Status of the STIS Auto-wavecal Exposures

Abstract

We discuss the behavior of the default ``wavecal'' spectra obtained together with most STIS spectroscopic exposures, which are needed for proper wavelength calibration of the science data. Because the Pt/Cr-Ne lamps used for the wavecals have been fading (especially at the shortest wavelengths), some changes in the default lamp and/or exposure time have been implemented in recent years to maintain accurate calibrations. To assess whether additional changes might be appropriate, we examine the distribution of the SHIFTA1, SHIFTA2 values derived from the wavecals (the x and y offsets of the spectral image on the detector), we re-visit the wavelength-dependent fading of the lamps, and we perform simulations to estimate the exposure times that would be needed to obtain accurate SHIFTA values. While the current wavecals do appear to yield reasonable SHIFTA, increases in the default exposure times for some of the shortest-wavelength settings would help to ensure reliable wavelength zero points as the lamps continue to fade.

Figures (15)

• Figure 1: Pt/Cr-Ne wavecal spectra at the G140M/1222 setting, obtained with the LINE lamp (top) and the HITM1 lamp (bottom). In each panel, the spectra in red were obtained in 1997/1998 (cycle 7), and the spectra in black were obtained in 2024 (cycle 30). While the emission lines in this setting were initially much stronger in the LINE lamp spectrum, they also have faded more (especially at the shortest wavelengths) -- so that the lines below $\sim$1240 Å are now stronger in the HITM1 spectrum.
• Figure 2: Pt/Cr-Ne lamp spectra at the G140M/1173 setting. In black is a 46s LINE lamp spectrum from cycle 18 (PID 12414); in red is a 150s HITM2 lamp spectrum from cycle 19 (PID 12775). Even then, the emission lines at these short wavelengths were stronger in the HITM2 spectrum -- and fading less rapidly -- which motivated a switch from LINE to HITM2 for the wavecal exposures in 2012.
• Figure 3: Pt/Cr-Ne lamp exposures at the E140H/1234 setting. Individual echelle orders run approximately horizontally, with wavelengths increasing to the right within each order; the longer wavelength orders are at the top. Many emission lines may be discerned, particularly in the upper half of the long (1000 sec) exposure (left panel), which was taken through the 0.2x0.09 aperture. Those lines are very difficult to discern in the default (13 sec) wavecal exposure, taken through the 0.2x0.2 aperture, shown in the right-hand panel -- which nonetheless yielded fairly reasonable SHIFTA values.
• Figure 4: SHIFTA $-$ MOFFSET values for E140H/1234 observations taken through the 0.1x0.03 and 0.2x0.09 apertures (upper and lower panels). The left-hand panels show SHIFTA2$-$MOFFSET2 vs. SHIFTA1$-$MOFFSET1; the center and right-hand panels show SHIFTA1$-$MOFFSET1 and SHIFTA2$-$MOFFSET2 vs. MJD. In the left-hand panels, most of the points fall within a main clump near (5, -2), but there can be one or more outlying sub-clumps and/or more isolated "outliers" as well. The sub-clumps appear to reflect some combination of increased scatter during the period when monthly offsets were applied to the echelle settings (MJD $\sim$ 50825--52500), changes in the slit wheel and/or mode select mechanism values adopted for each setting, and/or some non-reproducibility in the results of those values.
• Figure 5: SHIFTA $-$ MOFFSET values (as in Fig. \ref{['fig:stats1']}) for E140H/1234 ond 1271 observations taken through several apertures. The most recent points for E140H/1234/0.2x0.2 are all for eta Car, which is often discrepant in SHIFTA2$-$MOFFSET2.