Figure 3.2.5/1: A simplified view of the ISO Science Operations Centre (SOC).
ISO homepage http://isowww.estec.esa.nl
ISO is providing astronomers with a unique facility of unprecedented sensitivity at 2.5-240 µm for detailed exploration of the Universe, ranging from objects in the solar system right out to distant extragalactic sources. The satellite is dominated by a large cryostat that contained >2000 litres of superfluid helium at launch to maintain the Ritchey- Chrétien telescope, the scientific instruments and the optical baffles at 2-8 K. The telescope has a 60 cm-diameter primary mirror. A pointing accuracy at the arcsec level is provided by a 3-axis stabilisation system of reaction wheels, gyros and optical sensors. ISO has four scientific instruments:
These instruments were built by international consortia of scientific institutes and delivered to ESA for in-orbit operations. The highly elliptical operational orbit has an apogee of 70 600 km and a perigee of 1000 km. ISO is expected to remain operational beyond the end of 1997. In keeping with its role as an observatory, more than half of ISO's observing time has been made available to the general astronomical community via the traditional route of Calls for Observing Proposals, followed by peer review.
Advice on all scientific aspects of ISO is provided by an ISO Science Team, chaired by the Project Scientist. It includes the instrument PIs, a group of five Mission Scientists representing the interests of the general community, plus one representative from each of ISAS (Japan) and NASA (USA) ESA's international partners, who provide a second ground station (Goldstone) and resources to provide manpower for shift operations to maximise the time per day during which useful scientific data can be obtained. Support for the review process of the ISO-Science Operations Centre (SOC) was and is provided by a group of experienced and independent-of-ISO people known as the 'Rolling Review Board'.
During the first half of 1995, the system-level testing of ISO's Flight Model was successfully completed at ESTEC, and the ISO-SOC team who were at ESTEC for the development phase relocated to Villafranca for the preparations to support ISO's orbital operations. There were two full end-to-end tests involving the satellite and all of its ground segment in order to verify the complete operational chain. Following these tests, ISO was shipped to Kourou for its launch campaign while the simulations, training and final preparations were completed at Villafranca.
ISO enjoyed a perfect launch by an Ariane 44P into its planned elliptical transfer orbit, with departure from Kourou at 01.20 UT on 17 November 1995, in the first second of the launch window. The launch had been delayed from 10 November, which required a major replanning of the Performance Verification (PV) Phase, a significant extra workload for the ISO-SOC. The first 21 days after launch were devoted to the Satellite Commissioning Phase. During this period, the operational orbit was attained, the cryo- cover (needed to close the cryostat for work on the ground) was ejected, all spacecraft subsystems were tested and found to be in excellent condition, first light for all instruments was achieved, engineering checks were successfully made of all the four scientific instruments and the integrated ground segment was validated.
The performance of all the spacecraft's subsystems continues to be excellent - in many respects much better than specified. The cryogenic system is providing all the expected temperatures. From one direct measurement of the liquid helium remaining in the tank (made in September 1996) plus an estimated mass flow rate of the boiled-off gas, the predictions are that ISO's in-orbit lifetime will be 24±1 months, well above the requirement of 18 months. Regarding optical performance, images of point sources have been made with ISOCAM clearly showing up to the sixth Airy diffraction ring. Analysis of the data has shown that the ISO telescope is diffraction-limited down to at least 15 µm. All nominal modes of the Attitude and Orbit Control system have been successfully verified, with the pointing performance also substantially better than the specifications. The current estimate of the short-term jitter is about 0.5 arcsec (2 sigma, half cone over a 30 s period) as compared to the specification of less than or equal to 2.7 arcsec; the blind pointing is better than 2 arcsec (2 sigma, half cone) as compared to the specification of less than or equal to 11.7 arcsec.
During the PV Phase of 56 days (from 8 December 1995 to 3 February 1996), in which the instruments were operated sequentially on a 4-day cycle, a detailed assessment of their in- flight performance was made, their core calibrations established and planned operating modes validated. The scientific instruments are all, functionally, operating very well. Initial difficulties with the scanning mechanism of one of the LWS Fabry-Pérot interferometers were successfully overcome.
The biggest in-orbit issue has been the effects on the sensitivity of the instruments caused by impacts of high energy cosmic rays on the IR detectors. These glitches primarily increase noise, but in some cases the effects are so severe as to necessitate modifications to instrument settings and recommendations for changes in observing strategy. ISOCAM's sensitivity is very close to pre-launch expectations. ISOPHOT, SWS and LWS are more affected and their sensitivities are less than originally predicted by factors (depending on detector type) of up to 5. The operating conditions of the detectors and the data processing algorithms are being optimised to maximise the instruments' performances.
Some 16.75 h/day - the time ISO spends outside the main parts of the van Allen belts of trapped particles - is available for science use; this is 45 min longer than had been anticipated prior to launch.
