2.2. The Calibration of Wavelengths in NEWSIPS

  Before an analysis of the zero-point errors of wavelengths in the IUE cameras can proceed, it is necessary to review the steps taken in the wavelength calibration of NEWSIPS data; some of the following details have not been published in any form. The calibration of wavelengths in an echelle system requires the prior mapping of pixels from a two-dimensional detector surface to a series of one-dimensional arrays each of which is represented as a smoothly varying function of wavelength. In the raw detector geometry the echelle orders fall at unique angles relative to the electronic scan axis, and thus together they run nonparallel to one other. In addition, in certain places on the detector the spectral format can be displaced by local shears from slight nonuniformities within the optical coupling elements. All of these factors necessitate a detailed geometrical resampling of the raw echellogram image to a new surface in which the orders run parallel along a common dispersion axis and in which their fluxes have been reevaluated at a constant velocity increment. NEWSIPS maps the raw positions to a rectilinear geometry, thereby simplying the extraction of spectral fluxes (for details see Chapters 7-9 of Garhart et al. 1997). Wavelength calibration (``WAVECAL") observations of the three cameras were made monthly during the mission through the small aperture by means of on-board platinum-neon (Pt-Ne) lamp source. Early in the mission a tungsten flood lamp (``TFLOOD") exposure was added to the emission-line exposure in order to enable process software to determine the positions of reseau more easily. In April, 1990 this practice ceased when studies within the IUE Project demonstrated that these pre-exposures added a noise component to the signal which effectively reduced the instrumental dynamic range and thus the number of measurable emission lines.

With the linearized fluxes extracted, the wavelength system of each spectrograph camera could be calibrated by means of a reference calibration spectrum by remapping the (x,y) positions of its emission lines from the original geometry to the rectilinear detector surface. For this step one may think of the dispersion axis as ``x" and of the spatial (or echelle order m) axis as ``y." Wavelengths of the emission lines were taken from the study of Reader et al. (1990), which was commissioned for the purpose of calibrating wavelengths of the HST spectrographs. The first of four steps in the NEWSIPS wavelength calibration procedure was interactive and consisted of running the IRAF routine ecidentify to map the positions of a set of lines to wavelengths for a reference WAVECAL echellogram. In this step a quadratic Cheybshev polynomial functon was used to map the (x,y) pixel locations in the reference echellogram to a wavelength table. The second step was to utilize the IRAF routine ecreidentify Chebyshev solution determined for the reference echellogram in to determine predicted pixel positions for a few hundred lines in all the WAVECAL observations for a given camera. A polynomial fit was performed for each of the echellograms, resulting in a set of wavelength zero-points and mean pixel-to-pixel wavelength increments for each order. In practice, the wavelength zero-points for each ``science echellogram" were fit to a cubic polynomial in both time and camera-head temperature (``THDA"). These fits were adopted for all science data but of course not for the WAVECAL calibration observations themselves. This last step compensated for image shifts statistically by means of a previously derived least squares correlation of wavelength shift as a function of time and temperature. Note that the wavelength calibration of the WAVECALs technically applies to observations made through the small aperture. However, this calibration can be readily applied to large-aperture observations from prior measurements of the offset between the two apertures in the dispersion direction.

We should pause to point out that whereas the wavelength calibration for NEWSIPS was performed with a simultaneous polynomial fit in both the wavelength and echelle order dimensions, the number of WAVECAL emission lines was sufficient to permit a calibration of wavelengths for most echelle orders separately (Smith 1991). The first (``global") approach is clearly the preferred one if pixel-to-pixel spacings on the detector are regular because a large number of lines can contribute to the solution for any individual order. However, during the IUE lifetime the cameras suffered both electro-optical distortions and differential shifts arising from shears between fiber-optical bundles within the coupling-plate of the camera. A global wavelength solution tends to smooth over these small-scale distortions, causing correlated order-to-order errors in derived wavelengths. Consequently, with exceptions noted by Garhart et al. (1997), the ecidentify steps described above were implemented for individual spectral orders in the NEWSIPS calibration.