GLAST Update 15 July 2008

GLAST Update 15 July 2008

What is GLAST doing now?

We recently completed our first month on orbit!  The 60-day checkout period is already entering the home stretch, and routine science operations will start soon. 

Over the past week, the spacecraft and instruments verified their behavior during the different types of mode transitions we are expecting during routine operations.  For example, if an extraordinary astrophysical event takes place and we determine that GLAST should point all the time in one direction (instead of the standard sky survey mode, which covers all parts of the sky every three hours), a command can be sent quickly to enter the “Target of Opportunity” (TOO) mode.  In addition, the GBM started enabling gamma-ray burst (GRB) triggers, and the LAT updated onboard configurations that control how it operates.  We now have a few days of planned pointed observations.  

Lining up

The LAT is a massive detector, mounted to the spacecraft in four places.  As mentioned in a previous post, the orientation of the spacecraft with respect to the stars is determined by several sensors on board, including star trackers and gyros.  So, we know the orientation of the spacecraft, but how do we know where the LAT is actually pointing?  In other words, how do we measure the small rotations between the LAT sensors and the spacecraft?  There is a science requirement to know the pointing direction of the LAT to better than 10 arcseconds (or 0.0028 degrees).  You might think we did this alignment before launch, while the observatory was on the ground, but that wouldn’t be practical: the LAT has a mass of 3 TONS, and the ride on the rocket provides quite a shake, so maintaining the alignment to high precision would be difficult (just think about what happens to your car’s front-wheel alignment when you go over speedbumps!) … not to mention the fact that getting that precision isn’t easy in the first place.  Instead, we have a much easier way to do this on orbit, using bright celestial gamma-ray sources.  By looking at those sources in different instrument orientations, we are able to measure the LAT-to-spacecraft alignment rotations precisely, and that’s what we’re doing now. 

Toby Burnett and his group at the University of Washington have been working on GLAST since 1995, and one of the many contributions is the alignment analysis.   I asked Toby to describe how this works:

As we reconstruct the direction of each incoming gamma ray, we need to know the orientation of the LAT with respect to the stars, in order to transform the direction, measured in the instrument coordinate system, to the celestial one. The spacecraft has a special set of optical telescopes, known as “star trackers”, which recognize patterns of bright nearby starts to determine the orientation. However, there is likely to be a offset between the star tracker’s orientation, and ours. Fortunately, we have our own bright “stars”, point-like gamma ray sources, to look at, which allow us to make an independent measurement of the orientation of the LAT. Based on 15 such, we have used the first four days of first-light scanning, to determine that this angle is significantly less than 0.1 degrees, well within the error expected from surveys made before launch.  We will now use additional observations over the next few weeks to improve the precision to the required level.

Because we have an on-orbit way to measure this alignment precisely, the pre-launch requirements were modest, saving precious time and money.  We only needed a coarse physical alignment of 0.5 degrees (kinda like hitting the broad side of a barn!).

Coming up…

Getting ready for the first-light results, more news from the instruments, and the final set of calibrations leading to science operations.

GLAST Update 6 July

GLAST Update 6 July

Early operations continue to go remarkably smoothly and according to plan.  The GBM spent the week doing on-orbit calibrations and setting up the automatic onboard system that maintains the calibration.  The LAT team was very busy tuning up the instrument and doing detailed data checks.  Data taking and processing are becoming routine.

“When do we see pictures?”

Soon!  The first-light images will be released in just a few weeks, when NASA also plans to rename the observatory.  Arrangements are being made now for this event, which we expect will be at the end of July or in early August.  I’ll post the date when we have it.

In the meantime, the LAT has now posted single event displays, which show the detector information in graphical form.  These are really beautiful!!  It takes some time to learn how to read these pictures.  First, as referenced in previous posts, please see a brief description of the LAT and how it works.  Particles traverse the LAT many thousands of times every second, and the information reported by the sensors about that particle’s interaction in the LAT is called an event.  Notice that the charged particles in the events linked above are detected with very high efficiency in every layer.  Now, realize that the tracking detector has about 880,000 individual sensing strips and yet they are not registering anything where charged particles are not passing through them.  Like the dog that didn’t bark in the famous Sherlock Holmes story, this is profoundly important: the noise is remarkably low, while the detection efficiency is very high.

[These events help illustrate how the experiment works, but not all events look like these relatively simple, low-energy gamma-ray event candidates.  Very high-energy gamma-ray events are more complicated, because there is more energy available to make more particles.  An even more impressive display occurs when a cosmic-ray proton smashes into the spacecraft or the massive LAT calorimeter: hundreds of particles spew up through the instrument, and the event looks like a trainwreck.  The instrument and analysis software are carefully designed to handle all these types of events.]

Remember, the LAT takes “pictures” one particle of light (photon) at a time, and since we measure the incoming direction on the sky (as well as the energy and other information) for each, those photons can be added together over time to form an image of any part (or all) of the sky.  You’ll start seeing these in just a few more weeks.

Report from Japan

The event pictures linked above are just one representation of the beautiful performance of the instruments.  The LAT tracking detector was designed and built by a collaboration of scientists, engineers, and technicians from institutions in the United States, Italy, and Japan.  The charged particle sensors in the tracker were all built in Japan, and they are truly remarkable.  About 70 square meters of these precision sensors (called Silicon Strip Detectors, or SSDs) are onboard the LAT!  Without them, GLAST would simply be impossible.

Here is a note from someone who was deeply involved in the design and production of the SSDs.  Prof Takashi Ohsugi, from Hiroshima University, is one of the key GLAST collaborators in Japan.  He and many others there are also looking forward to doing science with GLAST.
A major Japanese newspaper and some web news sites in Japan covered the GLAST launch.  It was a very exciting day for us!  We are preparing an article for the monthly report of Astronomical society of Japan (The Astronomical Herald).  A special seminar is planned in Hiroshima University to make a more detailed briefing of the GLAST project and science for students and university staff.

To connect the breakthrough observations expected from GLAST with what is known about the universe in other wavelengths, coordinated “multi-wavelength” observations wordwide are important.  In Japan, the staff of the KANATA telescope (1.5m optical, near-infrared telescope) will start
a photo-polarimetric monitoring of some bright blazars for GLAST.

Coming up…

More details about the calibrations, observations, and getting ready for the first year of science operations.  Please check back for updates.  Thanks, also, for your comments!