SPIRAL Spectrograph

  • What is SPIRAL?
  • A brief despription of SPIRAL
  • First light for SPIRAL
  • Performance and signal to noise calculations
  • Software for SPIRAL and other IFU's

    What is SPIRAL?

    Spiral is a prototype integral field unit (IFU) connected via eighteen metres of fibre optics to a dedicated spectrograph on the floor of the AAT observatory. The name stands for Segmented Pupil/Imaging Array Lenses, and consists of two modes - integral field imaging and pupil segmentation imaging.

    High resolution (R ~ 8,000) work on faint stars/distant galaxies with large telescopes is difficult for a variety of reasons. The most prominent is the mismatch of the image scale in the focal plane of the telescope with the width of the entrance slit in ths spectrograph. Widening the slit does not help as your spectral resolution decreases at the same time.

    An IFU allows you to collect light over an extended area on the sky and also obtain spectral information for each spatial element in that area. This means you can produce maps of various emission lines, absorption lines, continuum and two-dimensional velocity maps. To do this you need to reshape the light from an extended area in the sky onto a long thin slit suitable for feeding into a spectrograph.

    There is an HTML-ised paper written for the "Optical Telescopes of Today and Tomorrow" symposium SPIE 2871 (1997). This paper covers all the equations concerned, complete with diagrams. If you have a printer, there is a postscript (376K) version of this paper available.

    A brief description of SPIRAL

    Light-path in SPIRAL (GIF, 12.4K) The diagram on the left shows the physical set-up of SPIRAL on the telescope. There is an optical table bolted onto the Cassegrain focus of the telescope on which the fore-optics and lens array are mounted. The fore-optics re-image the scale of the focal plane so that each lenslet corresponds to 0.5 arcsec on the sky.

    Picture of the optical table, with 
fore-optics and lens array shown (JPG, 111.4K) The picture on the right shows the aperture in the primary mirror at the top, with the fore-optics just below. Further down the optical table is the lens array unit, and the fibres pass through the black conduit into a strain-relief box in the bottom left and out of the Cassegrain cage.

    Picture of conduit from Cass cage to floor of Observatory (JPG, 
82.6K) The conduit now passes through the base of the Cassegrain cage which contains the lens array, and drops down to the floor of the telescope dome some twelve metres below. The spectrograph can now be seen with it's light-proof covers off. As the telescope swings around at night, we checked to our satisfaction that the conduit did not catch on any surrounding catwalks.

    Picture of the fibre-slit and light-proofed spectrograph (JPG, 
84.4K) The fibres pass into the spectrograph through the fibre slit where they are rearranged into a 1-D line. Due to the design of the spectrograph, the dewar with the CCD inside is situated next to the slit - it is a large red cylinder which is behind the grey box of electronics.

    Detail picture showing fibre slit and red 
dewar with first field correcting lens (JPG, 71.3K) Here is a detailed picture of the fibre slit (seen on the right) coming into the spectrograph. The light is reflected off a prism barely visible in the corner of the lens closest to the red dewar. The light passes down to the bottom left of the picture towards the grating, then back through the same lenses into the CCD.

    Whole spectrograph showing field lenses 
and grating on spectrograph table (JPG, 84.4K) Here is the whole spectrograph with no light-proof covers on. The fibre slit is hidden behind the red dewar and the grating is now easily visible. Another advantage of having small optic fibres as a 'slit' is that there is almost perfect matching of fibre diameter to pixel size on the CCD. This allows construction of a Littrow configuration spectrograph, where camera and the collimator are the same set of lenses, allowing the grating to be used most efficiently.

    First light for SPIRAL

    Picture of the spectrograph showing the
back of the grating, the field lenses and dewar (JPG, 68.6K) The comissioning run for SPIRAL was in the middle of February 1997 at the AAT. We suceeded in observing for one and a half nights and we lost one and a half nights to bad weather.

    The aim of the run was to measure the throughput of SPIRAL, and to see if we could detect Lithium absorbtion in a bright star, in preparation for examining candidate brown dwarfs. We also wanted to demonstrate the integral field capability and to this end we looked at the cores of nearby Seyfert II galaxies (the Circinus galaxy and NGC 1068) around 7500Å.

    Spectrum of the Lithium I line taken 
with SPIRAL (GIF, 12.6K) On the second night we obtained the Lithium absorption line (as seen on the right here) from a bright star.

    Flux map of the centre of Circinus 
galaxy (GIF, 5K) Velocity map of the same region of 
Circinus as the flux map (GIF, 5K) On the left is a flux intensity map of the continuum in the middle of the Circinus galaxy, an active galaxy. The centre can be seen in the lower left corner, together with some structure surrounding the core. On the right is the same region of sky, but this is a velocity map of the Hydrogen alpha line. The range is from -130 km/s to +140 km/s with blue signalling gas moving towards us. With an R of 10,000, there is an error of ±15 km/s.

    Performance and signal to noise calculations

    The efficiency of SPIRAL is shown in the table below as compared to some other spectrographs. The overall efficiency from top of atmosphere to the detector (including QE) is 11.8% for 6750 Angstroms with the telescope pointing at the zenith.

    Efficiencies for SPIRAL and other spectrographs
    Atmosphere 0.88 x x x x
    Telescope 0.61 x x x x
    Spectrograph slit (SPIRAL fibre optics + fore-optics) 0.68 0.36 0.41 0.40 x x
    Spectrograph optics 0.52
    Detector 0.62 x x x x
    Measured throughput 11.8% x x 12.5% 6.0%
    NOTE: Other spectrograph efficiencies were taken with a wide slit, usually 2 arcsec or more. SPIRAL throughput was with an effective slit width of 0.5 arcsec.

    Below are 5 sigma limiting fluxes for an emission line in a high-redshift starburst galaxy, and for pure continuum

    270R Grating
    Exposure time 0.5 hr 1.0 hr 3.0 hr
    Continuum in R band 18.0 18.8 19.9
    Emission line flux 2.05x10-17 18.8x10-17 19.9x10-18
    With 270R grating which gives R ~ 3600 (arc line width of 85 km/s) For a S/N of 5 in the continuum in the R band and an emission line with S/N of 5 that falls on one spectrally resolved element. A SLOW readout speed on the TEK, and a 6 day moon on the AAT.

    1200R Grating
    Exposure time 0.5 hr 1.0 hr 3.0 hr
    Continuum in R band 18.0 18.8 19.9
    Emission line flux 1.98x10-17 1.01x10-17 3.64x10-18
    With 1200R grating which gives R ~ 10000 (arc line width of 30 km/s) For a S/N of 5 in the continuum in the R band and an emission line with a S/N of 5 that falls within one spectrally resolved element. A SLOW readout speed on the TEK, and a 6 day moon on the AAT.

    Software for SPIRAL and other IFU's

    Snapshot of workstation desktop 
with IRAF IFU software running (GIF, 51.5K) I am currently writing IFU display software for SPIRAL and COHSI. The software can work with any hexagonal array geometry - the lens pattern is input via a simple numeric text file, and data to be displayed can be either by a text file, or more usefully, an IRAF multispec file.

    The IFU software is written entirely in SPP, the core language for IRAF. The two main reasons for this were: (i) IRAF is one of the most popular astronomical packages and (ii) the cross-platform capability of IRAF SPP means that the source code can be compiled on any IRAF platform with the minimum of fuss.

    Jim Lewis of the RGO and Alfonso Aragon-Salamanca (IoA) have helped and guided me through the programming of SPP and the astronomical requirements for the software. As of 21/7/97 the software can now display data files and can examine multispec files. In the next month I hope to start modifying 'dofibres' to work with SPIRAL and COHSI data frames.

    Comments on SPIRAL and on anything to do with this page to Matthew Kenworthy - email: mak@ast.cam.ac.uk

    URL http://www.ast.cam.ac.uk/~optics/spiral/spiral.htm - Revised: 20 Jul 97

    We are part of the Institute of Astronomy , which is part of the University of Cambridge