Category Archives: Calibration


With our large project and small team, at any given time each of us has a to-do list longer than we can read in one sitting.  For the most part, we each choose tasks to work on based on mood or momentum.  In the last few days, though, I’ve been trying to determine which of our myriad of checkboxes is limiting our progress towards the distant but looming goal of flight readiness.

The two most immediate items on that prioritized path are finalizing the detector connections and installing the shields.  Our heavy bismuth germinate (BGO) shields surround the NCT detectors on the bottom and the sides, reducing background from the atmosphere.  Installing them is a risky and dicey procedure, though, so we want to make sure our detectors are behaving as they should and the connections are all good before we wall them off.  All this takes time, consultation, and reference to archival data.  We’re making progress, but it’s not as clear cut as determining that things turn on.

For their part, the shields turned on fine, but we found higher background rates than we expected.  With ground backgrounds poorly characterized and influenced by everything from the presence of smoke detectors to the composition of the hangar cement, sorting this issue out has been a headache.  So far, though, careful tests by Jane and Zach suggest the shields are working as they should.

Steve has been busy building an electronic isolator to report the cryostat temperature.  Alfred and Ming-Zhe are working on the test solar panels.  I’ve made a few minor changes to the flight software, and Alan continues to improve our ground support software.  Alan and I are also discussing our calibration plans–if and when the sources we need arrive!

Keeping it all straight is a task in itself, but we’re helped by great support from team members still at home.

Bend, Don’t Break

To begin: news of the rotor.  When we found out last week that our rotor was not qualified for our gondola’s weight this flight, we returned it to Berkeley for a “pull test.” As the name suggests, the test consisted of gripping the rotor from both ends in a machine and pulling–with 50,000 pounds of force!  Yesterday, we learned that the rotor had passed the test without any catastrophic failure, so we can fly without cutting weight.  However, the yokes and pins we use to connect the rotor to the balloon above and the gondola below were bent during the test.  Accordingly, Berkeley is making new yokes and ordering new pins.  Hopefully the rotor will return to us very soon!

Update, 4/30:  A closer inspection of the rotor in Berkeley found a cracked component.  This is going to impose a significant delay…

As we wait, we’re dealing with lots of odds and ends.  On Monday, we got the flight computer interfaced with CSBF’s commanding computers (on the first try!).    Daniel has been heroically taping temperature sensors all over the gondola.  Alan and I are continuing to debug the flight computer and ground control program.  Justin is working on our analysis software.  Zhong-Kai is keeping track of the card cages.  And Mark is talking to all the CSBF personnel and keeping us on track.

Today the wind was a bit lighter, so we attached the solar panels and pushed the gondola outside (waaaaay outside) to test the solar power system and to calibrate our differential GPS.  Both systems performed flawlessly!

NCT explores the outside world.

NCT goes outside.

Our other task has been to calibrate the efficiency of the telescope.  We do this by placing radioactive sources in the field of view of the instrument.  These sources emit gamma-ray photons of specific, known energies.  Since we know the activity of the sources–the number of photons they emit per second–by placing them at a specified distance and counting how many photons we observe, we can calibrate the efficiency of our telescope.  (This is called the “effective area” of the telescope; its maximum is the geometric area of the detectors, which would happen if all photons hitting the detectors were observed.)

Since the efficiency varies with energy and throughout the field of view, we have quite a few calibration measurements to take.  We need fairly precise knowledge (~ a few centimeters) of the position of the source relative to the detector for these calibrations, which is hard since the source is several meters in the air!  To find the positions, we use a theodolite–a surveying instrument which measures angles.  The last few days, I’ve been training the other students to use the theodolite, and we’re all taking turns doing late shifts to take data.

Our calibration setup.  The theodolite is in the foreground, and the source is on the aluminum board overhead.

Our calibration setup. The theodolite is in the foreground, and the source is on the aluminum board overhead.

The latest CSBF weather reports indicate that turnaround–the period when the upper atmosphere winds are still enough for long flights–is still about a week and a half away.  There’s some definite jockeying for position in the flight queue happening–come on, rotor!

Fits, Starts

We’ve had some hiccups this week in our preparation for flight, but we’re still making solid progress. The biggest question at the moment is the status of the rotor; it flew back to Berkeley on Thursday to see if it can be rated to our weight for this flight. Assuming we don’t have to drop weight, our short-term schedule will mostly be shaped by the timing of the rotor’s return to us.

There’s plenty to press ahead with in the interim, though. The flight computer is safely ensconced in the electronics bay, and Yvette routed its myriad cables into something resembling order–a feat I thought impossible! We also put the sides of the ebay in place–they help regulate the temperature of the electronics in flight. Daniel is also fitting the mylar solar shields which keep the detectors themselves out of direct sunlight. Some of the others have been attempting to diagnose and reduce the electronic noise in the system.

The fully harnessed, much neater ebay!

The fully harnessed, much neater ebay!

Daniel and Alfred install the mylar solar shields.

Daniel and Alfred install the mylar solar shields.

For my part, my attention is divided. There are still improvements and bug fixes needed in the flight code. Today I found a nasty little bug in our pointing routines that caused an exponentially growing number of processes to be spawned! Needless to say, that was harmful to the stability of the system. It was a simple fix, thankfully. I also worked on interfacing our computer with the CSBF command link. Finding trouble, McBride determined that two of our serial lines have bizarre a hardware failure…

On the whole, though, the system is rapidly improving in stability and readiness. With most of the detailed harnessing and mechanical work done, we’ve moved the gondola up on its cart. With this mobility comes the ability to start our efficiency calibrations, which I’ll discuss more in a future post.