The Heart of SPT-3G

We have finished packing up the old SPTpol camera and are now working on assembling all the components for the SPT-3G camera.  My first job was to connect readout electronics to our detector wafers.  I'll talk more about how our detectors actually work in another post, but for now, they are basically extremely sensitive thermometers that measure the heat of incoming light. Each detector wafer is custom fabricated at Argonne National Lab and has 1600 superconducting detectors, making each one as sensitive as the entire SPTpol instrument.  We will have 10 wafers in the full camera, but for right now I'm prepping two for an initial commissioning.

We ship the detector wafers pre-packaged down to the pole.  Each one has an array of lenslets that will couple the incoming light from the telescope into the detectors, and then a protective holder.  You can't actually see the detectors when they are in this package, only the lenslets and back cover.  There are, however, gaps in the back cover that allow cables to come out of the package.  Conversely, these holes also allow things to fall onto the detectors, so doing the readout assembly is a job that requires a lot of concentration to prevent mishaps.  The picture below shows the work bench in our clean tent.  The hexagonal object in the white cylinder is the detector wafer (the white lenslets are visible).  Fun fact, just like on a pair of glasses, we anti-reflection coat our all of our optics, including the lenslets.  The white layer you see is actually that coating, tuned to millimeter wavelength light.  The other main object in the picture (green circuit boards with honey-colored wiring coming off) is the first set of readout electronics to be assembled.

The first SPT-3G detector wafer to have readout assembled onto it at pole!

The readout electronics go on the back cover of the detector assembly, where all the cables are.  So I have to flip the detector assembly over, and pull all of the cables out of the way.   It looks a little silly when it's prepped, but it's important to manage all the cables so that they don't get damaged.

The same SPT-3G detector wafer with all the cables ready to be hooked up.

Now the wafer is ready for me to plug in the readout boards one by one, and then screw them into the back cover.  Here's a picture of the first one in place.  Again, it looks silly because I've covered everything with sticky notes, but it's a great way to prevent any screws I might drop from falling through a hole and damaging the detectors.

One board in place, 11 to go!

Then I just work my way around the back cover until all the boards are connected and in place (12 total).  Both SPT-3G wafers are now fully assembled and once we've built up the receiver, they'll go straight into the middle section of the new camera. 

Both SPT-3G wafers, ready to go.

Top down view of one wafer assembly.

Maybe I'm just a little biased, but I think the finished products are awfully pretty.