1995 Update - In early 1995, concerns were raised about the accuracy of the extrapolations of the sensitivity corrections to times beyond the original database of observations. While no errors due to the extrapolated corrections were seen in NEWSIPS data, the IUE Project felt that updating the old sensitivity degradation analysis would result in more accurate fluxes for post-1992 and 1993 images. As a result of these concerns, the LWP and SWP sensitivity degradation analysis was updated in 1995 using data taken though the beginning of 1995. These new corrections were applied to post-01 January 1990 images while the old corrections were used for pre-1990 images. This 1995 update represents the first sensitivity degradation calibration update. No calibration update was required for the LWR camera, as it comprised a closed data set.
1996 Update - Towards the end of 1996, a changing trend in the late-epoch sensitivity degradation of the SWP camera was seen. Absolutely calibrated fluxes of post-1994 data showed systematic errors of approximately -3% (i.e., lower fluxes) when compared with images taken prior to 1994. This indicated that the 1995 updated calibration was under-correcting the post-1994 data and could occur if the rate of SWP camera degradation were increasing. In the case of the LWP camera, there was marginal evidence for a possible opposite effect, namely a slight overcorrection of post-1995 data, which could occur if the rate of camera degradation were tapering off. As a result, the SWP low-dispersion point source sensitivity degradation correction analysis was updated to include data taken through August of 1996 (Garhart 1997). In order to join the new late-epoch correction smoothly to the existing linear fit, a new linear fit was computed, joined to the old linear fit at a specific time (1993.0) and extended through a point defined by the average of the last three time-binned fluxes (i.e., an average of 1995.5, 1996.0, and 1996.5 bins). This resulted in a set of segmented linear fits (one for each wavelength bin) which are splicings of the previous linear fits and the late-epoch linear fit. A similar approach was tried with both SWP trailed and LWP point source data. However, in both cases the use of a segmented correction was no better than using an extrapolation of the old sensitivity degradation correction. Therefore the IUE Project decided to continue to use an extrapolation of the 1995 sensitivity degradation correction update for these two cases.