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.
Hardware
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.
 |
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.
***NOTE***
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)
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