next up previous contents index
Next: 4.4 Overview of the Up: 4. ISO Operations Previous: 4.2 The Ground Segment


4.3 Observing with ISO

4.3.1 Observing strategy

ISO was operated in a pre-planned manner without any significant routine real-time interaction. Thus, all observations had to be specified in full detail by the proposers. For a variety of reasons including efficiency and sky coverage constraints, observing time was distributed on a `per observation' basis as was the case e.g. for EXOSAT, and not on a `per shift' basis as it is the case for many ground-based observatories and for IUE. However, for operational reasons, no guarantees could be given that a particular observation would be executed.

Individual observations (or concatenated chains) were the basic building blocks for the observational schedule, which was built in units of `revolutions'. A revolution was one rotation of ISO about the Earth; as ISO's orbital period was close to 24 hours, a revolution was roughly equivalent to a day. In the schedule -- and also in the ISO Data Archive -- every observation is uniquely identified by an 8-digit number, called the TDT (for `Target Dedicated Time') number. The first three characters identify the revolution on which it was executed, the middle 3 digits specify the sequence number of the observation in the timeline of this revolution and the last two are an identifying number given to that observation by the observer in the original proposal.

4.3.2 Guaranteed time and open time programmes

Approximately 45% of ISO's time was reserved for those parties contributing to the development and operation of the scientific instruments and the overall facility, namely: the four Principal Investigators (PI) and their teams, who built the ISO instruments; the five Mission Scientists; the scientific staff of the SOC; and ESA's international partners in the mission, NASA and ISAS. Definition and coordination of these guaranteed-time observations started some eight years before launch with the definition of the so-called `Central Programme'.

The requirement upon this programme was that it would be a `balanced and coherent programme ......providing a solid scientific core for the ISO observing programme'. Its publication intended to `give an overview of the observing potential of the observatory to enable the submission of top quality, complementary and feasible observing proposals leading to the maximum output of the observation programme'. The programme was unanimously endorsed by the ISO Observing Time Allocation Committee (OTAC).

The complete guaranteed time programme was presented in three paper documents, namely `ISO Guaranteed Time Programme Part A: Overview', `ISO Guaranteed Time Programme Part B: Proposal Abstracts' and `ISO Guaranteed Time Programme Part C: Catalogue of Observations' in April 1994.

In addition to its scientific value, this early start was important both to help define observing modes and also to be able to publish worked examples to the community with the pre-launch call for proposals.

ISO -- as an observatory -- was open to the astronomical community including expert and non-expert users. Thus, the remaining more than half of ISO's observing time was distributed to the general community via the traditional method of proposals and peer review. Open time proposers could apply to use any of the spacecraft and instrument modes. Guaranteed time holders were also allowed to apply for open time.

The observations in the guaranteed time programme were reserved and open time proposals were not allowed to duplicate any of these. Checks for duplications were made during the processing of proposals. Duplications were determined by consideration of the target observed, the observing mode used, details of the observation parameters (e.g. size of rasters, wavelength ranges, filters) and the scientific objective.

Figure 4.7 shows the distribution of ISO observing time per participating group.

Figure 4.7: Distribution of observing time per participating group.

4.3.3 The ISO observing cycle Calls for proposals

In order to distribute the available open time among the members of the astronomical community, one `Call for Observing Proposals' was issued pre-launch (April 1994) and another `Supplemental Call' post-launch (August 1996). Both calls, pre- and post-launch, were in principle open only to proposers in ESA member states, the USA and Japan. Over 1500 proposals, requesting almost four times more observing time than available, were received in response to these calls. Some 40% of the proposals arrived in the last 24hours before the deadlines.

Pre-launch call

The Pre-launch call solicited proposals for:

Responses to this call consisted of a Letter of Intent and a so-called `Phase 1' proposal.

The following schedule was established for the pre-launch Call:

Issue of Call: April 1994
Letters of Intent: due 10 June 1994
Phase 1 Proposals (electronic version): due 19 August 1994
Phase 1 Proposals (paper version): due 23 August 1994
OTAC review complete by end November 1994
Start of Phase 2 data entry: December 1994
End of Phase 2 data entry: mid-1995
Launch of ISO: 19 September 1995 (finally, the launch took place on 17 November 1995)

Supplemental call

This supplemental call solicited proposals to be carried out in the period from December 1996 to the end of the mission (April 1998). Although completely new proposals were not excluded, priority was given to proposals aiming at continuing, completing and extending existing ISO observing programmes.

Responses to this call consisted of a `Phase 1' proposal only.

The following schedule was established for the Supplemental Call:

Issue of Call - 5 August 1996
Phase 1 Proposals (electronic version): due 7 October 1996
Phase 1 Proposals (paper version): due 9 October 1996
OTAC review complete by late November 1996
Start of Phase 2 data entry: end November 1996
End of Phase 2 data entry: end January 1996. Proposal evaluation and selection

From the observers' point of view the end-to-end observing process started with the preparation of the `Phase 1' proposals and their submission both in electronic and paper version to the OTAC.

