Echelle Spectroscopy

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Echelle Spectroscopy

• Dr Ray Stathakis, AAO

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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.

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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.

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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.

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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

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Designing your Experiment1) Pros and cons of
Echelles
R300

• Advantages
• Efficient at high spectral resolution R where
  Rl/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

R2500
R40,000
R1,000,000
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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.

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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.

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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

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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.

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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.

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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)