDB@O@AHKHSHDRHR/1(, R ability to measure astrometric angles of the geometry of the separation between two telescopes (the baseline ) combined with a measurement of the Figure 3.!XSGDDMCNESGD/1(,!HF!@MFEQHMFDR from stars with K ~ 6 were not only being detected, ATS@BSHUDKXSQ@BJDC3GDTOODQO@MDKRGNVRSGD phase delay (blue), group delay (red) and delay line NEERDSAK@BJHMLHBQNMRNENOSHB@KO@SGCDK@X@R@ ETMBSHNMNESHLDENQRS@Q'#VHSG 3R@S5+3( lower panel shows the optical path delay controller./#"rs@sdctqhmfsghrshldvhsg@ghfgrs@sd V@HSDCNTS The Messenger 134 December

3 SNQ +HMD #HEE %QHMFD 2DMRNQ,DSQNKNF X 2S@Q 2DO@Q@ SNQ #DK@X +HMD #HEE DQDMSH@K #DK@X+HMD %QHMFD 2DMRNQ!,DSQNKNF X Figure 4.2BGDL@SHBNESGD/1(, RXRSDLENQSGD 5+3(GHFGKHFGSHMFSGDL@INQRTARXRSDLR@KNMFSGD NOSHB@KO@SGRNESGDE@BHKHSX delay line setting can be used to estab- KHRG@ONRHSHNMNMSGDRJXNESGDNARDQUDC RNTQBD3GHRHRSXOHB@KKXKHLHSDCAXSGD atmosphere and, more importantly, the instabilities of the optical system, which, ENQ@VHCDKXCHRSQHATSDCRXRSDLKHJD@M HMSDQEDQNLDSDQ@QDBNMRHCDQ@AKD'NV - ever, by observing two sources simultaneously, and in particular, the instantaneous delay line settings for these two sources, this measurement may be done HM@CHEEDQDMSH@KRDMRD,@MXNESGD dominant error terms become common mode and drop out when using this approach, allowing unprecedented levels of precision to be attained in measuring the astrometric angle between two RNTQBDR3GHRSDBGMHPTDV@ROQDCHBSDCSN be useful by Shao & Colavita (1992) and demonstrated on an engineering basis by SGD/@KNL@Q3DRSADC(MSDQEDQNLDSDQ/3( +@MD"NK@UHS@/1(, VHKKADSGD jqrshmrsqtldmssnneedqsghrb@o@ahkhsxsn the general astronomical community on a QNTSHMDA@RHR 2DBNMCKX/1(, B@MKDUDQ@FDSGDSVHM interferometer setup to observe sources RHFMHjB@MSKXCHLLDQSG@MOQDUHNTRKX possible with single-source interferom- DSDQR!XNARDQUHMF@AQHFGSRNTQBDHM one of the two channels of the system, /1(, B@MTRDSGHRRNTQBDMNSNMKX SNSQ@BJSGD@SLNROGDQHBCHRSTQA@MBDRENQ the bright source itself, but to feedforward the error correction signals to SGDRDBNMC@QXBG@MMDK3GDRDBNMC@QX channel can then be steered off to a nearby, dim source, and interferometric obser vations can be carried out without the atmosphere obfuscating the interfer- DMBDEQHMFDR(MSGHRE@RGHNM/1(, operates as an interferometric analogue to single-aperture natural guide star adaptive optics: the bright source in this B@RDHRMNSNMKXADHMFTRDCSNS@JDNTS the corrugations in the wavefronts of the individual apertures, but is also used to remove pathlength errors that are HMSQNCTBDCAXSGDkTBST@SHMF@SLNROGDQD (commonly referred to as piston or feqhmfdihssdqt3nsq@bjsgdeqhmfdrsgd bright source is limited to observations no longer in duration than an atmospheric coherence time; however, the error RHFM@KREQNLSGDAQHFGSRNTQBDSQ@BJHMF can then be used to create a synthetic coherence time for the secondary source SG@SHRRHFMHjB@MSKXKNMFDQ3GDRDBNMC channel can then stare coherently TRDETKC@S@ F@HMSGHRV@RCDLNM - VHKKADSGDjQRSHMRSQT - ment to routinely offer this functionality %24RVHSG@CHLRNTQBD/1(, G@RADDM designed to provide off-axis dim source SQ@BJHMFSNSGDDWHRSHMF5+3(HMRSQTLDMSR,!$1@MC,(#( Thirdly, the dual-object nature of the interferometer can be used to construct high resolution images of objects upon SGDRJXTRHMFfOG@RDQDEDQDMBDCHL@F - HMFt SLNROGDQHBSTQATKDMBDBNQQTOSRSGD phase information relevant to the Fourier transform of the object image without such corruption, it would be possible for a single-object interferometer to construct, point by point, the full Fourier components of the object image, HR@AKDSNL@JDNAIDBSHL@FDOG@RD measurements by observing a science object simultaneously with a reference star; the reference star in this case being RDKDBSDCSNG@UD@MTKKOG@RDHDAD BDMSQNRXLLDSQHB!XL@JHMFLTKSHOKD observations over the course of a night, and with multiple baselines on different nights, many Fourier phase components for a science object can be built up, technique can be applied to faint 3GDRDSGQDDB@O@AHKHSHDRL@JD/1(, extraordinarily special and an exciting development for astronomical interferometry with either the ATs or the Unit Tele- RBNODR43R(SR@RSQNLDSQHBB@O@AHKHSX is a unique new tool, and its faint source capabilities will allow astronomers to reach past the barrier of sensitivity that has plagued interferometers and examine E@HMSS@QFDSRVHSGGHFG@MFTK@QQDRNKTSHNM Expected performance and limitations of /1(, R/1(, NODQ@SDRSGDQD@QDSVNL@INQ limitations familiar to all astronomers: SGD@SLNROGDQD@MCSGDHMRSQTLDMSHSRDKE 3GD@SLNROGDQDKHLHSR/1(, HM@ MTLADQNEV@XR RLDMSHNMDC@ANUD the atmospheric coherence time is the maximum time span allowed for attempt- HMFSNCDSDBS@MCSQ@BJEQHMFDRNMSGD AQHFGSDQNESGDSVNRNTQBDR3GHRU@KTDHR SXOHB@KKX@ANTSLRENQ/@Q@M@KVGHBG will limit the system (accounting for beam transmission losses, detector QE, and other system pitfalls) to sources of roughly K = 8 on the auxiliary telescopes (and correspondingly dimmer for UT observations, roughly K %NQSGDRDBNMCE@HMSDQRNTQBDSGDSQ@BJ- HMFNE/1(, NMSGDAQHFGSRNTQBD@KKNVR a longer, synthetic coherence time to be provided, but only if the atmospheric 8 The Messenger 134 December 2008

4 that is near the bright primary source K-band for an evening of median seeing is RNMDLHFGS angle decreases, the system performance improves, particularly in the area of deter- RSQN- as an astrometric reference increases as sources next to bright ones from existing For example, sources at K = 6 in the, 22@MC#$-(2RTQUDXRSDMCSNV@RG NTS@KKCHLLDQRNTQBDRNTSSN@QBRDB- NMCRTMENQSTM@SDKXL@JHMFSGDRDRTQUDXR unsuited for selecting bright dim pairs (although it is still quite useful for at least HCDMSHEXHMFSGDAQHFGSRNTQBDR The instrumental limitations are many, which is unsurprising in an instrument of SGHRBNLOKDWHSX.ESGDRDNMDSG@SHRENQD - most in many people s minds is that of system vibration, which serves to smear out interference fringes by introducing variations in system pathlength that are too fast and/or large to be followed by the EQHMFDSQ@BJDQRNARDQU@SHNMRNERS@QKHFGS EQHMFDR3GD/1(,$3LDSQNKNFXRXRSDL will operate to mitigate these effects by monitoring the pathlengths through most the system, coherently preserving the