Changes in Sensitivity of the Low Dispersion Modes

Significant changes in the STIS sensitivity are revealed by 2.3 years of observations of the same standard stars in the 52X2 arcsec slit. The largest changes are in G140L, where the sensitivity depends on both the time and temperature. The minor effect is a-0.25%/C temperature correlation, where the bulk of the observations are confined to the 34.5-38.5C temperature range. No significant change with temperature is detected for the near-UV MAMA or CCD modes. Following a correction for the correlation with temperature, the sensitivity of G140L decreases linearly with time. These losses vary with wavelength, ranging from a minimum of-0.8%/yr at 1325Å to a maximum of-2.8%/yr at 1575Å. For G230L, the mean sensitivity over all wavelengths increases by an average of 1.8 % in the first 1.3 years and then begins to drop. Two straight lines with different slopes fit the observed changes. The changes before 1998.7 also vary with wavelength, ranging from a minimum of +0.8%/yr at 2550 to a maximum at 2250Å of +2.1%/yr. The wavelength-dependent slopes of the currently decreasing sensitivity range from zero at the short wavelength end of the band to-1.9%/yr in the 2300-2400Å interval. Changes in the three low dispersion CCD modes are less severe. For G230LB and G750L, most wavelength bins are consistent within a three σ uncertainty of constant sensitivity over time. Only one wavelength bin for G230LB and two for G750L show sensitivity losses with more than three σ certainty. Those changes are small and lie in the 0.2-0.4%/yr range. However for G430L, the losses in every 200Å wavelength bin beyond 3300Å range from 0.5 to 0.7%/yr with a high statistical significance of 2-9 σ. After correcting the three low-dispersion modes for sensitivity changes, the 1 σ photometric broadband repeatability ranges from 0.2 % for G140L and G750L to 0.4 % for spectrophotometry in the other 3 modes.