Telescope and scanning strategy

The Archeops telescope is an off-axis Gregorian telescope with a 1.5 meter primary mirror that looks at 41 degrees of elevation. During the flight, the gondola rotates at 2 rounds per minute around its vertical axis and therefore performs scans on the sky at constant elevation. While the Earth rotates, these scans end up forming a wide annulus on the sky with scans crossing each other at various time scales allowing an efficient monitoring of systematic signals. The gondola is shown in Figure  and the scanning strategy in Figure . The gondola is hanged below a stratospheric balloon that floats at an altitude of around 40 km. The data is taken during 24 hours nighttime winter arctic flights.

Figure: Schematic view of the Archeops gondola. The telescope observes at 41 degrees of elevation and rotates at 2 rounds per minute.

Figure: Scanning strategy on the sky (Molleweide projection): a circle is plotted every hour. After 24 hours, the covered region has the shape of an annulus.