In the context of reported low and decreasing throughputs of DIS, it has several times been suggested (most recently by Michael Strauss) that the lens monitoring group use its repeated observations of the lens fields which have been obtained regularly starting in December 1994 to look for such a decline in sensitivity. We have now done such an exercise and the results are displayed in the 4 gzip compressed postscript file plots available in the elt/:disthruput subdirectory of the anon ftp area on astro.princeton.edu . These plots show raw count rates for each of the 9 secondary photometric comparison stars in our 0957+561A,B field, separately for the g and r bands. In each case the count rates have been divided by the average value for the star in question over all of the frames displayed (a convenience for display purposes since the actual rates vary over about 2 orders of magnitude). In the g.ps and r.ps plots, the data for all the stars is superimposed, while for the gpara.ps and rpara.ps the data for each star is offfset by an additive constant to make the nearly identical behavior of each more apparent. These data are not so suitable as one might first think for this purpose because our programs concerns itself only with *relative* photometry of the lens images with respect to the secondary standards in the field. This allows us to take data in (sometimes grossly) non-photometric conditions, to ignore air mass corrections and to use aperature photometry despite variations in seeing. All these factors, which cancel out for purposes of relative photometry, do affect the count rates plotted. Thus, for any given star, we see a large spread in count rates depending on observing conditions; these variations cannot be interpreted as DIS throughput variations on a point-by-point basis. Nevertheless, there are suffciently many observations that long term changes are still apparent. Basically, the upper envelope of the distribution of count rates for a given star represents observations made in relatively good (photometric and seeing) conditions. Points taken with air mass > 1.2 have also been excluded from the plots, although this makes no qualitative difference, it does decrease the scatter somewhat. The data show a definite decrease in the DIS throughput (really the total efficiency of the whole system including telescope, etc.) in the r band between the 1995 and 1996 0957+561 seasons. The amplitude of the decrease is at least 30% or so. Moreover, the decline appears to be continuing and perhaps accelerating in the 1996 data. In the g band, any difference between 1995 and 1996 is less apparent, but there is an apparent decline during 1996. The history of the counting rate for different stars is very similar which gives one some confidence that the measurement is meaningful. While these data do support the contention that the DIS system throughput (in imaging mode, at least) has dropped substantially since the instrument went into regular use, the g vs. r difference is puzzling. Typically, reflectivities degrade more rapidly in the blue than the red. Tomislav Kundic Ed Turner APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 17 in the apo35-dis archive. You can find APO the archive on http://astro.princeton.edu:82/apo35-dis/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-dis@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO