Anytime that the grating position changes (or at least once per night), wavelength calibration data should be taken by either observing lines of the CSHELL (Ar, Kr, Xe) lamps or by observing sky lines in the data itself. Since CSHELL uses vacuum wavelengths, be sure to correct rest wavelengths for any velocity shifts in your objects. The first step is to select which lines to use in the calculation of the spectral zero point and dispersion at each observational wavelength (grating position) selected. An auxiliary program, cal_lines, is used to determine which calibration lamp lines are to be used at a given wavelength selected for observation.
Since CSHELL's spectral range is only 1/400 of the central wavelength, calibration lines rarely fall on the arrays when the instrument is set up to observe lines of astronomical interest. We can trick CSHELL into observing these calibration lines at the same grating position as used for astronomical lines by selecting different orders of the same grating setting. This is accomplished by moving the CVF filter (CVF Wlen box NOT the Wavelen boxes) to select different orders after the grating has already been positioned for the desired astronomical wavelength. NOTE: Whenever changing the CVF or central wavelengths, the associated button must be clicked). In practice, observing approximately four lines that span at least half of the array (128 columns) for each grating setting will allow for good determination of the dispersion and zero point. The observation of a line of astronomical interest and the calibration lines should be observed without movement of the grating.
The general steps are:
Flat-fielding with Cshell requires 10-30 flat frames (depending on desired S/N) on a continuum source with counts similar to those in your object frames. This should be done at the beginning and end of the night and whenever standards are measured for each wavelength observed. In order to minimize wavelength shifts and ripples in the spectra, the grating and CVF should not be moved between observations of objects and flats. The continuum lamp is reccomended for λ < 3 μm and dome flats for λ > 3 μm.
All exposures include a dark current contribution from the detector arrays. This current can be a significant fraction of the signal from astronomical objects since CSHELL operates at very high resolution. Objects are normally observed in AB pairs (on and off the source OR moved along the slit) which are subtracted to remove this dark current contribution from the data. We must observe separate dark frames to remove this component from the flat fields, however. This is done by turning off the calibration lamps and closing the shutter before acquiring frames of exposure time equal to the flat field frames. Again, many (5-10) dark frames should be acquired at a time in this manner. They must be acquired with the same detector bias setting as your object and flat field data.
The general steps are:
LAST UPDATE: February 1, 2010