OQDBHNTRRS@QKHFGSENQSGDEQHMFDSQ@BJDQ A second thorny instrumental issue is SG@SNEA@RDKHMDJMNVKDCFD%NQ@RSQNLD - level, the geometry of the two telescopes QDK@SHUDSND@BGNSGDQMDDCRSNADJMNVM SN@M@BBTQ@BXNEQNTFGKX LNUDQ RDO@Q@SHNMRNElLDSQDR3GHR problem separates out into two components: the wide-angle baseline, which is the average separation between the two telescopes during the observation, and the narrow-angle baseline, which is the differences in wide-angle baseline seen by the primary and secondary sources due to residual non-common paths and mechanical imperfections in SGDRXRSDL3GDVHCD@MFKDA@RDKHMDB@M be established to the required accuracy AXNARDQU@SHNMRNERS@QRVHSGVDKKJMNVM astrometric positions, such as Hipparcos S@QFDSR3GDM@QQNV@MFKDA@RDKHMDHR determined through monitoring of the system mechanical structure, and is a OQHL@QXLNSHU@SHNMENQSGD/1(,$3RTA - RXRSDL'NVDUDQ/1(,$3HRTM@AKDSN monitor all of the beampath, in particular SGDSDKDRBNOD"NTC!SQ@HMR$RS@AKHRGHMF a full solution for the narrow-angle baseline problem remains an outlying chal- KDMFDENQSGD/1(, SD@L%NQSTM@SDKX while this particular problem is an issue ENQ/1(, R@RSQNLDSQHBODQENQL@MBDHS CNDRMNSHLO@BSSGDE@HMSRS@QRBHDMBD 6NQJHMFVHSGHMSGDRDKHLHS@SHNMRVD expect to be able to reach magnitudes of K = 8 on the bright source in reasonable seeing conditions with the ATs, and companion faint source when a bright RNTQBDHROG@RHMFTOSGDRXRSDL2XM - SGDSHBBNGDQDMBDSHLDR@SSGD/3(DWODQH - ment of 1 2 seconds some times longer than the atmospheric coherence time bear this expectation MDBCNS@KDUHCDMBD EQNLD@QKX%(-(3.NODQ@SHNMRSG@SDUDM longer synthetic coherence times are possible under excellent seeing conditions will be explored to establish the full RDMRHSHUHSXDMUDKNODNESGDRXRSDL(MHSH@K astrometric performance of the system, pending a solution to the narrow-angle baseline problem, will be limited to the RTOONQSDCAXSGDOQDUHNTRVNQJNMSGD SNOHB3GDETKKXNODQ@SHNM@KRXRSDLVHKK although this will require not only nearperfect operations, but also almost unrealistically well-suited pairs of bright sources with almost similarly bright secondary astrometric reference sources situated well within an isoplanatic angle (of VGHBGSGDQD@QDMNJMNVMDW@LOKDRHM SGDRNTSGDQMGDLHROGDQD.TQDWODBS@ - tion is that, for a reasonable science programme with a sample size of at least ten, the median performance limit of the system (accounting for instrument, good but not great atmospheric conditions, realistic source brightnesses, and realis- Clearly, our goals are to push beyond SGDRDKHLHSR'NVDUDQDUDM@SSGDRD 2BHDMBDB@RDENQ/1(, The general science case that motivated SGDCDUDKNOLDMSNE/1(, HRCHRBTRRDC HM#DKOK@MBJDDS@K OQHL@QX CQHUDQENQSGD@RSQNLDSQHB@RODBSNE/1( - MA s functionality is the detection and BG@Q@BSDQHR@SHNMNEDWSQ@RNK@QOK@MDSR At a distance of 10 parsecs, a star with (Figure 5) clearly within the reach of DUDM/1(, RHMHSH@KB@O@AHKHSHDR3GDRD objects are, by design, particularly well RTHSDCENQ/1(, RHMBDSGDRS@QNEHMSDQ - R bright channel with a strong signal upon VGHBGSNEQHMFDSQ@BJ Many of the extrasolar planets that are nearby have already been detected through the efforts of teams using the Q@CH@KUDKNBHSX15SDBGMHPTD6GHKDSGHR could be considered by some to be a frbnnotne/1(, RNOONQSTMHSXSNCHR - cover new worlds, it in fact greatly expe- CHSDR/1(, R@AHKHSXSNBNMSQHATSDRHFMHj - B@MS@RSQNOGXRHB@KJMNVKDCFDSNSGDjDKC A major limitation of the RV technique is the uncertainty in planetary mass due SNSGDK@BJNEJMNVKDCFDNENQAHS@KHMBKHM@ - tion: RV is essentially a one-dimensional SDBGMHPTD@MCG@RSGHRKHLHS@SHNMATHKSHM /1(, R@RSQNLDSQXAXBNMSQ@RSHR@M inherently two-dimensional approach that G@RMNRTBGQDRSQHBSHNMR RSGDjDKCNE extrasolar planetary science progresses from the discovery phase to the more CDS@HKDCBG@Q@BSDQHR@SHNMOG@RDRODBHjB JMNVKDCFDNESGDOK@MDS@QXL@RRDRVHKK help open up the considerations of planetary composition and structure that are MNVADHMFONMCDQDC3GD15CDSDBSHNMR made to date provide a roadmap for /1(, SNOQNUHCDBNMSQHATSHNMRSNSGD jdkcq@ohckxvghkdhsadfhmrsgdlnqd lengthy process of exploring its own TMHPTDCHRBNUDQXRO@BD+@TMG@QCSDS@K (SRGNTKCADMNSDCSG@SQDBDMSQDRTKSR probing the limits of large single-aperture telescope astrometry are starting to push The Messenger 134 December

5 (Pixels) Figure 5. Apparent astrometric signature of our Sun as a function of time at a distance of 10 parsecs, DWGHAHSHMFSGD@RSQNLDSQHBQDkDWLNSHNM@RRNBH@SDC VHSG@KKNESGD2NK@Q2XRSDLOK@MDSR)TOHSDQCNLH - nates, followed by Saturn and the other two gas FH@MSR$@QSGRRHFM@STQDHRL@RJDCAXSGDOKNSKHMD QDFHLD+@YNQDMJNDS@K'NVDUDQ these experiments are limited to dense jdkcrners@qrnerhlhk@qaqhfgsmdrrkhlhs - ing their utility, particularly for extrasolar planetary investigations (although they are exciting new tools for exploring glob- TK@QBKTRSDQR@MCNSGDQRTEjBHDMSKXCDMRD QDFHNMRNESGDRJX Many other astrometric applications exist ENQ/1(, ADXNMCITRSDWSQ@RNK@QOK@MDSR Determination of parallax is possible with proper selection of the secondary QDEDQDMBDRS@QCDkDBSHNMNERS@QKHFGSCTD to general relativistic effects could be DWOKNQDCDF@R)TOHSDQO@RRDRBKNRDSN NMDNESVNRS@QRADHMFEDCHMSN/1(, orbits of small Solar System bodies could ADSQ@BJDCVHSGTMOQDBDCDMSDC@BBTQ@BX VHSG/1(, CXM@LHBRNENAIDBSRMD@Q SGD&@K@BSHB"DMSQDBNTKCADSQ@BJDCVHSG accuracy beyond existing studies and additional applications surely exist that G@UDMNSXDSADDMBNMRHCDQDC For faint object and phased-referenced HL@FHMFRBHDMBD/1(, NODMRTO@ realm of phase space that, up until now, had been off-limits to optical HMSDQEDQNLDSQX&@K@BSHBBNQDRBNMRSHSTSD an obvious class of objects that is of considerable interest for high resolution NARDQU@SHNMR%QHMFDSQ@BJHMFKHLHS@SHNMR G@UDTMSHKMNVKHLHSDCRTBGVNQJSN sources that could be self-referenced, RDQUHMF@RSGDHQNVMEQHMFDSQ@BJHMF RNTQBDATS/1(, VHKKAD@AKDSNRHCD - step this limitation through use of its faint RNTQBDBG@MMDK3GDMDDCENQ@MD@QAX AQHFGSRNTQDENQEQHMFDSQ@BJHMFHRHSRNVM KHLHS@SHNMATS@PTHBJRTQUDXNESGD appropriate catalogues shows it to be far less of a one than the previous limits (van!dkkdds@krdddw@lokdhm%hftqd.sgdqnaidbsbk@rrdrhmbktcdxntmf stellar objects (especially those that are deeply embedded in dusty shells), asteroid shape mapping and density determination, imaging of evolved stars, and ONRRHAKXAQNVMCV@QE@MFTK@QCH@LDSDQR As with the astrometric possibilities, L@MXNESGD@OOKHB@SHNMRNE/1(, E@HMS object mode await the creativity of the ESO community to exploit it in new and TMDWODBSDCV@XR Description of the system 3GD/1(, HMRSQTLDMSG@R@RTEjBHDMSKX DWO@MRHUDENNSOQHMSTONMSGD5+3(HMEQ@ - structure that we consider it to be more NE@E@BHKHSXSG@MLDQDKX@MHMRSQTLDMS RHFMHjB@MSBNLONMDMSNE/1(, HRSGD existing infrastructure, including the ATs, (Pixels) Figure 6. Example faint source for observation with /1(, -332.%(HL@FDNE$2.3GDNAIDBS HR@2DXEDQSF@K@WXSNNCHLENQCHQDBSEQHMFDSQ@BJ ing at K > ~ ATSMD@QAXSGDQDHR@AQHFGSRNTQBD '# KSG@SB@MADTRDCSNOG@RDTO SGDHMSDQEDQNLDSDQ UTs, standard delay lines (DLs), tele- RBNODUHRHAKDKHFGSSHOSHKSSQ@BJHMFRXRSDL 231 /,TKSH OOKHB@SHNM"TQU@STQD Adaptive Optics (MACAO) on the UTs, HMSDQEDQNLDSDQHMEQ@QDCSHOSHKSSQ@BJHMF RXRSDL(1(2U@QH@AKDBTQU@STQDLHQQNQR (VCMs) on the delay lines and now also part of the new star separator subsystem, the DL alignment subsystem, and when /1(, HRTRDCVHSGSGD43RUHAQ@SHNM BNMSQNK 2HFMHjB@MSTOFQ@CDRSNSGD5+3(HMEQ@ - RSQTBSTQDSG@SADMDjSSGDDWHRSHMFHMRSQT - ments are a new, improved alignment source MARCEL (which replaces the pre- UHNTRTMHS+DNM@QCN@MC@MDVQDkDB - SHUDLDLNQXMDSVNQJ1,-SG@SED@STQDR improved throughput and reduced K@SDMBXSHLDR /1(, RODBHjBRTARXRSDLRSG@S@QQHUDC en masse during and in advance of the /1(,!HF!@MFADF@MVHSGSGD2S@Q 2DO@Q@SNQR232R3GD232RRDO@Q@SD the light of two astronomical objects with separations 1 60 arcseconds and feed it HMSNSVNO@Q@KKDK5+3(NOSHB@KAD@LSQ@HMR 3GD232BNLODMR@SDRENQjDKCQNS@SHNM stabilises the beam tip-tilt and adjusts the 10 The Messenger 134 December 2008

6 Chopping and/or counter-chopping on the bright or faint source has also been implemented in the STS design; two units RODBHjBSNSGD The Differential Delay Lines (DDLs) were are responsible for providing slight delay offsets between the primary and quality cat s eyes in vacuum, displaced on parallel beam-mechanics by means of two-stage actuation, with a precision NEMLNUDQ@RSQNJDKDMFSGNELL Over the full range, a bandwidth of about 'YHR@BGHDUDC/DODDS@K The Fringe Sensor Units (FSUs) are de - signed to provide high precision fringe phase measurements with a goal of 1 nm rms (corresponding to 3N achieve this, careful calibration procedures were developed, with special attention given to the achieved measure- LDMSKHMD@QHSX@MCQDOD@S@AHKHSX3GDPT@K - ity of the FSU calibration is crucial in order to achieve the ultimate Central to successful FSU calibration HRSGD/1(, LDSQNKNFXRTARXRSDL /1(,$3+DUDPTDDS@KCDRHFMDC AX$2.