Subject: An unexpected warming of the DSC dewar


Submitted: Thu, 18 Apr 96 02:47:15 -0500

Message number: 32 (previous: 31, next: 33 up: Index)

	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

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