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Echelle Spectroscopy. Dr Ray Stathakis, AAO. What is it?. Echelle spectroscopy is used to observe single objects at high spectral detail. The spectrum is mapped as a 2-dimensional array onto the detector, providing large wavelength coverage. How is it done ? 1) The Echelle grating.

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echelle spectroscopy

Echelle Spectroscopy

Dr Ray Stathakis, AAO

what is it
What is it?
  • Echelle spectroscopy is used to observe single objects at high spectral detail.
  • The spectrum is mapped as a 2-dimensional array onto the detector, providing large wavelength coverage.
how is it done 1 the echelle grating
How is it done ? 1) The Echelle grating.
  • Gratings produce a double series of repeated spectra through diffraction.
  • The spectra closest to the centre are the 1st orders. Conventional spectrographs usually operate in 1st or 2nd order.
  • Echelle gratings are specially designed to operate at very high orders (70 - 150).
  • Each order has a blaze efficiency function.
  • The rulings are coarse.
  • The light reflects off the short face.
  • The Echelle grating operates at large angles.
how is it done 2 the cross disperser
How is it done ? 2) The Cross-Disperser
  • Successive orders overlap with constant Ml.
    • M = order number, l = wavelength
    • e.g. red light at 8000A in order 71 falls on top of blue light at 4000A in order 142.
    • the peak of the blaze function goes bluer for larger orders.
  • A prism or grating is used to disperse light in the perpendicular direction to the Echelle grating to separate the orders.
  • The result at the detector is a stack of spectra from successive orders, which goes from blue at the bottom left corner to red at the top right corner.
  • The FSR is the range of wavelengths most efficiently observed at each order.
how is it done 3 spectrograph design
How is it done ? 3) Spectrograph design
  • The main Echelle spectrograph at the AAT is UCLES.
  • It is floor-mounted at the Coude focus.
  • Two configurations:
    • 31 g/mm gives wide wavelength range, full FSR coverage
    • 79 g/mm gives 2.5 x sky coverage
designing your experiment 1 pros and cons of echelles
Designing your Experiment1) Pros and cons of Echelles

R=300

  • Advantages:
    • Efficient at high spectral resolution R where R=l/Dl = 30,000 -1,000,000or resolving 10 - 0.3 km/sec at 6000A
    • Accurate removal of sky features
    • Large wavelength coverage.
  • Disadvantages
    • Limited magnitude range
    • Complex instrumental profile
    • Small sky coverage
    • Slow turnover time

R=2500

R=40,000

R=1,000,000

designing your experiment 2 getting it right
Designing your Experiment2) Getting it Right
  • Check whether you need larger sky coverage.
  • Check the location of important regions of the spectrum
  • Choose the optimum detector.
  • Check integration times using the S/N calculator.
observing technique
Observing Technique
  • The detector is rotated and focused, and the grating is shifted to locate the wavelength region.
  • The beam is continuously rotated to align the slit with the direction of atmospheric dispersion.
  • A ThAr arc lamp exposure is taken to calibrate wavelengths.
  • An optional iodine cell provides even more accurate wavelengths.
data processing
Data Processing
  • Special packages exist to handle the format, e.g. DOECHELLE in IRAF & ECHEMOP in Starlink
  • Data reduction steps are:
    • standard detector correction
    • location and identification of orders
    • straightening of orders & forming “echellogram”
    • wavelength calibration
    • location of target and sky in each order and correction of sky
    • combination of orders into continuous spectra
examples of echelle science
Examples of Echelle Science
  • Searching for planets by finding stars which wobble.
  • Observing atmospheres of stars which pulsate.
  • Observing halo stars to determine the chemical history of our galaxy, and even the universe.
other echelle techniques
Other Echelle Techniques
  • UHRF - provides single order observations at up to R = 940,000.
    • UHRF is ideal for studying cool clouds in the ISM.
    • Other projects include atmospheric lines from Mercury and isotopes in stars.
  • The Semel polarimeter is used with UCLES. The main project is Zeeman Doppler Mapping of the magnetic structure of stars.
  • The Manchester Echelle provides single order observations over a large area, and is ideal for ISM emission line studies.
useful sites and references
Useful sites and references

Useful technical information can be obtained at the AAO web site:

  • http://www.aao.gov.au/astro/instrum.html

under UCLES and UHRF. See the on-line manual, the S/N calculator and on-line ThAr arc atlas.

Further reading includes:

  • “Astronomical Optics” by Daniel Schoeder, 1987, Academic Press Inc. (General)
  • Walker, D. D. & Diego, F. 1985, MNRAS, 217, 355-365 (UCLES)
  • Barlow, M. J. et al., 1995, MNRAS, 272, 333-345 (UHRF)
  • Diego, F., et al. 1995, MNRAS, 272, 323-332 (UHRF)
  • Diego, F. & Walker, D. D. 1985, MNRAS, 217, 347 (UCLES & UHRF)