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UCLES Iodine Cell

Availability of an Iodine Cell for Precision Velocity Measurements

In Semester 1998B, an Iodine Absorption Cell was made  available for use with the UCL Echelle Spectrograph (UCLES) at the 3.9-m Anglo-Australian Telescope. The Iodine cell is used to provide a fiducial wavelength scale for making precise Doppler velocity measurements.


The Iodine cell is mounted directly in the telescope beam, immediately behind the entrance slit of UCLES. The Iodine cell is a cyclinder of pyrex, 10 cm by 5 cm, filled with 0.001 atmospheres of molecular I2. The absorption cell is temperature stabilized at 50 C (+/- 0.1 C) with a temperature controller. The cell imprints a rich forest of molecular Iodine lines (see example in figure below) from 5000 A to 6000 A directly on the incident starlight.

Sample iodine spectrum Two Angstroms of I2 Spectrum

The intrinsic I2 absorption spectrum, (R~1000000) is shown at the top. Below is an intrinsic stellar spectrum, (R~100000), and below that a stellar spectrum observed through an Iodine cell (R ~ 60000). The dots on this trace are observed data points, and the line a model fit.

The very bottom trace is the residuals between the observed spectrum and the model,  multiplied by a factor of 10.

Observing with the Iodine cell

The AAT staff can set up and align the Iodine cell for use with UCLES. The Iodine cell is now located in the same filter wheel as the focal modifier and can be moved in/out of position using the UCLES GUI in the control room.

The reduction in throughput due to the Iodine cell is roughly 30% (~0.4 mag). A plot showing the measured reduction in throughput versus UCLES order can be found here.

Spectra of Iodine cell

Spectra of the AAT Iodine cell taken with the NIST-FTS (National Institute of Standards and Technology-Fourier Transform Spectrometer) have been provided by Paul Butler. These spectra are taken at a resolution of R~1,000,000. The two spectra (in FITS format) are for the Iodine cell at a temperature of 50 C and 60 C. Two NIST-FTS lamp scans were taken on the same day as the Iodine cell spectra and can be found here and here.


It is expected that any users of the AAO Iodine cell should make an acknowlegement to Paul Butler of the contribution in making the Iodine cell and NIST-FTS spectra available in any papers they publish (e.g. "The authors acknowledge the contribution of Paul Butler in donating the Iodine cell and NIST-FTS atlas for use with the UCLES instrument on the AAT.").

Data Reduction

There are no standard (eg. IRAF) packages for the analysis of spectra with embedded Iodine lines. Precisions of 50 to 100 m/s can be obtained by a relatively simple cross correlation routines (Star shift - Iodine shift). Higher precision requires complex spectrograph modeling, using the embedded Iodine lines to recover the point-spread-function (PSF). Untreated, small changes in the PSF will lead to systematic velocity errors of 25 to 100 m/s.

The following three postscript papers outline reduction strategies that have been successfully used for Iodine data. The first paper discusses the use of Iodine for variable star studies. Typically precision of 50 to 100 m/s is sufficient for this work. The other two papers discuss full spectral modeling, which is required to achieve precision at the 3 to 5 m/s level.

Further information can be obtained from Paul Butler.

1993 ``Cepheid Velocity Curves from Lines of Different Excitation and Ionization: I. Observations'', Astrophys. J., 415,323. (R.P. Butler)

1996 ``Achieving Radial Velocity Precision of 3 m/s '', Pub. Ast. Soc. Pac, 108, 500. (R.P. Butler G.W. Marcy, E. Williams, C. McCarthy, and S.S. Vogt)

1995 ``Determining Spectrometer Instrumental Profiles using FTS Reference Spectra''. Pub. Ast. Soc. Pac, 107, 966. (J. Valenti, R.P. Butler, and G.W. Marcy)

Original page by Paul Butler. Last modified 4 June 2009 by Stephen Marsden
Format updated 24 January 2005  cgt
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