Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.mrao.cam.ac.uk/projects/OAS/pmwiki/uploads/MROIDelayLine.Requirements/INT-402-MIS-0000.pdf Äàòà èçìåíåíèÿ: Mon Jun 25 18:43:27 2007 Äàòà èíäåêñèðîâàíèÿ: Sat Mar 1 04:04:52 2014 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: ï ï ï ï ï ï ï ï ï ï ï ï

A System Design for the MRO Interferometer
Revision: 1.31 Date: October 2002 Contents
1 Intro duction 2 Aims and ob jectives 2.1 Strategic aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Science ob jectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Overall concept 4 Functional requirements 4.1 Sensitivity . . . . . . . . . . 4.2 Wavelength range . . . . . . 4.3 Spectroscopic resolution . . 4.4 Spatial resolution . . . . . . 4.5 Imaging and number of teles 4.6 Automation . . . . . . . . . 4.7 Observing . . . . . . . . . . 4.8 Scientific scope . . . . . . . 5 The 5.1 5.2 5.3 5 6 6 6 8 10 10 10 11 11 11 12 12 12

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site 14 Seeing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Seismic issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 16 16 16 17 17 17 18 18 19 20 20 20 22

6 Error budgets 6.1 The interferometric signal-to-nois 6.2 Global wavefront error . . . . . . 6.2.1 Wavelength dependencies 6.3 Throughput . . . . . . . . . . . . 6.4 Limiting magnitude . . . . . . . 6.4.1 Assumptions . . . . . . . 6.5 Fringe-tracking SNR . . . . . . . 6.6 Tip-tilt sensor . . . . . . . . . . .

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7 Array configuration 7.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Meeting the requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


8 Uni 8.1 8.2 8.3 8.4

t telescop es - UTs Requirements/issues Proposed solutions . Budget . . . . . . . . Questions for further

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9 Tip-tilt correction 30 9.1 Requirements/issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 9.2 Proposed solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 9.3 Questions for further study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 10 Ada 10.1 10.2 10.3 11 Bea 11.1 11.2 11.3 11.4 11.5 32 ptive optics Requirements/issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Proposed solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Timeline and budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 35 35 35 36 36 37 38 38 38 39 39 39 39 41 41 41 41 42 42 42 43 43 43 43 43 44 44 44 44 45

m transp ort Requirements/issues Simplest solution . . Alternative designs . Budget . . . . . . . . Questions for further

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12 Delay Lines 12.1 Requirements/issues 12.2 Proposed solutions . 12.3 Budget . . . . . . . . 12.3.1 Equipment . 12.3.2 Staff costs . . 12.4 Questions for further

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13 Beam combination 13.1 The overall beam combination room 13.1.1 Requirements/issues . . . . . 13.1.2 Overall concept . . . . . . . . 13.1.3 Questions for further study . 13.2 Beam compressor . . . . . . . . . . . 13.2.1 Requirements/issues . . . . . 13.2.2 Proposed solution . . . . . . 13.2.3 Questions for further study . 13.3 Fringe tracking beam combiner . . . 13.3.1 Requirements/issues . . . . . 13.3.2 Proposed solution . . . . . . 13.3.3 Questions for further study . 13.4 Science beam combiners . . . . . . . 13.4.1 Requirements/issues . . . . . 13.4.2 Proposed solution . . . . . . 13.4.3 Questions for further study .

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14 Detectors 14.1 Requirements . . . . . . . . 14.2 Proposed solution . . . . . . 14.2.1 Visible Wavelength D 14.2.2 Infrared Detectors . 14.3 Budget . . . . . . . . . . . . 14.3.1 Equipment . . . . . 14.3.2 Staff costs . . . . . . 14.4 Risks . . . . . . . . . . . . . 14.5 Questions for further study

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15 System Alignment 51 15.1 Requirements/issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 15.2 Proposed solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 15.3 Questions for further study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 16 Control System 16.1 Requirements/issues . . . . . . . . . . . . 16.1.1 Control system lifetime . . . . . . 16.2 Meeting the Requirements . . . . . . . . . 16.2.1 Organization of the controls group 16.2.2 Personnel resources . . . . . . . . . 16.3 Example MROI control system . . . . . . 16.4 Control architecture . . . . . . . . . . . . 16.4.1 Observatory Control System (OCS) 16.4.2 Telescope Control System (TCS) . 16.4.3 Trolley control . . . . . . . . . . . 16.4.4 Fringe Tracker (FT) . . . . . . . . 16.4.5 Science beam combiner . . . . . . 16.4.6 Data handling . . . . . . . . . . . 16.4.7 Alignment . . . . . . . . . . . . . . 16.4.8 Monitors . . . . . . . . . . . . . . 16.5 Action sequences . . . . . . . . . . . . . . 16.5.1 Observing queue . . . . . . . . . . 16.5.2 Startup . . . . . . . . . . . . . . . 16.5.3 Shutdown . . . . . . . . . . . . . . 16.5.4 Observing . . . . . . . . . . . . . . 16.6 Budget . . . . . . . . . . . . . . . . . . . . 16.7 Potential Compromises . . . . . . . . . . . 16.8 Questions for further study . . . . . . . . 17 Observation preparation and 17.1 Requirements/issues . . . . 17.2 Proposed solution . . . . . . 17.3 Questions for further study data ... ... ... reduc .... .... .... ti . . . 3 52 52 54 54 54 54 55 55 55 55 57 57 57 57 58 58 58 58 58 61 61 61 61 62 63 63 63 64

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18 Information management 65 18.1 Requirements/issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 18.2 Meeting the requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 18.3 Questions for further study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 19 Interferometer infrastructure 67

20 General infrastructure 69 20.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 20.2 Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 20.3 Unresolved problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 21 Technical risks assessment A A Science Case for the MRO Interferometer A.1 The need for an imaging array . . . . . . . . . . . . A.2 Imaging the birth of stars and planets . . . . . . . . A.2.1 Discs, gaps, and accretion flows . . . . . . . . A.2.2 Jets and outflows . . . . . . . . . . . . . . . . A.2.3 Residual dust discs . . . . . . . . . . . . . . . A.2.4 Uncovering low-mass companions to cool stars A.2.5 The role of angular momentum . . . . . . . . A.3 The life-cycle of stars . . . . . . . . . . . . . . . . . . A.3.1 The physics of mass loss . . . . . . . . . . . . A.3.2 Dynamical studies . . . . . . . . . . . . . . . A.3.3 Mass-loss in binary systems . . . . . . . . . . A.4 Imaging the hearts of active galaxies and quasars . . A.4.1 The broad-line region - BLR . . . . . . . . . A.4.2 The obscuring torus . . . . . . . . . . . . . . A.4.3 The inner narrow-line region - NLR . . . . . A.4.4 Jets and the radio-loud/radio-quiet dichotomy A.4.5 AGN overview . . . . . . . . . . . . . . . . . A.5 Other programmes . . . . . . . . . . . . . . . . . . . 73 74 74 74 75 76 77 77 78 78 78 79 79 80 81 81 82 82 83 83

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1

Intro duction

The following is a working draft of a system design for the Magdalena Ridge Observatory Interferometer. It attempts to proceed from a set of well-defined science goals to an implementation which is capable of meeting these science goals within the resources available. The presentation is mostly in the form of top-down design, even though the actual design process has been an iterative one in which the top-level requirements have been changed in the light of implementation constraints. This document is of necessity incomplete since it represents a work in progress, and some parts of the design have not reached the same state of maturity as other parts of the design. Those items covered in detail have been considered in some depth, but where possible we have attempted to highlight areas that require further study. In many cases a final decision has not been made on the direction to take in a given design, but what is presented is either a set of design choices or a route towards making the design choice. Some attempt has been made to ensure that the different parts of the design are at least consistent with one another, but this has not been always been possible.

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2

Aims and ob jectives

For the purposes of this document we have assumed that funding is available to build an interferometer at the Magdalena Ridge Observatory (MRO) in New Mexico. Hereafter we will call this interferometer the MROI. Discussion of other developments on this site, e.g. a fast-tracking single dish telescope, is left to other documents and will not be considered further here.

2.1

Strategic aims

The context for the MROI is principally determined by a number of other large interferometric pro jects that are underway at present. These are summarized in the table below, where we have made what we feel are reasonable assessments of where these pro jects will be on the timescale for initial operations with the MROI. Facility & location CHARA: Mt. Wilson, USA ISI: Mt. Wilson, USA NPOI: Flagstaff, USA SUSI: Narrabri, Australia KECK: M. Kea, Hawaii VLTI: Paranal, Chile N 6 3 6 2 2+4 4+3 D/m 1.0 1.7 0.5 0.1 10.0 + 1.8 8.0 + 1.8 B /m 350 85 440 640 140 200 Primary operational roles in 2005+ Optical and near-IR imaging 10 µm astrophysics Optical astrometry and imaging Optical visibilities Near-IR astrometry and 10 µm visibilities Near-IR astrometry and 10 µm visibilities

Table 1: Ma jor optical/infrared astronomical interferometer arrays. In this summary only ma jor facility instruments which we expect to be operating in the 2005+ time-frame are included. N , D, and B refer to the number of telescopes in the array, their aperture size, and the maximum baseline achievable. Both the Keck and VLTI arrays have the possibility of using a mixture of large and more modestly sized telescopes. All figures are approximate.

