Dust shutdowns I have looked at Gordon Richards plots, and would like to point out a number of issues. 1. While Gordon, for the period 05/22-07/24 would infer an average shutdown percentage of about 10, the actual night time shutdown percentage to dust only (not including other weather) for that period, according to Gloria's graphs is at least 15%. Since weather and dust may go together, I suspect that the shutdown criterion is actually reached many more times during the nights than 15% of the time. Either we have more dust (or humidity ....) at night, or we shut down below 1600 a significant number of the time. I would expect, with winds dying down in general after sunset that dust content would be less at night, not more. 2. The lack of correlation between 1 and 0.3 micron dust implies that our shut downs appear to be completely ineffective at avoiding small dust particles. However, I do not advocate shutting down based on counts for 0.3 micron particles, not knowing which particles settle down most and are most damaging. But this does illustrate that we may not know what we are doing. 3. I would rephrase Gordon's conclusion regarding the effectiveness of the shutdowns. In spite of shutting down 10 to 15% of the time for dust only, we appear to avoid no more than 20% to 30% of the possible total dust accumulation. Even if we assume that 1 micron dust accumulation is the primary cause for a drop in throughput of, say, 15% per year, this implies that by shutting down 10 to 15% of good observing time, we stretch realuminization from every 2 yrs (if we want to realuminize when we reach 70% or so of optimum throughput) to maybe every 2.5 to 3.0 years or so. If realuminization takes 1 month, then we save 1 month every 5 to 6 years, but we shut down the telescope for dust at least one month per year, so we have lost 6 months over that 6 year period due to dust shutdowns. Even if I am off by factors of two, we appear not to be doing very well under the current scenario. Even if mirror degradation happens faster than in the example above, we would be losing out under the current situation. What would be worth doing is repeat Gordon's analysis for the night time data, and add up exactly for the dust shutdowns how much dust we avoided. One reason for doing this is that my analysis ignores dust accumulation avoided for shutdowns other than dust (so weather, instruments, etc.) What we really need is accumulation during time that we actually observed, and compare that with the fraction of dust avoided by shutdowns only for dust conditions. I agree with his final conclusion: In my mind we have not much reason to believe that we are doing anything better than a regular flash light test at this point, and we are likely shutting down too much. There are many unknowns in this whole issue. Given this, I see no reason why we should use a different criterion for dust shut downs than used at other observatories, until we actually understand the dust monitoring figures. I am in favor of continuing the dust monitoring to learn more what we may be measuring. But I suggest that we readdress our shutdown policy based on dust. Rene Walterbos APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 181 in the apo35-general archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-general/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-general@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO