Thursday, 29 November 2012

An End to Calibration

 Yesterday was our last day for calibration measurements.  First thing in the morning we went out and unplugged the calibration source.  Everything was packed up onto the back of our snowmobile and we brought it back to SPT.   There will be a second round of calibration measurements made in late January, but we can't leave the source outside for that long.  The source is about 3 km away from the telescope.  To get an idea of the scale, that's me in front of SPT (10 meter dish).  Looking out at the source from the top of the telescope, you can only see a tiny black dot at the end of the road.   Zooming in,  it resolves into a wooden fence (that we use
for shielding) and a big metallic square reflector.  The reflector itself is 25 feet high, and in the middle is actual calibration source.  Once we took the calibration box out, it was an opportune spot to take a picture!

 As soon as we finished calibrations, the telescope was docked to lower the receiver!  Basically, the telescope slews around and down until the boom (the long straight white structure that extends from the dish) is facing the building and is right above it.  There is a sliding door in the roof to the building that opens and the entire boom lowers down onto the hole.  There are two more doors into the telescope boom that open onto the receiver cabin.  
 If you look straight up into the receiver cabin, this is what you'd see.  On the left is all of the readout electronics (all the grey cables and racks).  The large white round cryostat on the right contains all of the secondary optics, and the red and black one in the middle contains the focal plane and detectors.  The optics cryostat is about the size of a smart car.
 In order to take out the detectors for upgrading, the whole thing has to come down into the laboratory space below.  It took about three hours total, during which we lowered both cryostats on the four chain hosts you can see in the picture.  It's a tricky job, because you want to keep it level, and there are places that it can scrape against three of the cabin walls. 
Eventually it was down, with no problems!  Now it's just a matter of time until the receiver team can take out the focal plane.  The coldest part detectors inside was around 300 millikelvin when we stopped observations and both cryostats are under vacuum.  We have to wait until it is around room temperature to open the cryostat, or else risk condensing any residual atmospheric water vapor that is inside on the detectors and ruining them.  Sometime on Friday, the focal plane will come out!

Now that the calibration is over, I'm moving on to my last task here for SPT, upgrading part of the readout electronics.  

1 comment:

  1. Do the calibrations from year to year have to be changed due to snow drift? If so how do you account for that? Joel