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