Aeolus's telescope requires periodic collimation and refocussing. To this end, the Instrument Telescope Refocussing activity aims to improve the radiometric performance of the Atmospheric Laser Doppler Instrument's telescope, ALADIN, by optimising the throughput at the field stop, the smallest aperture where the return light has to pass. This is done through a change of the temperature settings on the telescope.
The ALADIN telescope is a 1.5-metre diameter Cassegrain telescope. By using Silicon Carbide, an overall low mass of 75 kg was achieved for the telescope while maintaining high stiffness. The secondary mirror of the telescope is mounted on a tripod of three evenly placed struts, at a distance of about 1.3 metres from the large primary mirror. The focus of the telescope is controlled by thermal adjustment of the heaters on the struts. Changing the temperature of the struts results in expansion or contraction, thus altering the distance between the two mirrors of the telescope and affecting its focus. During the telescope refocussing activity, an extended temperature range is used to scan the signal throughput at the field stop.
The Instrument Telescope Refocussing activity was first performed after ALADIN's initial switch on during Aeolus's commissioning phase in September 2018, repeated in summer 2019, and started this year on 25 May.