All the Panels are on the reflector!

(The following is simply a transcription of Jeff's e-mail to the collaboration announcing this milestone.)

Hi all,

Last night we placed the last panel on the bus, and here is the
picture to prove it. The dish may look flat, but that is just the
fish eye undoing its curvature- every indication is that it is very
close to the correct shape. Just wanted to send out a picture so
you can share in our enthusiasm.

-Jeff


ps. The sun dog picture is from right after we finished.

The (almost) fully populated reflector


Auspicious omens in the sky on the occasion of the placement of panel #218

The Telescope & Reflector Begin to Take Shape

A quick disclaimer before today's post: The SPT is not just the primary mirror, and the collaboration does not consist solely of the reflector assembly team. It may seem that way at the moment (at least to readers of this blog), but that will change rapidly as more members of the team --- and more pieces of the instrument --- show up at the Pole. January will bring lots of new faces and much more emphasis on the receiver (i.e., the part of the telescope that actually detects and records the incoming light). But for now, you're going to keep hearing about the telescope infrastructure and the primary mirror.

OK, now back to the post.

Though it may seem slow, the progress on the telescope infrastructure and primary mirror continues --- pretty much on schedule even. A couple of days ago, the first piece of the telescope lower boom (the structure that extends out from the primary mirror and holds the secondary and receiver) went into place, giving the first hint of the scale and final shape of the telescope.

Telescope with part of lower boom


Meanwhile, the reflector assembly folks have spent the last few days in furious panel-placing and -aligning mode. In fact, we're up to a pace of over 20 panels a day placed and aligned to (we hope) less than 100 microns. If we can keep this up, we'll have all the panels placed a few days before Christmas, at which point we spend a couple of days taking care of some wiring and mechanical details, then it's on to testing the surface. The first big test of how well we managed to align the surface will use a technique called photogrammetry. I'll defer a detailed explanation of this process to a later post; for now, it suffices to know that one takes a bunch of pictures with a specially outfitted camera then hands those pictures to an incredibly expensive piece of software that picks out the locations of hundreds of little reflective targets stuck to the mirror surface (and some bigger targets too) and uses that information to determine the exact shape of the mirror surface. You can see the photogrammetry targets (and the progress we've made laying panels) in the picture below. In this photo, Steve P. is using a feeler gauge (a precision set of metal shims) to check the job that Jeff, Ryan, and I did on the latest set of panels. The large metal bars layed across the middle of the reflector are to give the photogrammetry software a known length scale to work with, the foam pads with pressboard on top allow us to walk across the panels, and the shrouded piece of equipment in the center of the reflector is a theodolite (a surveying instrument we use in panel alignment).

Steve checks the alignment job.

Panel Alignment

As of last post, we had assembled the reflector backup structure (BUS) and were ready to start placing reflector panels. The first step was to get the BUS from where we assembled it (next to the airstrip, right where the pieces came out of the LC130 they arrived in) to where the rest of the telescope was going together --- namely the Dark Sector Lab (DSL), about 1km from the South Pole Station. We had assembled it on a sled just for this purpose.

The BUS on the move


At DSL, the BUS was dragged into a large tent built to shelter us from the wind as we worked, and we could begin the prep work needed to place panels. This work mostly fell to Jeff, who is in charge of understanding how one makes an off-axis paraboloid by placing 218 irregular shapes at different heights above an on-axis paraboloid. (If "off-axis" and "on-axis" mean nothing to you, just believe me that this is a crazy idea.) After lots of thinking on Jeff's part and some manual labor by the rest of us, we had seven panels placed in the innermost ring.

Prep work on the BUS inside the tent


Perhaps not surprisingly, how things looked after this initial placement resulted in lots more brainwork by Jeff (with a little help from the rest of us), sniffing out small non-idealities in certain parts of the system that have to be taken into account for everything to fit correctly. After a few iterations of this, panel placement can now begin in earnest.