T. Bradshaw

Planck pre-launch status: The HFI instrument, from specification to actual performance

Context. The High Frequency Instrument (HFI) is one of the two focal instruments of the Planck mission. It will observe the whole sky in six bands in the 100 GHz-1 THz range. Aims: The HFI instrument is designed to measure the cosmic microwave background (CMB) with a sensitivity limited only by fundamental sources: the photon noise of the CMB itself and the residuals left after the removal of foregrounds. The two high frequency bands will provide full maps of the submillimetre sky, featuring mainly extended and point source foregrounds. Systematic effects must be kept at negligible levels or accurately monitored so that the signal can be corrected. This paper describes the HFI design and its characteristics deduced from ground tests and calibration. Methods: The HFI instrumental concept and architecture are feasible only by pushing new techniques to their extreme capabilities, mainly: (i) bolometers working at 100 mK and absorbing the radiation in grids; (ii) a dilution cooler providing 100 mK in microgravity conditions; (iii) a new type of AC biased readout electronics and (iv) optical channels using devices inspired from radio and infrared techniques. Results: The Planck-HFI instrument performance exceeds requirements for sensitivity and control of systematic effects. During ground-based calibration and tests, it was measured at instrument and system levels to be close to or better than the goal specification.

Receivers for ALMA: preliminary design concepts

The Atacama Large Millimeter Array (ALMA), a joint project between Europe and the U.S. and at present in its design and development phase, is a major new ground based telescope facility for millimeter and submillimeter astronomy. Its huge collecting area (7000 m2), sensitive receivers and location at one of the driest sites on Earth will make it a unique instrument. We present preliminary design concepts for the overall receiver configuration. Optics and cryostat design concepts from OSO, OVRO, RAL, IRAM, NRAO and SRON and their main features are described.

Closed cycle coolers for temperatures below 30 K

Abstract Single-stage Stirling coolers providing refrigeration at ≈ 80 K have been developed for space use and are now being produced commercially. Development work is now concentrating on multistage coolers for temperatures below 30 K. This paper describes results from preliminary tests on a two-stage cooler built at the Rutherford Appleton Laboratory and work in progress to build a closed cycle 4 K cooler.

The susceptibility of incoherent detector systems to cryocooler microphonics.

Mechanical cryocoolers will be used to provide 4 K cooling for the HFI bolometer instrument in the Planck Surveyor mission for measurement of the anisotropies in the cosmic microwave background. They may also find application in future Earth-observing satellites. We describe an experimental programme to investigate the effects of microphonic vibrations on infrared detectors. Our initial tests have demonstrated the feasibility of operating far-infrared photoconductive detectors in conjunction with a 4 K cryocooler for Earth observing applications. The microphonic susceptibility of a bolometer at 100 mK has also been investigated, using a modified variation of the Planck Surveyor zero gravity flight dilution system. The results of these tests gives us confidence in the proposed use of sensitive bolometers with mechanical cryocoolers for space missions. We also describe development of the bolometers, the readout systems and temperature regulators for improved systems level performance of such instruments.

Mechanical and thermal architecture of an integrated payload instrument for the Exoplanet Characterisation Observatory

The Exoplanet Characterisation Observatory (EChO) is a space mission dedicated to undertaking spectroscopy of transiting exoplanets over the widest wavelength range possible. It is based around a highly stable space platform with a 1.2 m class telescope. The mission is currently being studied by ESA in the context of a medium class mission within the Cosmic Vision programme for launch post 2020. The payload instrument is required to provide simultaneous coverage from the visible to the mid-infrared and must be highly stable and effectively operate as a single instrument. This paper presents the architectural design for the highly interlinked mechanical and thermal aspects of our instrument design. The instrument will be passively cooled to approximately 40K along with the telescope in order to maintain the necessary sensitivity and photometric stability out to mid-infrared wavelengths. Furthermore other temperature stages will be required within the instrument, some of which will implement active temperature control to achieve the necessary thermal stability. We discuss the major design drivers of this complex system such as the need for multiple detector system temperatures of approximately 160K, 40K and 7K all operating within the same instrument. The sizing cases for the cryogenic system will be discussed and the options for providing the cooling of detectors to approximately 7K will be examined. We discuss the trade-offs that we are undertaking to produce a technically feasible payload design which will enable EChO’s exciting science.

