The iodine absorption cell is used to create a fiducial wavelength scale for very precise (m/s) radial velocity measurements. Most iodine features are located between 5000 and 6200 Angstroms, ideal for radial velocity determinations of late-type stars. The iodine cell is physically located in front of the decker plate and can moved in and out of the optical beam via keywords or using XHIRES. To use the iodine cell, it must first be turned on from the HIRES control pulldown menu (right click in background, HIRES control menu, start iodine cell). This will begin warming the cell to its nominal operating temperature. Two temperatures are displayed in an XSHOW GUI: TEMPIOD1 and TEMPIOD2. The former is typically around 65 C and represents the temperature at the outer edge of the cell. The latter is maintained near 50 C and represents the temperature near the center of the cell. Allow at least 45 minutes for the iodine cell to warm up to these nominal values before use. The cell can be turned off from the pull-down menu or by initiating the end of night shutdown script when observing is complete.
The iodine cell was scanned using the Fourier Transform Spectrometer at the National Institute of Standards and Technology in March 2009. The scans were made at resolutions of 500,000 or greater to improve the modeling efforts of the iodine spectrum. The FTS scans are available upon request by e-mailing the instrument master.