This movie (46.8M) shows the number of field transits (so for both fields of view).

• The coordinates are in ICRS, longitude increasing to the left.
• The ecliptic plane in this system is given by the blue warped line.
• The sun is represented by a yellow dot.
• The spin axis of Gaia is represented with a black dot

The movie has three intervals, interesting features to note are:

1. From 0 to 2 days:
   • The two fields of view scan great circles.
   • Due to slow precession of the spin axis, different great circles are scanned.
   • The 'holes' between the great circles are real, since the across scan rate is too high there to overlap everywhere with the previous great circle. You will see that these holes are observed at later times.
2. From 2 to 183 days:
   • The 63 day precession period of the spin axis can clearly be seen, and the spin axis is obviously in the middle of the great circles that are being scanned.
   • The spin axis is always at an angle of 45 degrees with the sun.
   • It shows that after half a year we have covered the whole sky at least once.
3. From 183 days to 5 years:
   • It shows how the particular NSL scan pattern is being built up.
   • It shows that the overabundant regions at +/- 45 deg latitude in ecliptic co-ordinates are caused by the 45 deg angle between the spin axis and the sun (remember that the ecliptic plane is indicated by the blue line).

The movie was made by scanning the central positions of each pixel of a healpix map of depth 8 (3.1 million pixels) with the scanner in AGISLab and taking the number of SM-observations (of which there is just 1 per field transit). Then the map was projected into a Hammer-Aitoff plot using the graphics library in GaiaTools.

The picture, in particular, shows the number of field transits in ICRS after 5 years.

Copyright: Berry Holl

[Published: 12/03/2012]