HERITAGE OVERVIEW: 20 YEARS OF COMMERCIAL PRODUCTION OF CRYOCOOLERS FOR SPACE

The paper presents an overview of the results of industrialization of the ISAMS Stirling cooler technology, achieved through cooperation with the Rutherford Appleton Laboratory. Seven batches of coolers have been produced and delivered to international customers for various space applications. A summary of in-orbit flight heritage is presented showing flight telemetry data for detector and mounting interface temperatures, vibration and other drive parameters. Thirteen of the 50–80K cooler models have been launched, with many of the coolers having already exceeded their required lifetimes. Highlights of cooler commissioning and performance in orbit are presented, with discussion of lessons learned, focusing on the last three production batches of the 50–80K Stirling cooler.

The effects of cryocooler microphonics, EMI and temperature variations on bolometric detectors

The use of mechanical coolers for space-based infrared telescopes is becoming a reality with the development of the Planck spacecraft, which will obtain full sky maps of the temperature anisotropy and polarisation of the cosmic microwave background (CMB). The High Frequency Instrument is one of two instruments aboard Planck and will use 48 bolometric detectors operating at 0.1 K. We summarise the performance of the RAL 4 K Joule-Thomson (J-T) system which will precool these detectors, and describe integration aspects of the sensitive bolometric detectors with cryocoolers at system level, in particular the effects of cryocooler vibration, EMI and thermal fluctuations. Full understanding of these systematic sources of noise is critical to enable the microkelvin level scientific signals to be cleanly extracted from the raw data.

Design and Characterization of a Miniature Pulse Tube Cooler

The design of an advanced Miniature Pulse Tube Cooler (MPTC) for long-life space applications is presented. The cooler system incorporates a balanced compressor, with an in-line pulse tube configuration, yet retains the flexibility of a transfer line. The ‘split’ configuration provides obvious advantages for instrument design, while maintaining economical options for drive electronics and maximising potential for vibration isolation between compressor and detector. It also potentially allows the pulse tube end to be separately placed within a pre-cooled enclosure at temperatures beyond the range of operation for typical compressor mechanisms.

CubeSats for infrared astronomy

This paper investigates the potential role of small satellites, specifically those often referred to as CubeSats, in the future of infrared astronomy. Whilst CubeSats are seen as excellent (and inexpensive) ways to demonstrate and improve the readiness of critical (space) technologies of the future they also potentially have a role in solving key astrophysical problems. The pros and cons of such small platforms are considered and evaluated with emphasis on the technological limitations and how these might be improved. Three case studies are presented for applications in the IR region. One of the main challenges of operating in the IR is that the detector invariably needs to be cooled. This is a significant undertaking requiring additional platform volume and power and is one of the major areas of discussion in this paper. Whilst the small aperture on a CubeSat inevitably has limitations both in terms of sensitivity and angular resolution when compared to large ground-based and space-borne telescopes, the prospect of having distributed arrays of tens (perhaps hundreds) of IR-optimised CubeSats in the future offers enormous potential. Finally, we summarise the key technology developments needed to realise the case study missions in the form of a roadmap.

Recent progress in the Elegant Breadboard supra-THz activities for LOCUS and a view to an astronomy application

We report on ongoing technology activities on the Elegant Breadboard for the LOCUS telescope and its 1.15THz receiver. LOCUS is a Low-Earth orbit small satellite (150kg category) with the objective of linking observations of climate, the upper atmosphere and space weather by performing simultaneous high spectral resolution measurements of molecular signatures of molecules which drive the thermal exchange in the highest layers of our atmosphere. These measurements are performed both in the THz frequency region and in a photometric narrow-band near and mid-IR channels allowing to decouple molecular abundances from temperature and pressure profiles. The elegant breadboard under development will reproduce representative optics and explore the thermal implications of a compact optical-bench with a small cryo-cooler and radiator stage for a small LEO satellite. An extensive test programme will be undertaken to raise the payload and receiver system and we will seek opportunities to test the system in an observational campaign. Technology developments will benefit future technology activities related to both a future Earth-Orbit mission and potential astronomy missions.