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THE FOCAL PLANE UNIT OF

THE HETERODYNE INSTRUMENT FOR FIRST: HIFI

N. D. Whyborna

, Th. de Graauwa

, H. van de Stadta

V. Belitskyb

, R. Kruisingac

, S. Torchinskyb

, H. Visserc

, K. Wildemana

a space Research Organisation Netherlands (SRON), PO Box 800, 9700 AV Groningen, The Netherlands

E-mail: nick@sron.rug.NL; Tel: +31 50 363 4074; Fax: +31 50 363 4033

b Onsala Space Observatory, Chalmers University of Technology, S-412 96 Giiteborg, Sweden

C TNO-TPD, Stieltjesweg 1,2600 AD Delft, The Netherlands

ABSTRACT

ESA's Far-IR and Sub-millimetre-wave Telescope, FIRST, is an astronomy space mission which will provide

an unobstructed view of the universe in the last major unexplored region of the electromagnetic spectrum.

The satellite is planned to be launched in mid 2006 and will carry a payload of three instruments spanning

the wavelength interval 80 and 800 pm. ESA has recently released an Announcement of Opportunity (AO)

asking for interested parties to make proposals to provide these instruments and this paper will describe our

proposed front-end for a Heterodyne Instrument for FIRST - HIFI. HIFI will be built by a large consortium

of European, American and Canadian institutes which began work defining a heterodyne instrument in 1996.

HIFI will cover the frequency interval 480 - 1250 GHz in 5 bands using pairs of SIS tunnel junction mixers

to receive both polarisations. The frequency ranges 1600 - 1900 GHz and 2400 - 2700 GHz will also be

covered using single hot-electron bolometer mixers. A modular construction will be used with 6 mixer

assemblies, one for each of the 5 lower bands and one for the two high-frequency bands. Each mixer

assembly will contain optics for local oscillator injection and the first stage of IF amplification. The local

oscillator signals are generated outside the cryostat in a separate unit and pass through dedicated windows in

the cryostat wall. The local oscillator unit is described elsewhere. Low noise InP HEMT's with very low

power consumption will be used in the IF preamplifiers and will provide a 4 GHz IF bandwidth. A suite of

spectrometers in the warm service module of the spacecraft will analyse the IF signals with frequency

resolutions ranging from 100 kHz to 1 MHz.

The mixer assemblies slot into a housing containing the optics common to all bands. The common optics

performs the functions of refocusing the beam from the telescope, splitting the focal plane amongst the 6

mixer assemblies, chopping, and calibration.

1 INTRODUCTION

The proposed Heterodyne Instrument for FIRST,

HIFI, has been optimised to address a number of

key themes in modern astrophysics related to

understanding the cyclical interrelation of stars and

the interstellar medium of galaxies. This interplay

between stars and the ISM drives the evolution

and, thus, the observational characteristics of the

Milky Way and other nearby and far away

galaxies, all the way back to the earliest

protogalaxies at high z.

By combining the high spectral resolving power

capability of the radio heterodyne technique with

quantum noise limited detection from super- conductor physics and with the state-of-the-art in

microwave technology, IHFI will provide unrivalled

spectral resolution and ultimate sensitivity over the

frequency ranges 480 to 1250 GHz (in 5 bands),

1410 to 1910 GHz, and 2400 to 2700 GHz. The

instrument will be able to perform rapid and

complete spectral line surveys with resolving

powers from 103

up to 107

(300 — 0.03 km/s) and,

will complement the spectroscopy capabilities of

the two incoherent instruments on FIRST.

This instrument will fully exploit the recent rapid

pace of development in sub-mm wavelength mixer

technology to give sensitivity close to the

theoretical limit. The first five frequency bands

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FPU sis

Main optics

Cal source

Mechanisms

Mixer Assemblies

with

6x2 Mixers

Preamps

FCU

Control

Bias Supply

LOU s/s

'Power Amplifiers

ILO Assemblies

j with

ILO Sources

Back-End s/s

IF Pre-Processor

Matrix Switch

WBS IF Processor 1

4 GHz

1 MHz

4 GHz

1 MHz

HRS IF Processor

<1 GHz

variable

<IGHz

variable

BE Controller

LCIJ

!Control

Bias Supply

Freuency

Synthesizer

will each contain a pair of mixers using

superconductor-insulator-superconductor (SIS)

tunnel junctions. Both polarisations of the

astronomical signal will be received for maximum

sensitivity. Channel 6 will contain two mixers

based on the recently-developed fast hot-electron

bolometers (FMB) using thin superconducting

films — each mixer will cover one of the sub-bands.

The instrument will operate at one frequency at a

time, i.e. only one of the frequency bands will be

active.

