Tropospheric Airborne Fourier Transform Spectrometer (TAFTS)

Introduction

A novel far-infrared Fourier Transform Spectrometer designed to make direct, differential spectral measurements of up-welling and down-welling radiation  from a high-altitude aircraft flying near the tropopause.  The principal purpose of these measurements is to measure the radiation balance in a region where water vapour has many imperfectly-characterised absorption features; this is directly relevant to global warming research.  TAFTS has the unique capability of being able to measure the differential spectral flux directly by looking up and down at the same time and optically subtracting the two incoming spectra before the result is detected.  The instrument was built from scratch by the author (left) and Dr Jon Murray (right) at Imperial College, with invaluable help from the Applied Optics and main Physics workshops and Prof Peter Ade's group at Queen Mary College.

Jon Murray (right) and me wondering what went
wrong with the instrument this time...

Technical

TAFTS is based upon a Martin-Puplett polarising interferometer which has two input ports (looking up and down) and two output ports (inside the liquid-helium cooled detector optics).

Interferometer optics

This is the interferometer section outside its vacuum enclosure: on the right (vertical post) is a collimator and injection mirror for the helium-neon reference laser.  Next to the collimator is the large round polarising beamsplitter composed of a thin membrane on which is deposited a very fine grid of conductive stripes (thanks to the skilled magicians then at Queen Mary College).  At the rear (centre) the moving double-roof mirror can be seen on a precision movement: this  component alters the path lengths in both arms of the interferometer at the same time and also rotates the polarisations which allows efficient coupling from the input to the output.  The movement employs precision V-grooves and three large ball bearings to achieve highly linear travel.  The motion is transmitted from a micro-stepping motor by stainless steel fishing line!  The reference laser and output mirror are located underneath the base plate.  The laser had to be modified to allow heatsinking and operation in a vacuum without high-voltage discharge!

Being a thin membrane, the beamsplitter can act as an excellent microphone unless steps are taken to isolate it from acoustic noise, although the reference laser  signal should (and does) permit compensation for residual vibrations.  The path lengths in the interferometer are relatively long, and if filled with water vapour they would act as internal filters, obscuring the very signals that the instrument is looking for.  For this reason the interferometer is housed in a vacuum chamber.

TAFTS with teh covers off: pointing / calibration box (left), vacuum chamber and interferometer (centre) and helium cryostat housing detectors and analyser optics (right).

The interferometer can be seen here in its vacuum chamber (with anti-vibration mounts attached).  The vacuum chamber also serves to mount the blue cryostat (containing analyser optics and detectors with associated electronics) and the "pointing optics box" at the front.  To achieve a good seal and structural integrity, it was machined from a single "sheet" of HE30 aluminium alloy some 11 inches thick using a drill press, a CNC mill and a copious amount of swearing!  Of the original 176 kg lump, about 19 kg remained in the finished piece.  The windows on the vacuum chamber must be strong enough to resist atmospheric pressure (maximum 1 bar) whilst still permitting the free passage of far infrared light: this is not a trivial requirement to solve! The instrument actually uses plyproplylene film of the type used to manufacture capacitors as windows.  The material is between 13 and 25 microns thick and remarkably strong.  The deflection when under stress is however very scary!

The pointing optics contains motorised steering mirrors which allow each of the two input beams to look either at the atmosphere or at one of two calibrated, home-built black-body radiation sources (total of four). The black body sources are maintained at two different temperatures so that each channel can be calibrated directly in terms of brightness temperature.  An external precision black body source was designed and constructed with whch to calibrate the entire instrument.

