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.

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 Successive orders overlap with constant

M. – M = order number, = 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

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 Experiment1) Pros and cons of Echelles

Advantages:– Efficient at high spectral resolution R

where R=/ = 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=300

R=2500

R=40,000

R=1,000,000

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

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 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

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

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 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)