Gaia scanning law

• 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.

• 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.

• 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

Date: 25 November 2008; updated 1 February 2012

Download: mov (46.8M)

Gaia Vodcast

In this vodcast Rebecca Barnes discovers the motions of the stars, learns how astronomers measure their distances and looks at the new European mission that will really get to grips with our place in the Universe: Gaia.

For more information, visit the ESA International Year of Astronomy website.

Copyright: ESA

Date: 26 February 2010; updated 20 January 2012

Download: mov; mv4; wmv; mpeg

Gaia Payload Module: X-axis vibration at qualification level

In late June 2011, mechanical testing of the Gaia Payload Module was performed at the facilities of Intespace in Toulouse, France, under the direction of the Prime Contractor, Astrium. The results have been analysed and the testing was declared successfully completed.

This a picture of the Structural Model of the Gaia Payload Module undergoing X-axis swept-sine vibration testing at qualification level. The frequency is sweeping from high to low; at the higher frequencies, the movement of the test item is very small.

More information about this topic is available on the Sci-Tech website.

Copyright: Astrium, France

Date: 27 June 2011

Download: mov (1.3MB); mov (3.6MB); avi (5.6MB)

Gaia Payload Module: Z-axis vibration at qualification level

In late June 2011, mechanical testing of the Gaia Payload Module was performed at the facilities of Intespace in Toulouse, France, under the direction of the Prime Contractor, Astrium. The results have been analysed and the testing declared successfully completed.

This a picture of the Structural Model of the Gaia Payload Module undergoing Z-axis vibration testing at qualification level.

More information is available on the Sci-Tech website.

Copyright: Astrium, France

Date: 27 June 2011

Conjugate-gradients method for the global astrometric solution

AGIS, the software system to produce the global astrometric solution for Gaia, uses a block-iterative scheme to perform the global astrometric parameter adjustment. In the framework of ELSA (the Marie Curie Research and Training Network associated with Gaia) it was investigated whether the usage of the conjugate-gradients method instead of a block iteration might give more pleasant convergence properties.

For this purpose, the conjugate-gradients method was implemented in AGISLab, a simplified simulation environment to mimick the Gaia global adjustment problem. From the experiments done so far Alex Bombrun concludes that the conjugate gradients are an efficient algorithm to compute the global astrometric solution. The convergence rate is higher than that of the (un-accelerated) block iterations.

The pictures and the video display show - for one particular experiment - the successive error maps during the iterations. The quantity shown are the differences between the computed right ascensions (of the simulated stars) and their true values, averaged over small bins on the sky. The map itself is a full-sky projection in equatorial coordinates.

The sky used in this simulation is composed of 60000 single stars, isotropically distributed, which have been observed according to the Gaia scanning law. The simulated measurements were of homogeneous precision except in a a small sky area, dubbed the P region, having the size of one field of view and containing 252 stars.

In that region the precision was simulated to be five times higher, and a large systematic error was added to the initial position and parallax values used to start the iterative process. On the rest of the sky, onlya white normal noise has been added to the true values (the P region was introduced to investigate whether special adjustment methods are needed for big, bright open star clusters in the sky).

The conjugate gradient scheme is seen to quickly converge to the correct solution, and in just a few iterations it removes the systematic errors. As expected, the solution has a higher precision in the P region than in the rest of the sky. Note that we can also observe the effect of the scanning law on the solution: The precision is better near the ecliptic poles (top right and bottom left) than along the ecliptic plane (wavy band through the center).

More ABHeidelberg videos are available on Youtube.

Copyright: A. Bombrun

Date: 11 March 2009

Download: initial error map (png, 602KB); error map after 2 iterations (png, 501KB); error map after 4 iterations (png, 519KB); error map after 6 iterations (png, 568KB); error map after 9 iterations (png, 592KB); error map after 11 iterations (png, 591KB); error map after 16 iterations (png, 588KB); video (avi, 9.54 MB)

Light bending by the Sun

In this animation the positions of background stars are seen to move as a star in the foreground passes along the line of sight of the observer.

Copyright: ESA

Date: 26 August 2005

Download: avi (14.2MB); mpg (3.5MB)

The scanning law for Gaia

Three animations illustrating the scanning law envisioned for Gaia have been created. Each animation shows the celestial sphere as viewed from the "outside". Gaia is at the centre of the box, which just encloses the celestial sphere. (The image above shows extracts from the animations - the original animations (in mpg format) can be viewed by following the links below.)

The first animation shows, in red, the path of the Sun in one year. The blue line is the vector from Gaia towards the Sun.

The second animation shows the motion of the Sun as before, but also (in purple) the path of the spin axis of Gaia. The spin axis is always 50 degrees away from the Sun and moves in a circle about the Sun. If the Sun had remained fixed on the sky, the spin axis would have traced a cone around the solar direction. But because of the Sun's motion, the resulting path of the spin axis is a series of loops on the sky. Each loop takes about 70 days.

The third animation shows, in addition to the Sun and the spin axis, the path of one of the Astro fields (in green). Only the first two months of the scanning is shown, and the spin of the satellite is shown with only one revolution per day (instead of four rev/day, as will be used for Gaia). Already after two months a large fraction of the sky is covered with scans that cross each other in different directions. After six months, the whole sky gets at least a three-fold coverage. Gaia is designed to scan the sky in this manner for at least five years, after which every point has been thoroughly criss-crossed, which allows to measure the positions, motions and parallaxes of the stars.

Copyright: Lennart Lindegren

Date: 25 October 2004

Download: first animation (mpg, 1.3MB); second animation (mpg, 2.3MB); third animation (mpg, 5.4MB)

Light bending in the Solar System

At microarcsecond accuracy levels, such as those measured by Gaia, relativistic effects begin to play a considerable role. In particular, general-relativistic gravitational light deflection due to Solar System bodies will have profound effects on Gaia measurements over the planned 5 year lifetime of the mission. This is vividly illustrated in an animation, created by Jos de Bruijne, from which the image above has been extracted.

This animation depicts an all-sky map of the sky as seen from L2 - Gaia will perform its observations from here. Ecliptic coordinates (increasing from left to right, with 0 degrees in the center) are used in a Hammer projection. The integrated amount of light bending due to all planets in the Solar System, plus Ceres and our Moon, is shown with a colour scale (see object names and time scale on image). Contributions from the Sun are excluded. Further details on the contributions from Solar System bodies, and on how the animation was created, can be found in the technical note (Gravitational light deflection; GAIA-JdB-001) prepared by de Bruijne (available on request from the author).

Copyright: ESA/Jos de Bruijne

Date: 28 July 2003

Download: animated gif, (6.7MB); avi, (16MB)

Note on copyright:

Some images contained in this Media Gallery have come from sources other than ESA (industry, the scientific community, etc), and this is indicated in the Copyright notice. For re-use of non-ESA images please contact the designated authority.

Many other images in the Gallery have been released publicly from ESA. You may use ESA images for educational or informational purposes (non-commercial). The publicly released ESA images may be reproduced without fee, on the following conditions:

• Credit ESA as the source of the images:
  Examples: Photo: ESA; Photo: ESA/Gaia; Photo: ESA – C. Carreau
• ESA images may not be used to state or imply the endorsement by ESA or any ESA employee of a commercial product, process or service, or used in any other manner that might mislead.
• If an image includes an identifiable person, using that image for commercial purposes may infringe that person's right of privacy, and separate permission should be obtained from the individual.

If ESA images are to be used in advertising or any commercial promotion, layout and copy must be submitted to ESA beforehand for approval.

Contact

If further information or assistance is needed, please contact us via the Gaia Helpdesk.