Cryogenic optical / detector system

The cryostat contains a gold-plated optics and detector assembly mounted on a 5-inch diameter copper disk which is bolted to the liquid helium tank and surrounded by thermal radiation shielding - the whole thing is designed to work at only 4 K.  I am especially proud of this assembly which I designed and which was manufactured by Paul Brown of the Applied Optics Workshop despite the quality of my Autocad drawings!  In this view (which is actually upside-down) the input beam enters abovethe base plate from the lower right and passes through the central circular analyser towards a small circular off-axis paraboloid at therear (upper left).  From there the beam double back and is directed towards the base plate where it is caught by a second off-axis mirror (out of view behind the small wedge-shaped mount at lower right).  The beam is then passed to the analyser and each of the two complementary outputs are taken to band-defining filters and detectors (in low, rectangular, chanfered housings, two of which are visible in the foreground left of centre).  The detectors are placed in small reflective integration chambers and fed via offaxis paraboloids and home-made hyperbolic non-imaging concentrators.  This arrangement makes for a highly compact yet efficient light collector with a well-defined field of view (which is essential for the rejection of stray radiation).

The detectors are tiny crystals of semiconductor mounted on small pins and connected to preamplifier / buffers (essentially just warmed field-effect transistors!) which are mounted in the rectangular box (right) to prevent stray radiation.  Connections to this assembly were made using a micro-miniature D connector and a home-made ribbon cable (silver tape) containing multiple 42-SWG manganin wires to avoid un-necessary thermal conduction from the outside world.  Talk about eye-strain!

The cryostat was custom designed and fabricated by Oxford Instruments to a very high quality indeed.  It was not inexpensive!  The cold optics was surrounded by a gold-plated radiation shield, multi-layer radiation shielding, a second shield at liquid nitrogen temperature and finally more mylti-layer shielding. Multi-layer shielding comprises alternating layers silvered mylar foil and bridal veil material - an interesting purchasing experience for a young researcher!

An externl precision black body source "powered" by dry nitogren from cryosta boil-off being used to calibrate TAFTS.  The black body is a home-brew design with excellent emissivity.

This picture shows the TAFTS instrument mounted in its (dark-blue) flight frame and with surface heaters / insulation fitted (after the unfortunate incident with an o-ring).  The cryostat is attached to a turbo-pump, and the large red cylinder is myhome-designed and built calibration black body source.  I designed this for use when the instrument is mounted in the aircraft as well as in the lab.  It takes dry nitrogen gas from a cryostat boil-off vent via insulated flexible tubes and passes it though regulated heaters to set the temperature.  The gas passes through a spiral track in the outer wall of the black body (which is a deep cylinder) and then over, around and through a concentric cone assembly mounted at the base.  All internal components are painted with NEXTEL black.  The gas is swirled and vents through the open end of the black body where it fills th e TAFTS pointing optics box, thus excluding moisture.

The performance of the cavity has been modelled by the NPL using a Monte-Carlo raytrace program.  Emissivities have been computed both at distinct wavelengths and integrated over the whole waveband in which TAFTS operates. Assumptions have been made regarding the emissivity and diffusivity of NEXTEL black at the longer wavelengths where NPL is unable to make direct measurements (allowing its reflectivity to rise as far as 26%!).  Thermal non-uniformities along the cavity have also been modelled.
The results indicate that the emissivity at 15 microns and 55 microns wavelength under all conditions  exceeds 0.9997, and that the integrated emissivity (out to 120 microns wavelength) exceeds 0.9965.  Excellent performance for a home-brew design!

Met Office C-130 "Snoopy" at its retirement (courtesy of Mike Grierson, G3TSO)

This is "Snoopy", the Met Office C130 research aircraft which acted as the first platform for TAFTS (photo credit to Mike Grierson G3TSO who used to fly in this fine aircraft!).  The instrument was capable of fitting into either of the outer wing "pods" (actually converted drop-tanks).  Despite the fact that this is not exactly a vibration-free environment, the extremely low temperatures outside (-50 C) and despite having run out of liquid helium part-way through the first flight, the instrument worked on its first deployment and produced useable results.  Quite an achievement!  The lack of helium was caused by the entire flight crew watching the two beleaguered instrument operators whilst tapping their watches and muttering loudly about what time the local pubs would close: the instrument operators took the hint and left the cryostat only part-filled!  It is not easy to get liquid helium up to the wing and still less easy to tell if the tank is full in the blowing winds of Boscombe Down!