feb-08

The remainder of the summer people board the last LC-130 aircraft
for the season leaving winter polies isolated for 8-1/2 months
(photo by keith vanderlinde)

coming soon
A preliminary CMB-SZE image map of
galaxy cluster AS1063 taken with SPT

For the first two weeks of winter since the summer team has left, Keith and I have been tweaking parameters on the refrigeration cycle. The bolometer array (sensor) is cooled down to about 0.25 degrees above absolute zero (0.25 Kelvin) using as series of helium coolers. A pulse tube cooler takes us down to about 2.7 Kelvin and then a subsequent Simon Chase helium 10 cooler allows us to take the bolometers and SQUIDs down to transition and superconducting temperatures as low as 1/4 degree Kelvin. The entire cooling system is closed cycle and requires no transfer of expendable cryogens. The Simon Chase cooler is called a helium 10 cooler because the three stages, a helium 4 stage and two helium 3 stages adds up to 10. Helium 3, with an atomic mass af about 3 is an isotope of standard helium 4 and its properties allow us to get to lower temperatures. We have been tweaking the refrigeration cycle in order to optimize the cycle time. During observation time, liquid helium 3 eventually all vaporizes and is collected in the charcoal adsorption devices. The helium is then recondensed as liquid by recycling (heating) the charcoal and driving off the helium, allowing it to condense at the the other end of the cooling stage. Minimizing the re-cycle time and maximizing the helium 3 vaporization time at 0.25 Kelvin before running out of liquid helium 3 results in the maximum CMB observation time. We are getting total cycle times on the order of 39 hours, although we would like to push it to 48 hours, in order to make the winterover's sleep cycle a bit more normal.

In addition, we have been running focus scans schedules sent to us in order to optimize the optical bench positions as a function of telescope orientation. The University of Chicago, Berkeley, Case Western Reserve and the other collaborators are working hard to send us focus scans, CMB scans and to troubleshoot every aspect of this complex instrument. Keith and I participate in two teleconferences per week with the entire team (http://pole.uchicago.edu/) using the Iridium satellite system.

We have also had numerous mechanical issues to deal with which is typical in this harsh climate. It is still February and the temperature has already dropped to -67 F (-55C) and it will continue to drop over the next months to temperatures that can exceed -110 F(-80C) and that is the static temperature, not any wind chill nonsense. Just this week, the 2000 lb optical bench failed to move to the correct position during a focus scan. The optical bench has 6 actuators driven by 6 servo motors and gear trains. One of the six actuators ceased moving, so Keith and I quickly aborted the current focus scan, docked the telescope and crawled up into the receiver cabin using the hydraulic man-lift. After 7 hours of troubleshooting and repair in this extremely tight space we fixed the actuator and got the telescope back in operation. The telescope was brought down by a 1/4-inch set screw that had loosened. one of the six servo motors and optical bench actuators removing the servo motor measuring the precise depth for the coupling checking out the coupling repair closing the 5000 lb roof after undocking the telescope Other mechanical tasks included replacement of the rollers on the 5000lb rolling docking roof. finishing replacement on the roof

The mechanical aspects of the telescope are a diversion from the operation of the telescope with its many computers. sweeping snow out of the primary mirror ground shield

Some parting shots of SPT while the sun is still up. SPT-1 SPT-2 SPT-3 many summer days are grey