While the primary operational roles of the arrays in Table 1 span a broad range of capability, it is noticeable that none of them combines large numbers of 1 m-class collecting apertures. With this background in mind, the basic aims for the MROI have been established as follows: · That it be a unique, world class interferometer array. A principal feature of the array must be an ability to undertake important and topical science programmes that other arrays are unable to pursue. · That it have a broad rather than specialist science remit. This is required so as to attract support from a wide community both in the US and the UK, and is likely to be a necessity for securing support for the continued operation and maintenance of the array once it has been commissioned. · That it advance education in New Mexico. To do this it must deliver "spectacular" science so as to attract and enthuse both student and general public audiences. It should also allow for the training and development of undergraduate and graduate scientists, both to support the astronomical facilities and research programs in New Mexico, and to provide a pool of skilled technical personnel for the wider labor market. · To further the US Navy's strategic goals it must also provide the capability to investigate combining adaptive optics with interferometry.

2.2

Science ob jectives

To meet these aims, we have developed a preliminary science case which is detailed in an appendix to this document. Scientists from over a third of the Astrophysics Departments in the UK contributed 6


towards this overview, and their number gives some indication of the range of possible applications of interferometric imaging to contemporary astrophysics. Their initial document was augmented with additional programmes that further matched the interests of astronomers in New Mexico. A key feature of the resulting case is that it demonstrates the breadth of science possible with an interferometer whose sensitivity, angular resolution and imaging capability are appropriately chosen. The broad remit of the case covers three primary topical branches of astrophysics: 1. Active galactic nuclei: resolved imaging of the nuclear dust component of AGN, the broadline-region (BLR), synchrotron jets and nuclear and extra-nuclear starbursts. 2. Stellar accretion and mass loss: via winds, jets, outflows, and Roche-lobe overflow. Studies of examples in single and binary systems in themselves and as analogs for AGN jets and beams. 3. Star and planet formation: the detection and characterization of protostellar disks. Accretion, disk-clearing, fragmentation and stellar duplicity over all mass ranges. Stellar rotation in clusters and angular momentum distribution.

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3

Overall concept

To address the three primary science missions outlined above, i.e. studies of active galactic nuclei, stellar accretion and mass loss, and star and planet formation, our overall design concept for the Magdalena Ridge Observatory Interferometer is that of a state-of-the-art facility imaging array. A critical assessment of existing interferometric arrays suggests that the principal distinguishing feature of the MROI will be its ability to image faint and complex sources in a model-independent manner. This capability has been singularly lacking from the current generation of optical/infrared interferometers, and it has undoubtedly been the ma jor stumbling block to optical/infrared interferometry being championed by the broad astronomical community. Within this framework the ma jor features we envisage for the MROI can be broadly described as follows. For information, brief statements of the technical and scientific drivers for these are included below as well: · A basic unit-telescope size large enough to provide the sensitivity to permit fringe tracking on reasonable samples of all of the primary targets defined in the top-level science mission. · An approach to adaptive optics that satisfies the desire to explore the combination of interferometry with adaptive optics beyond tip and tilt. We expect this to be pursued with the goal of impacting only positively on the top level science mission of the MROI. · Coverage in wavelength so as to allow imaging in key optical diagnostic lines and with access to the photometric bands in which the top-level science targets are bright and where the best interferometric sensitivity can be realized. · A sp ectroscopic capability that allows useful isolation of atomic lines and molecular features from their nearby continuum in stellar sources, and velocity resolved imaging in the very nearest AGN. · An interferometric field-of-view to allow imaging telescopes, when bandwidth constraints allow this. be accommodated without replacement of the beam targets identified in the current version of the science combiner. across a single primary beam of the unit Larger fields of up to ±1 arcsecond may relay optics, but are not required for the case and would require a specialized beam

· A choice of interferometric baselines matched to the full range of science targets in the top-level science mission. This requires a reconfigurable array, with movable telescopes, and a probable mode of operation where the array configuration is adjusted as required, similar to what is performed for the VLA. The array should have a low-resolution configuration to allow comparison of its interferometric observations with complementary data from facilities such as HST, ALMA, NGST, Keck AO etc. · A sufficient numb er of telescopes so as to allow at least 5 â 5 pixel model-independent imaging for fully resolved ob jects. For sources with a strong unresolved component, images as large as 10 â 10 pixels should be achievable. Since many of the sources identified in the top-level science mission evolve on monthly or weekly timescales, the number of telescopes must be sufficient to allow imaging without relocation of the array elements. · A level of automation and reliability to allow exploitation of the times of best seeing for the most challenging astronomical pro jects, and to minimize overall staffing and running costs. The large numbers of replicated subsystems in the array make this a far more demanding task than for a single telescope observatory. · An op erational mo del that envisages only facility service observing. Trained telescope operators would run the array, overseeing the automated sequencing of observations with minimal intervention. 8


· A scientific prioritization that identifies the delivery of model-independent imaging of faint sources as a mandatory requirement for the MROI in its first phase of operation. Other capabilities, such as polarimetry and wide-field imaging should be accommodated where possible. · An overall technical design philosophy that allows for potential expansion in basic capability in the longer term. We expect that additional technical capabilities will be added to the array on the basis of maintaining its position as a world-class astronomical facility. One good example of this is a polarimetric capability, where we would not wish to restrict future development of this area if it became a focus in the longer term. In summary, our concept for the MROI is a reconfigurable multi-element moderate-sized aperture optical/infrared interferometer. Its primary function will be to deliver reliable imaging of faint and complex astronomical targets. We believe that this deliverable alone can realistically be expected to engage the public, politicians, funding agencies and the wider astronomical community, all of whom will be needed to support the MROI beyond its initial commissioning phase. Exactly how this can be achieved will be explored in the following section.

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4

Functional requirements

The goals highlighted above imply a number of overall design parameters which are summarized below and expanded on in later sections of this report. While later sections of this document divide the array into a number of subsystems, many of the arguments relating specific parts of the design to specific requirements necessarily cross subsystem boundaries. As a result in some cases the justification for a particular design feature refers both forwards and backwards within the document.

4.1

Sensitivity

The extra-galactic component of the top-level science mission for the MROI sets the basic requirement for its desired sensitivity. At K-band magnitudes fainter than about 11 the very closest and brightest active galactic nuclei just start to become visible. However, it is not until a K-band sensitivity of 14 is reached that of order 100 targets become visible in the Northern celestial sky. This is a large enough sample that even if as many as 75% of these sources are too large to be studied interferometrically a large enough number of sources amenable to detailed study will remain. It is worth noting that at this limiting sensitivity many tens to hundreds of examples of al l of the sources categories enumerated in the top-level science mission will be observable. For example at a K magnitude of 14 all condensed sources down to the hydrogen burning limit will be detectable at the distance of the Taurus-Auriga star forming region. As will be expanded on later, the concept of sensitivity for an optical/infrared interferometer has a very precise meaning. In particular, what we mean by a "sensitivity" of 14th magnitude in a given band is that some form of fringe tracking -- either phase or envelope tracking -- be possible with a source of that brightness. Once the interferometer is stabilized against the atmospheric fluctuations in this manner, science observations at much lower light levels can be made in bandpasses not being used for the fringe tracking. In practice, this sensitivity requirement for the array is particularly challenging and will require careful attention to detail at all stages of the array design. Most important will be the optimization of throughput and wavefront quality along the whole optical train. The analysis presented in the subsequent section of this report shows that this top-level sensitivity requirement can be met with unit telescopes employing tip-tilt correction with an aperture diameter of 1.4 m when the seeing is better than 0.75 arcseconds by tracking the atmospheric fluctuations in the H-band. The reduction in thermal background in going from 2.2 µm to 1.65 µm is sufficient to allow existing near-infrared detectors to be used to achieve this goal, and thus eliminates one of the few ma jor risk elements associated with the array subsystems.

4.2

Wavelength range

From an astronomical perspective most scientists would agree that the MROI should attempt to exploit as broad a range of wavelengths as possible, perhaps from the ultraviolet at 350 nm to the mid-infrared at 10 µm. However, many years of experience with existing interferometric arrays has made clear that at short wavelengths the rapid deterioration in r0 and t0 means that it is essentially impossible to compensate for the atmosphere usefully. Similarly, towards the mid-infrared it is well established that the huge thermal background forces the use of very large collecting apertures if good sensitivity is to be achieved. With these boundary conditions in mind, we are proposing a top-level design goal that the MROI operate between 0.6 and 2.4 µm. This will allow observations at H-, a key line for active galaxies and other energetic sources, but will also give access to the K-band which will be crucial for penetrating dust in AGN and YSOs. Unless aperture sizes comparable to those being use for mid-infrared observations at the Keck and VLTI arrays are envisioned, there is no compelling scientific case for extending this range to longer wavelengths. Measurements at wavelengths shorter than 0.6 µm should be explored on 10


a best-effort basis. i.e. with due regard to not compromising the array's capabilities between 0.6 and 2.4 µm. In practical terms, the wide wavelength range required for the array means that the coatings along the optical train will need to have excellent broadband amplitude and phase performance. Furthermore, it is likely that the ma jority of the optics should be reflective. If existing technologies are to be used, then at least two types of detector types will be needed.

4.3

Sp ectroscopic resolution

The spectroscopic resolution of the MROI that we are proposing is fairly straightforwardly set by the type of astronomical programmes we have in mind. Faint source imaging will require rather broad continuum bandpasses, while studies of