An Argon lamp is used for wavelength calibration with UIST. Plots of Argon arc lines obtained with the UIST grisms are available from the table below. A 2-pixel wide slit was used with all data, except for the IJ, JH and HK grisms, where a 4-pix slit was used. Lamp lines are labeled in each plot (with VACUUM wavelengths). Arc line designations are also available from the CGS4 arc line web pages.
|* Arc spectra taken with the Short-L, Long-L and M-band grisms use the J- and H-band spectral blocking filters. These only give an APPROXIMATE wavelength calibration; see the discussion below.|
Short-H and short-K Grisms
Arc spectra taken with the short-K and short-H grisms exhibit unusual internal reflections, which are manifest as large circles with long-slit spectroscopy, and faint, shifted “ghost” lines in IFU data (see examples below). These features are not seen with the other grisms and are independent of the blocking filter used. These reflections seem to be associated with the calibration unit itself, since similar features are not seen from sky lines with long exposures. They should therefore not affect your data.
Wavelength calibration in the thermal IR
NOTE: short-L, long-L and M-band spectra should be wavelength calibrated with sky lines; using arc spectra will only give an approximate scale.
The optics associated with the calibration unit are quite bright in the L and M bands. Consequently, arc lines are routinely observed through the J- and H-band spectral blocking filters in second (short_L) or third (long_L and M) orders. However, these filters also introduce a wavelength shift, so that simply calibrating a target spectrum (observed through the L or M-band blocker in first order) with these second or third order J-/H-band arc lines gives an incorrect wavelength scale.
Estimates of the wavelength shift – between spectra properly calibrated with sky lines and spectra calibrated with argon lines observed in a different order – are given below.
|*A short-L spectrum calibrated first with an argon arc spectrum (yellow) and then with telluric absorption lines (red). The latter is shifted by -0.04 micron, though there is some wavelength dependence to this value.|
**Plots showing emission-line spectra (Orion) calibrated using an argon arc spectrum (observed in 3rd order); the emission lines in each spectrum consequently appear at incorrect wavelengths.
Note that the orac-DR pipeline will calibrate IFU spectra using Argon arc spectra (long-slit spectra will be assigned an estimated wavelength scale). To shift spectra to the correct (to first order) wavelength the offset noted in column 2 should be applied. However, best results will always be obtained when sky lines are used for wavelength calibration.