When Optical Benches Fly

Building up the receiver is not the only work that has been going on at SPT the last couple weeks.  There has been a separate team of people working on installing new optics.  We have a brand new optical bench and two new mirrors that bring the light from the primary dish into the receiver.  The optical bench is shown in the picture below, with the front surface of the secondary mirror showing.   The mirrors themselves are pretty special as they are machined from single pieces of aluminum.  The secondary mirror is a little over a meter in diameter.

The new SPT-3G optical bench before it was installed.

The entire optical bench & mirrors are also covered with the little black and white pieces of paper.  These are actually targets used in a process called photogrammetry.  The idea idea is that you take many pictures of the mirrors and the bench from different angles.  You calibrate the position of the camera, and a computer program then determines exactly how the mirrors are lined up and if they need to be adjusted. Now that the mirrors are aligned, someone will have to take all the targets off and clean up the surface.

The SPT-3G secondary mirror.

The other fun thing to notice is that our mirrors are not as precise as optical mirrors and the surface is rougher.  That is just fine to longer wavelength millimeter light.  You can still see your reflection in the surface of the mirror, but it is very blurry.

My reflection in the SPT-3G mirror.

The next step is actually installing the entire bench into the telescope. It sits at the top of the boom arm, right under where the person standing on the telescope is.  The team doing this had to remove a bunch of panels from the receiver cabin, pull out the old bench and some supports, and then put the new bench in.  The particular day this picture was taken was the day of installing the new bench.  You can judge how much help was needed based on the number of snowmobiles parked out front.

Snowmobile-jam!

Once the cabin was ready, it was time to lift the bench into the telescope using the crane.  First it came out onto the porch of the tent it was in, and then was lifted off the ground, swinging up and around until it was lined up with the hole in the receiver cabin.  It was pretty impressive to watch the entire operation, given the necessary precision of the crane operation.

The optical bench as it is first lifted off the ground.

Up and away.

Coming into the cabin.

All lined up and almost in place.

As soon as the receiver was assembled we turned on the vacuum pump, and then the mechanical refrigerators that cool the detectors down to 0.25 Kelvin.   It took about a week of continuous cooling to get there.  During that time, we were also working on assembling the external readout electronics.  In the picture below, the control electronics for the detectors are in the box on the upper right of the cryostat.  Now that the detectors are cold and the electronics are in place, we've begun turning on everything and trying to understand the performance of the receiver.

Almost a full receiver now that it has electronics!

The other major accomplishment last week was testing the mechanical fit of the receiver into the cabin.  The whole thing weighs somewhere upward of 2500 pounds and gets lifted up by 4 chain hoists until it mates with the optical bench.  With the bigger new cryostats, the fit is really tight, and it is a delicate dance of careful lifting to make sure nothing collides.  

Prepping for the lift.  The first chain hoists are attached and the receiver was tilted into the vertical position.

 

Just barely lifted out of its cart.

Part way up.  You can see it looks like it is at an angle.  This is to keep a part on the side of the receiver from colliding with some wiring on the cabin wall.

It fits!

Getting it up took all afternoon.  After a break for dinner and some more quick checks for fit and alignment, we spent the evening bringing it back down so we could continue detector testing. All in all, a  pretty wild day.  At the end of it, I captured some of the team as they were winding down.

They are actually happy everything worked, just tired from operating chain hoists for 8 hours.