The execution of these critical initial phases of the mission went extremely smoothly, with schedules and timelines - laid down in the months before launch and including the replanning due to the launch delay - being followed very accurately. It was thus possible to start routine observing, with all instruments able to operate on every orbit, on 4 February 1996, in part testament to the quality of the work performed by the full ISO-SOC team in the hectic years of development prior to launch.
The majority of ISO's observing time, allocated on a 'per observation' basis, is made available to the astronomical community by the traditional route of Calls for Observing Proposals, followed by peer review. A pre-launch Call was issued in April 1994 and a post-launch Supplemental Call followed in August 1996. In addition to this Open Time, there is Guaranteed Time reserved for the groups involved in the preparation and operation of the ISO mission. These groups are: the four PIs and their teams, who built the ISO instruments; the five Mission Scientists; the Science Operations Team; NASA and ISAS, Japan.
As ISO is operated in a pre-planned, service observing mode, full details of all observations need to be prepared in advance. Entry of these full details for both Guaranteed and Open Time observations was made by the observer visiting the Proposal Data Entry Centre (PDEC) at ESTEC and set-up and run by SOC staff. (US observers were supported in a similar way by the Infrared Processing and Analysis Center). PDEC opened in December 1994 and, to July 1995, supported a total of 540 visitors.
Following completion of the PV Phase, the SOC provided observers with information on the in-orbit performance of the instruments and with advice on the best course of action. Nearly all of the 35 000 observations in the mission data base needed some tuning to take account of demonstrated, in-orbit performances. The majority of these changes were made by SOC staff following specifications from observers; some observers made their own updates using a remote login facility to PDEC and a relatively small number made return visits to PDEC.
The Supplemental Call for Observing Proposals was released in early August 1996, with all information being made available via ftp and the WWW, rather than the pre-launch paper form. By the deadline in early October, a total of 551 proposals had been received with an over-subscription factor in time of about 3.5. These proposals were reviewed by an ISO Observing Time Allocation Committee (OTAC). Recommendations were made for time for 330 proposals and proposers were notified of the results in early December 1996. The entry of full details of the observations will take place during the first months of 1997.
An overview of the main software modules and data flow in the SOC is shown in Fig. 3.2.5/1. As seen by the observer, the process starts with the preparation of a phase 1 proposal for submission to the OTAC, using an SOC-developed piece of software. Successful proposers use Proposal Generation Aids (either at ESTEC or IPAC, or by remote login) to enter full details of all their observations into the SOC's data bases. Checking of their inputs is done both by the Proposal Generation Aids and the Proposal Handling modules. When all is correct, the observations are stored in the Mission Data Base, a critical concept in the overall SOC architecture as it contains full details of all observations that ISO will make. Calibration observations are also stored in the same data base but can enter by a different route. The setting up of a daily timeline of observations is carried out by the Mission Planing Phase 1 system, which also uses a set of routines (Astronomical Observing Template logic) to convert automatically the user-entered parameters (right ascension, declination, wavelength, flux, spectral resolution, observation time or desired signal to noise, etc) into detailed commands to be passed to the Spacecraft Control Centre (SCC) for transmission to the spacecraft.
Figure 3.2.5/1: A simplified view of the ISO Science
Operations Centre (SOC).
As ISO has no onboard storage, the data are transmitted from ISO in realtime to the ground station. The SCC monitors essential housekeeping from the instruments for safety while the SOC uses a set of realtime modules (Real Time engineering Assessment/Quick Look scientific Assessment, RLA/QLA) to monitor the state of health of the instrument and to make an initial assessment of the data quality. Offline, each observation is processed into a set of three levels of products for storage in the archive and distribution to the observer. These are (a) essentially reformatted raw telemetry, suitable for the expert user, and (b) two levels of data, processed to a level suitable for the use of a general astronomer. The data products are distributed to users on CD-ROMs.
In addition to the Project Scientist, two ISO Instrument and Calibration Scientists come from the complement of SSD. The Astrophysics Division has the responsibility for ISO's scientific operations through the ISO-SOC team, comprising supernumerary staff, contact staff and persons seconded by the PI teams. This responsibility included firstly the development, integration and test of all the necessary software and operational procedures prior to launch and now the conduct of the operations themselves. Some 250 man-years of effort went into the ESA ISO-SOC development and operational preparations before launch. The size of the ISO-SOC in routine operations is some 80 people, with somewhat more in place during the PV Phase. The structures of the ISO-SOC team during the latter part of the development and operations phases are shown in Figs. 3.2.5/2 and 3.2.5/3, respectively.
Figure 3.2.5/2: The structure of ISO-SOC during the latter
stages of the development phase.
Figure 3.2.5/3: The structure of ISO-SOC during routine operations.
