Fibre-feed tests at Izana in 2017 September

Abstract

The new fibre-based spectrometer was updated from 2017 September 1 to 11. The detector electronic noise and atmospheric scintillation noise were both measured. The scintillation noise was found to be approximately 105 times higher than the electronic noise, and so the system is dominated by atmospheric noise. It is unlikely that the LCD-based system switching polarisation states at 5 Hz will be able to equal the performance of a typical Pockel’s-cell-based system switching at 100 Hz. An overall reduction in noise power by a factor of almost 5 times is required to match the performance of Fred in Sutherland. This difference is of the order difference in scintillation noise between 5 Hz switching and 100 Hz switching.

Two fibre-feeds were trialled. A 30 mm focal length lens to replace the exiting 80 mm, and so focusing a smaller image of the Sun onto the end of the fibre, and a telescope arrangement coupling quasi-parallel light into the fibre. The shorter focal length lens resolves both the low- and high-frequency issues experienced since initial commissioning. The quasi-parallel telescope fibre-feed suffers some Doppler-imaging problems and also produces higher noise levels due to the lower light throughput.

The primary cœlostat mirror was replaced, and resulted in an increase in counts for Mark-I by approximately 3%. The PMT HV-PSU was replaced, removing a temporary 2kV unit and reinstating the normal 3kV supply resulting in a further increase in counts for Mark-I by approximately 18%.