@MCSGD(MRSHSTSDNE,HBQNSDBG MNKNFXNE-DTBGSDK(,33GD/1(,$3 source, based upon a frequency- RS@AHKHRDC-Cl8@FK@RDQOQNUHCDCAX(,3 and calibrated with the help of the Max- /K@MBJ(MRSHSTSEXQ0T@MSDMNOSHJ@KKNVR nanometre-level pathlength measurements to be attempted with an imper- EDBSKXRS@AKDHMSDQEDQNLDSDQ3GDNUDQ@KK BNLOKDWHSXNE/1(, HRB@QDETKKXADHMF addressed from software standpoint with comprehensive operations software and astrometric data reduction software (Elias DS@K3TAARDS@K@RVDKK ETQSGDQRHFMHjB@MSDEENQSHMRTOONQSNE /1(, SG@SG@RMNVADDMQDSHQDC but bears special mention, is the Fringe 3Q@BJHMF3DRSADCSG@SV@RDLOKNXDC extensively over the past two years to 2@GKL@MMDS@K3GHRSDRSADC E@BHKHSXV@RATHKS@S,/$K@ANQ@SNQHDRHM &@QBGHMFVHSGSGD@HLNERHLTK@SHMFSGD 5+3(@MCHMBKTCDC%24R@MNOSHB@KO@SG CDK@XBNMSQNKKDQ/1(,$3@MCHMGNTRD ATHKSCDK@XKHMDR 6GNHR/1(, More than the collection of hardware and software mentioned in the previous RDBSHNM/1(, HR@O@QSMDQRGHOVNQJHMF towards the goal and rewards of dual- AD@LHMSDQEDQNLDSQX/1(, HMBKTCDR RHFMHjB@MSBNMSQHATSHNMREQNLANSG$2. &@QBGHMF@MC$2./@Q@M@K@MC$2/1( O@QSMDQRSGD&DMDU@.ARDQU@SNQXSGD,@W/K@MBJ(MRSHSTSEXQ RSQNMNLHDHM Heidelberg, and the Landessternwarte - butions from Leiden University, the Ecole /NKXSDBGMHPTD%!C!Q@KDCD+@TR@MMDSGD NMSGD/1(, OQNIDBSHMBKTCD3-.@MC 3G@KDR KDMH@2O@BD 2BGDCTKDENQ/1(, "NLLHRRHNMHMFNE/1(, HRRBGDCTKDCSN NBBTQSGQNTFGNTS/DQHNCBNUDQHMF ENTQQTMRNEQNTFGKXSDMC@XRD@BG/ observing will concentrate on simple system operations involving feeding only a RHMFKDRS@QHMSNSGDRXRSDL.OSHLHR@SHNM NEEQHMFDSQ@BJHMF@KFNQHSGLRVHKKKD@C to fundamental system characterisation, including night-to-night repeatability, absolute visibility amplitude tests and LD@RTQDLDMSRNEKHLHSHMFL@FMHSTCD /DQHNCVHKKG@UD@RHLHK@QRDSNEBNL - missioning runs, but will expand testing to full dual-star operations, with initial SDRSRNESGD@RSQNLDSQHBNARDQUHMFLNCD (ERTBBDRRETKRBHDMBDUDQHjB@SHNMNARDQ - U@SHNMRVHKKRNNMENKKNV@MC/1(, astrometry will be released to the com- LTMHSXSGDQD@ESDQ imaging Already there are plans to expand the scope of dual-beam interferometry at the 5+3((MO@QSHBTK@QSGDRDBNMCFDMDQ@SHNM 5+3(HMRSQTLDMS&1 5(38&DMDQ@K 1DK@SHUHSX M@KXRHRUH@5+3(MSDQEDQNLDSQ8 $HRDMG@TDQDS@KHRRODBHjB@KKXCD RHFMDCSNNODQ@SDHM@/1(, KHJDE@RGHNM but using not just two, but all four UTs - etry on six baselines on faint (K RNTQBDR@SSGD&@K@BSHB"DMSQD2TBG observations have the potential to probe highly relativistic motions of matter close to the event horizon of Sgr A*, the mas- RHUDAK@BJGNKD@SSGDBDMSQDNESGD,HKJX 6@X %NQ/1(, MDVB@O@AHKHSHDRHSOQNUHCDRSNSGD5+3(ENQ both astrometry and faint object science open up wide new frontiers in astronomi- B@KHMSDQEDQNLDSQX$WODQHDMBDG@RRGNVM that in such circumstances, the most interesting results come from unexpected PT@QSDQR 4RDETK/1(, )@QFNM Term ADRS DDL ESPRI FSU PRIMET {DV STS References Description Astrometric Data Reduction Software Differential Delay Line $WNOK@MDS2D@QBGVHSG/1(L@ Fringe Sensor Unit /1(,,DSQNKNFX micro-arcsecond Star Separator #DKOK@MBJD%-DV RSQNMNLX1DUHDV 52, 199 #DKOK@MBJD%DS@K O22 $HRDMG@TDQ%DS@K2/($"NMEDQDMBD 2DQHDR $KH@R-,DS@KHM( 42XLONRHTL 5NK +@MD!%"NK@UHS@,, ) +@MD!%DS@K2/($"NMEDQDMBD2DQHDR 4006, 452 +@TMG@QCS1DS@KHM( 42XLONRHTL 248, 417 +@YNQDMJN/%DS@K +DUDPTD2 DS@K/QNB2/($ -HIDMGTHR)DS@KHM2/($"NMEDQDMBD 2DQHDR /DOD%DS@K2/($"NMEDQDMBD2DQHDR 2@GKL@MM)DS@K@HM2/($"NMEDQDMBD 2DQHDR 2@GKL@MM)DS@KAHM( 42XLONRHTL 5NK 2G@N,"NK@UHS@,, 3TAAR1DS@KHM( 42XLONRHTL 5NK U@M!DKKD&3DS@KHM2/($"NMEDQDMBD 2DQHDR5NK The Messenger 134 December