Planck scanning the sky

As Planck spins on its axis it observes a ring around the sky.  Over the course of the year, the axis around which Planck spins moves around the sky, ensuring that the satellite is never looking towards the Sun or the Earth.

Around once every hour the thrusters fire a short burst to push the satellite round a tiny amount.  The angles by which it turns are very small, and add up to only around 1o per day, but over the course of a year Planck goes the full 360o around the Sun.  Since every ring has two sides it scans almost every part of the sky twice in that year.

The detectors of Planck don’t point quite at 90o to the spin axis, but instead around 85o.  This means that to see the whole sky, the orbit of Planck must wobble a little bit – this is helped by the fact that Planck is actually orbiting the L2 point, rather than sitting in one place.

All of this means that Planck makes a slightly convoluted map of the sky, as shown in the movies below.  The orientation of the map affects how it looks – the two movies below are shown in Ecliptic coordinates (aligned with the Earth’s orbit around the Sun) and Galactic coordinates (aligned with the plane of our Galaxy).  Both are shown in the “mollweide” projection, which is the standard projection for the CMB.  This kind of projection maps the entire sky into a single oval shape, keeping all sizes and areas correct but distorting the shapes, particularly around the edges.

Click here for more views of the way Planck scans the sky

There are more animations available here, including in in different projections such as orthographics and cartesian, including some with the locations of the planets shown.