CC-POL Components: Focal Plane Mask
Required when observing extended sources
- If no mask is used e and o rays can overlap, scrambling the polarised signal
Required when background emission is significant
- If no mask is used e and o rays can overlap from ‘sky’, reducing S/N due to sky noise
Must be placed upstream of Wollaston prism
Exact spacing of mask dependant on angular dispersion of Wollaston
- may require one or two iterations due to slight changes in birefringence of cold Wollaston or movement of internal components