HIFI will have an instantaneous IF bandwidth of

4 GHz analysed in parallel by two types of

spectrometers: a pair of wide-band spectrometer

(WBS), and a pair of high resolution spectrometer

(FIRS). The wide-band spectrometer will use

acousto-optic technology with a frequency

resolution of 1 MHz and a bandwidth of 4 GHz for

each of the two polarisations. The HRS will

employ either a digital auto-correlation

spectrometer (ACS) or a chirp transform

spectrometer (CTS) and will provide two

combinations of bandwidth and resolution: 1 GHz

bandwidth at 200 kHz resolution, and at least

500 MHz at 100 kHz resolution. The HRS will be

divided into 4 or 5 sub-bands each of which can be

placed anywhere within the full 4 GHz IF band.

HIFI will consist of three major sub-systems and

an instrument controller:

1. The focal plane sub-system comprises the

focal-plane unit (I-TFPU) inside the cryostat,

containing relay optics, mixers, low-noise IF

HEMT pre-amplifiers, a focal plane chopper,

and a calibration source; and the HFPU control

unit (HICU) which supplies the bias voltages

for the mixers and IF preamplifiers in the

HFPU and controls the frequency diplexers,

the focal plane chopper mechanism and the

calibration source.

2. The local oscillator sub-system comprises: the

local oscillator unit (HLOU) located on the

outside of the cryostat generating the LO signal

which is coupled into the liFPU via a window

in the cryostat wall; and the local oscillator

control unit (HLCU) in the service module

(SVM) which controls the frequency of the

local oscillator with a precision of 1 part in

108.

3. A back-end sub-system (HBES) within the

SVM. This contains the IF processor, WBS,

HRS, and backend control system (BCS).

4. An instrument control unit (HICU) within the

SVM which interprets commands from the

Instrument Control Unit

Figure 1. Block diagram of the HIFI instrument

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satellite telecommand system, controls the

operation of the instrument, and returns

science and housekeeping data to the satellite

telemetry system.

Figure 1 is a block diagram of the HIFI showing

the relationship between the various units of the

instrument.

2 FOCAL PLANE SUB-SYSTEM

2.1 General Description

In the HFPU the sub-mm wavelength signal from

the telescope is mixed with radiation from a local

oscillator and the "beat frequencies" are generated

in a cryogenically cooled mixer. The instrument

measures radiation in two polarisations in 5

contiguous bands covering the frequency interval

480 to 1250 alz and in a single polarisation in

two sub-bands around 1.7 THz and 2.5 THz. There

are 6 optical beams coming from the telescope, one

for each of the 5 dual-polarisation bands and one

for the two high-frequency sub-bands.

The HFPU employs a highly modular design

consisting of:

• a common optics assembly (COA) which

serves as the support structure for the other

FIFPU modules and contains the optical

elements which are common to the 6 optical

beams,

• 6 mixer assemblies (MA) containing the

optical elements, mixers and IF components

specific to each of the 6 instrument bands,

• a chopper assembly containing a nutating

mirror and drive mechanism,

• a calibration assembly containing a black-body

calibration source and refocusing optics.

These elements are described in the following

sections.

Extremely flat and stable spectral baselines are a

stringent requirement for the HIFI instrument in

order to be able to study the very weak broad

emission and absorption lines of distant galaxies

and to perform broadband spectral surveys. Thus,

special emphasis is placed on the design of the

optics in the I-IFPU to avoid generation of standing

waves. It also includes a chopper mechanism to

switch between two positions on the sky, and will

allow the standard dual beam switch techniques for

standing wave elimination. In addition, the

specification of the intermediate frequency

processing from the first IF amplifiers down to the

spectrometer backends are driven by these

requirements, resulting in particular in the

necessity of mounting the first IF amplifiers very

close to the mixers (see below) and carefully

controlling the thermal environment of critical

components with regard to stability.

2.2 Common Optics Assembly

The functions of the instrument optics and their

order of implementation are indicated in Figure 2.

The first function is calibration (CAL) followed by

focal plane chopping (CHOP), band splitting

(CHAN), polarisation separation (POL), and LO

injection (LO).

The Common Optics Assembly (COA) contains

the optics from mirror M3 in the telescope focal

plane through to but excluding the Mixer

Assemblies (MA) which are described in Section

2.5 (see Figure 3). The COA also includes the

calibration assembly.

The telescope focal plane mirror (M3) acts as a

folding mirror and also as a field mirror to reduce

the optical path length towards the pupil image (=

image of the telescope secondary mirror) for

packaging reasons. The telescope focal plane is re- imaged in the main optics by means of a Gaussian

telescope at unit magnification implemented by a

collimating mirror and an imaging mirror, both

with a focal length of 280 mm. Between these two

Figure 2. HIFI Common Optics functional block diagram

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