Following each of the two calls above mentioned, the `Phase 1' open time proposals received were evaluated scientifically by an Observing Time Allocation Committee (OTAC) consisting of approximately 35 external scientists. This committee was appointed by the ESA Director of Science, following the recommendation of the Astronomy Working Group. It was headed by an overall Chairman and consisted of seven panels, one for each of the following scientific areas: Solar System, Interstellar Matter, Stellar Physics, Circumstellar Physics, Normal and Starburst Galaxies, Active Galaxies and Quasars, and Cosmology. Each panel consisted of a Panel Chairman and, depending on the scientific area, 3 or 4 members.

In parallel, the electronic versions were imported into a proposal database for statistical analysis and checking for, inter alia, duplication with the guaranteed observing programme and with each other, or the distribution on the sky of the requested observations in view of the uneven sky coverage of ISO. The scientific staff of the SOC made also an assessment of the technical feasibility of the proposed observations and sent the results to the OTAC.

After peer review, the ISO OTAC made recommendations on the observing programme to be carried out by ISO via assigning grades either to entire proposals or to individual observations.

The following items were taken into account during the review process:

scientific case described in the abstract and justification,
scientific merit and relevance of the proposed observation(s),
technical feasibility of the observation(s),
the necessary observing time, and
oversubscription in particular parts of the sky.

The output of the OTAC review was a list of proposals/observations ranked by scientific merit. OTAC assigned a cut-off grade below which proposals and observations were not carried forward into Phase 2 for further processing.

However, for efficient timelining of observations, the scientific scheduling software needed to work from a pool of observations which significantly overfilled the time available. All these observations had to be fully specified in the database from which the mission planning system worked. Proposal data entry

The Principal Investigators of proposals/observations with grades above the cut-off value (see previous section) moved on to the next phase of the process, the so-called `Phase 2'. Here they had to enter full details of their observations into the SOC's databases. Checking of their inputs was done both by the `Proposal Generation Aids' (PGA) and the `Proposal Handling' (PH) modules.

Prior to launch, this typically involved a visit of around a week to the `ISO Proposal Data Entry Centre', set up in ESTEC (for USA ISO observers, IPAC was designated by NASA as the support centre). The European centre at ESTEC was co-located with the Science Operations Centre during its development phase, prior to moving to Spain and it was operated by scientific and technical staff from the SOC, who assisted visitors in the entry of their data and who checked, as thoroughly as possible, that the observations were both feasible and optimised. It was only after completion of this Phase 2 data entry and checking process that the pre-launch feasibility of the observations was verified. Over 500 astronomers visited ESTEC in the first six months of 1995 and were assisted by resident astronomers and technical assistants in finalising and entering their observational programmes.

Successful proposers of polarisation observations were assigned a Resident Astronomer (if one was not already present on the proposal) who contacted the proposers and discussed with them the details of the observations. These observations were entered via a SOC expert.

After completion of the Phase 2 data entry, and when all was correct, the observations were stored in the `Mission Data Base', a critical element of the overall SOC architecture as it contained full details of all observations that ISO would make. Calibration observations were also stored in the same database but entered by a different route.

The daily timeline of observations was then set up by the `Mission Planning Phase 1' system. This software system used a set of routines (`AOT logic') to convert automatically the user-entered parameters ($\alpha $, $\delta$, wavelength, flux, spectral resolution, observation time or desired signal to noise, etc.) into detailed commands to be finally passed to the Spacecraft Control Centre for transmission to the spacecraft.

Post-launch as experience and confidence grew, visits to ESTEC and IPAC were almost completely replaced by remote logins across the Internet.

During the operational life of ISO, observers were permitted to tune up their programmes -- via Internet communications with the Community Support Team at the Science Operations Centre -- to take full advantage of results from previous observations and of improving knowledge of how best to use the instruments. In some cases, the different in-orbit detector sensitivities led to changes in the values of observing parameters and in the worst case rendered some observations unfeasible. The visibility constraints were closely related to the launch date and the target lists also had to be adjusted accordingly. The facility was widely used, with -- averaged across the entire set of observations -- each programme being updated around three times. Because scientific judgement often had to be involved, checking that updated observations did not duplicate existing ones was a very labour-intensive task.

4.3.4 Community Support

The overall community support task during operations was intended to facilitate scientifically-effective use of ISO and included not only handling all requests for observing time as above described but also providing concise and up-to-date information.

Prior to launch, user documentation (such as observers manuals, data reduction manuals, information notes, etc.) was mainly paper-based. However, during the operations, this completely switched to being web-based. The ISO WWW site ( opened in 1994 and had over 1 million hits (from non-ESA machines) during operations. Internet rapidly became the essential way of communicating with observers, also through the ISO Helpdesk ( service. The ISO WWW site has been continually upgraded since its creation, e.g. with the addition of galleries of science results and of tools for detailed monitoring of execution of observing programmes. It currently serves every week more than 3000 HTML pages, 6000 images and 2000 documents to 4000 distinct hosts. The ISO Helpdesk has received and answered more than 31000 e-mails as of November 2003 and it is still used by the ISO archive users to retrieve information on the archived data products or to ask questions to the calibration experts on data reduction.

During Post-Operations the ISO Data Centre welcomed and supported more than 135 visitors who wanted to analyse ISO data with the help of an IDC support astronomer and it also organised several workshops and conferences (see Section 1.2).

next up previous contents index
Next: 4.4 Overview of the Up: 4. ISO Operations Previous: 4.2 The Ground Segment
ISO Handbook Volume I (GEN), Version 2.0, SAI/2000-035/Dc