ISO's ground segment is working very smoothly, as confirmed by the Rolling Review Board's deliberations at the ISO-SOC mid- term review in October 1995. Most of the planned main observing modes have been successfully commissioned; the remaining modes (absolute photometry with ISOPHOT and one of the LWS Fabry- Pérot modes) were released to the community in early 1997. The polarisation capabilities of ISOCAM and ISOPHOT are expected to be released around mid-1997. Very efficient observing schedules with highly graded observations are regularly planned and executed; during the 16.75 h/day available for science use, an average of 45 observations is made, using 90-95% of the available time. The remaining time is used mainly for slews and engineering activities. Shipping of all data products to all observers started in early May 1996. By the end of 1996, all data products from ISOCAM, SWS, the LWS grating and the ISOPHOT spectrophotometer had been declared scientifically valid; the remaining products will be validated during the course of 1997. This is slower than originally planned but is a consequence of needing time to understand the details of the instruments' performance more fully. A data reprocessing capability has been added so that, as additional products become scientifically validated in a new release of the pipeline software, the older data can be reprocessed and reshipped to observers.
A post-operational phase of 3.5 years is foreseen to produce a homogeneous initial archive of all ISO data plus a set of software tools for fully-optimised processing of individual data sets. These tasks will involve the PI teams, national data centres and the ESA SOC. Also, following a period of concentration on the in-orbit operations, detailed definition of the post-operation phase is resuming. Sketch plans also exist for an 'active' archive phase for the 5 years thereafter.
Following the first ISO Science Workshop in ESTEC in May 1996, ISO's initial scientific results were published as a set of 91 letters in an ISO-dedicated issue of Astronomy & Astrophysics, November (II) 1996, the bulk relating to observations made early in the mission and particularly in the PV Phase. General features of these results include use of ISO's wide photometric capabilities to address energy budgets, use of its varied spectral capabilities to examine the chemistry and physics in detail, and extensive application of its mapping abilities at all wavelengths.
Wide-ranging sets of sources are being observed across ISO's entire wavelength range of 2.5-240 µm, providing significant insights into the ways that different objects emit energy, as well as specifying their bolometric luminosity. Early examples of sources with extensive emission in the previously-inaccessible 200 µm region include the cold Cham A01 cloud and the galaxy NGC 6090.
Spectacular spectra, covering up to six octaves of the electromagnetic spectrum, have been taken of many objects, ranging from NGC 7027 to M82 and the Circinus galaxy (see Fig. 3.2.5/4). A rich variety of atomic, ionic, molecular and solid state spectral features is being studied in great detail, many for the first time.
Figure 3.2.5/4: A 2.5-45 µm spectrum of the Circinus
Galaxy obtained with the SWS (Moorwood et al, A&A, 315, L109,
1996).
Spectra at the shorter ISO wavelengths are providing many detections of H2O, CO2,CO,CH3OH and CH4 features in absorption, allowing characterisation of the ice mantles on grains with unprecedented accuracy. Absorption features corresponding to solid 13CO2 have also been detected.
Emission from the well-known polycyclic aromatic hydro-carbon (PAH) features at 3.3, 6.2, 7.7, 8.6 and 11.3 µm dominates the mid-IR spectra of many galactic sources. The integrated emission from all types of galaxies, including spirals, starbursts and Seyferts, shows PAH features. Distinct and complicated variations in the relative strengths of the features are seen, thereby providing a method for examining formation and destruction mechanisms, size and hydrogenation ratios.
Both spectrometers have seen water in a wide variety of sources, including W Hya, NML Cyg, GL2591 and HH 54B. Both emission and absorption lines have been detected. So many transitions have been observed different wavelengths that theoretical analyses can applied to deduce physical conditions in the emitting regions and show whether, in fact, thermal water vapour emission is a prime coolant of cold shocked regions.
ISO has detected emission from molecular hydrogen, including the long-sought S(0) pure rotational transition 28 µm, in sources ranging from Jupiter and Saturn, through Cep A, out to M82 and NGC 6946. These detections provide a more direct method of determining the molecular hydrogen content of these objects than was previously available. The 56 µm line of HD has also been detected.
ISO's imaging capabilities have already been widely applied. Examples include ISOCAM's very detailed IR images of the M51 Whirlpool Galaxy, which was also ISO's first light object. Subtle differences are seen between the images at 7 µm and 15 µm. Longer wavelength images have been made by ISOPHOT. Other galaxies addressed to date include M101, NGC 6946 and the Antennae. Maps covering 0.75x0.75° of the RO Oph cloud at 7 µm and 15 µm show a rich pattern of interactions in the complicated region, including, inter alia, regions opaque even to ISOCAM. Images of Supernova Remnants have been obtained with ISOCAM and ISOPHOT. The circular variable filters in ISOCAM have been used for spectral imaging, for example, of the planetary nebula NGC 6543, with very clear differences being found in the structure of the object at different wavelengths.
Other early results include detections of water in absorption towards the Galactic Centre, ISO maps of the Hubble Deep Field, additional ISOCAM images of M83 (Fig. 3.2.5/5) and a detection of the gravitationally-lensed arc in Abell 370.
With new instruments offering unprecedented combinations of wavelength coverage, sensitivity and spatial and spectral resolutions in the IR region, ISO is making significant impacts on nearly all areas of astronomy.
Figure 3.2.5/5: An ISOCAM image, at 7 m with a 6 arcsec per
pixel field of view, of the spiral galaxy M83 (from M. Sauvage).
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