FIRST: the Far-InfraRed and Submillimetre Telescope

G. Pilbratt

FIRST homepage http://astro.estec.esa.nl/SA-general/Projects/First/first.html

The Far-InfraRed and Submillimetre Telescope (FIRST) was selected as Cornerstone 4 (CS4) in ESA's Horizon 2000 long-term science plan by the Science Programme Committee in November 1993. It is an observatory-type mission targetting 85-900 µm, with emphasis on the shorter wavelengths. This region of the spectrum is primarily sensitive to continuum and line radiation from relatively cool, diffuse media, such as interstellar and circumstellar dust and gas. Black bodies at 5-50 K peak here, and gases of between 10 K and a few hundred kelvin emit their brightest molecular and atomic emission lines in this wavelength range. Broadband thermal radiation from small dust grains is the most common continuum emission process in this band.

These low temperatures are a characteristic of a significant fraction of the visible mass in the Universe, including dense interstellar clouds and embedded protostellar condensations, planets, comets, outer atmospheres of evolved, cool stars and nuclei of active galaxies. As galaxies in the early Universe generally had a larger fraction of interstellar matter than present-day galaxies, it is very likely that such objects are very prominent in the far-IR and sub-mm band. The key scientific topics to be addressed by FIRST include:

• 150-500 µm deep broadband surveys and related research. The main goal of research in this area will be a detailed investigation of the formation and evolution of galaxy bulges and elliptical galaxies in the first third of the present age of the Universe. Furthermore, the possibility of discovering new classes of objects is great.
• follow-up spectroscopy of interesting programme objects discovered in the survey. The far-IR and sub-mm band contains the brightest cooling lines of interstellar gas, which give very important information on the physical processes and energy production mechanisms (e.g. AGN vs star formation) in galaxies.
• detailed studies of the physics and chemistry of the interstellar medium in galaxies, both locally in our own Galaxy, as well as in external galaxies, including objects at high redshift. This implicitly includes the important question of how stars form out of molecular clouds in various environments.
• observational astrochemistry (of gas and dust) as a quantitative tool for investigating the physical and chemical processes involved in star formation and early stellar evolution (e.g. cloud collapse, freeze out, disc formation, dust coagulation and planetesimal formation).
• detailed high resolution spectroscopy of a number of comets, high resolution molecular spectroscopy of the cool outer planets, and searches for Kuiper belt objects.

The mission selected for implementation as CS4 was based on the outcome of an industrial study performed in 1992/93. It employed a spacecraft carrying a payload module (PLM) with a 3 m diameter Cassegrain telescope inside a sunshade and a 4.5 K science payload environment created by mechanical cryocoolers, and a service module (SVM) providing the necessary infrastructure.

As the selected mission was costed over budget, the prime motivation for subsequent work has been to refine the FIRST concept in such a way as to lower the overall mission cost to fit within the cornerstone allocation without negatively impacting its science capabilities. This has included technical development of identified critical technologies and a reassessment of the cryostat vs cryo-cooler choice in the light of the developments on both fronts since the CS4 decision, as well as studying operational aspects and possibilities of international collaboration.

Following the decision by the SPC, two different satellite concepts have been studied. On the one hand, the design of the cryocooler spacecraft has been refined, taking the current increased technical maturity and performance of the coolers into account. On the other hand, a 'wet' cryostat concept based on the (now well proven) ISO cryostat technology has been studied. A cryostat with 2560 litres of superfluid helium would give an operational lifetime of 3 years. For both concepts it is presently being studied whether the (re)use of the XMM SVM would be advantageous. The two concepts are being studied in parallel on a competetive basis; they employ the same model payload and share the same science objectives and ground segment philosophy.

FIRST will have instruments for high and medium resolution spectroscopy, imaging and photometry over the sub-mm and far-IR range. The model payload as presently defined has evolved to consist of three instruments and has been used to define requirements, interfaces, operation and performance of the spacecraft for study purposes. The actual payload to be flown will be supplied by PIs representing consortia of instrument building institutes, selected from the response to an AO issued by ESA. The model payload comprises:

• a heterodyne instrument, referred to as the 'HET'. It performs high to very high resolution spectroscopy in approximately the 500-1200 GHz (250-600 µm) band, using a multichannel SIS (superconductor-insulator-superconductor) mixer receiver with solid state local oscillators and ('hybrid') digital autocorrelator and/or acousto-optical spectrometers. The SIS mixers need to be operated at a temperature of around 4.5 K or lower.
• an incoherent photoconductor instrument, referred to as the 'PHOC'. It performs spectroscopy and photometry in the 85-200 µm region using a 16 16 stressed 'bulk' Ge:Ga photoconductive detector array and Fab-ry-Pérot interferometers. The photoconductors need to be cooled to around 1.7 K.
• an incoherent bolometer instrument, referred to as the 'BOL'. It performs photometry in the 200-900 µm region and, in addition, spectroscopy in the 200-400 µm region using two bolometer detector arrays and Fabry-Pérot interferometers. The bolometers must be cooled to around 0.2 K.

The model payload is being optimised with respect to the identified key science topics, and for minimising technical complexity and risk, cost, operational effort, while at the same time complying with the spacecraft constraints.

FIRST has been conceived as a multi-user observatory, and it will be open to the general astronomical community. An operations scheme is being considered in that each instrument will have its own Instrument Control Centre (ICC) geographically separated and detached from the spacecraft control centre. Each ICC will be responsible for the operations of that instrument and also for the generation of calibration and data reduction tools for all its data. The ICCs will be under the responsibility of the corresponding PIs, just like the actual instruments. The overall science planning, coordination, data archiving and central observer interface will be provided by the FIRST Science Centre (FSC).

During the 3 years of nominal lifetime, the observation time will be shared between guaranteed and open time. (For the cryocooler spacecraft, the onboard consumables will be budgeted to allow a total lifetime of 6 years, enabling a decision to extend the mission lifetime beyond the nominal to be taken at a later date.) The guaranteed time will be implemented in the form of a 'core programme' defined by the guaranteed time holders. The open time will be allocated to the general community on the basis of calls for observing proposals. The formation of large observer collaborations collectively addressing key topics will be actively encouraged. The proposals will be evaluated and selected by a time allocation committee on the basis of scientific merit and technical feasibility. All scientific data will be archived and made available to the general astronomical community after a proprietary time has elapsed.

FIRST is now in a pre-Phase B technical development stage. The main areas of development involve the telescope and the cryocoolers. It is currently scheduled to finalise the technical design of the FIRST spacecraft and mission, and to make a choice between the two spacecraft concepts, in the spring of 1997. Subject to approval by the SPC, it is then planned to issue the AO for the instruments in the autumn of 1997, with the selection taking place the following year. Having proved the critical technology and selected the instruments, Phase B would then start in 2001.