A study of the wavelength calibration of NEWSIPS high-dispersion spectra

Ma. Smith, A study of the wavelength calibration of NEWSIPS high-dispersion spectra, PUB AST S P, 113(785), 2001, pp. 882-897
Citations number
Categorie Soggetti
Space Sciences
Journal title
ISSN journal
0004-6280 → ACNP
Year of publication
882 - 897
SICI code
In this study we cross-correlate many IUE echellograms of a variety of well -observed stars to evaluate systematic error sources in the wavelength zero points (velocities) of all three cameras. We first evaluate differences be tween the final archived ("NEWSIPS") and the originally processed ("IUESIPS ") echellograms. These show a marked time dependence in zero point for the SWP camera resulting from several revisions of wavelength calibration coeff icients used for IUESIPS. Smaller offsets are present for the LWR camera be tween the two processings. We also evaluated small-amplitude fluctuations i n the zero points of the NEWSIPS wavelength calibration spectra themselves. In the case of the SWP camera, these variations are too complicated to hav e been completely removed in the NEWSIPS wavelength calibration. We also ex amine wavelength zero-point disparities between data obtained both through the small and large entrance apertures as well as for observations made by different target acquisitions of faint and bright stars. We also find that statistical differences between these alternative observing modes are virtu ally nil. For large-aperture observations, the dominant error source is the placement of the target in the aperture. These give rise to non-Gaussian, extended "tails" in apparent velocity. We also searched for spurious trends . Except for a possible trend for faint objects with SWP camera data, we ca nnot detect significant dependences with time. Additionally, we discovered a trend with telescope focus for data sets derived from intensive monitorin g campaigns of bright stars. These exhibit a repeatable, 1 day "radial velo city variation" with a semiamplitude of nearly 3 km s(-1). This pattern app ears to be a by-product of fluctuations in telescope focus caused by operat ional procedures to maintain the ambient instrument temperature. In the sec ond part of the paper, we measure the mean zero-point errors of NEWSIPS ech ellogram data with respect to laboratory results by using the Goddard High Resolution Spectrograph (GHRS) spectral atlas of the O9 V spectral standard 10 Lacertae as an intermediary reference. We find that the derived apparen t velocity difference for this star is essentially zero: -1 +/- 3.5 km s(-1 ). Several less precise comparisons lead to similar results, including cros s-correlations of (1) spectra of 10 Lac and two stars with similar spectra, HD 93521 and HD 60753; (2) lines in common with the SWP camera and GHRS an d STIS atlases of Arcturus and Procyon; and (3) interstellar lines in the G HRS spectrum of the white dwarf G191-B2B. The zero points of the NEWSIPS-pr ocessed long-wavelength cameras are evaluated and are also found to be near ly zero (+/-5 km s(-1)) relative to the Arcturus and Procyon atlas calibrat ions and relative to one another. In general, these results do not support the suggestion by Gonzalez-Riestra and coworkers that corrections should be introduced to the wavelength scales of various NEWSIPS high-dispersion dat a products. Despite our optimistic results, it is obvious that using small IUE data sets from large-aperture observations of arbitrarily chosen stars can contain velocity errors of at least a few km s(-1).