The DSC dewar warmed up unexpectedly on Wednesday morning. Because of the possibility that the warming was caused by a (catastrophic) loss of dewar vacuum, this event was treated as a matter of great concern. Eddie Bergeron noticed the DSC was warm when he went to fill it at sunrise, and alerted the rest of the APO staff. Later in the morning Jon Brinkman, Steve Knapp and Tim Mckay removed the dewar and brought it down to the lab to warm up. Both dark frames and dewar temperature measurements taken during the night showed the dewar warming up soon after the DSC was taken off the telescope. After consultation with us back here at Fermilab, Jon and Tim tested the dewar to try to understand why it warmed up. They hooked the DSC up to a vacuum pump, evacuated the hose to levels below what could be in the dewar (< 10^-5 torr), and opened up the DSC vacuum valve. The maximum rise in pressure at the pump was to a few 10^-4 torr. We take this as showing that the DSC was still under a reasonably high quality vacuum. This then suggests that the warming up of the dewar was caused by exhaustion of the LN2. The dewar was filled at sunset of the previous night; we can only assume that somehow it was not completely filled. We don't understand the details, but will investigate over the next few days. Nevertheless, it is currently the most likely reason for the warming of the dewar, and of course, the most benevolent of all the possible causes. We are pumping on the dewar over night, and if everything continues to check out positively, the DSC will be available on Thursday night. The best part is that the system we put into place to avoid contaiminating the CCD worked as planned: the outboard temperature sensor allowed the observer to confirm his fears that the dewar was warm and not fill it; the staff, alerted, brought the dewar to the lab and allowed it to warm the rest of the way up; and we at Fermilab were brought into the loop very early on. This was the way it was planned to work, and it worked smoothly. Jim Annis APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 32 in the apo35-dsc archive. You can find APO the archive on http://astro.princeton.edu:82/apo35-dsc/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-dsc@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO