Visibility of Prominences using the HeI D3 Line Filter on PROBA-3/ASPIICS Coronagraph

We determine an optimal width and shape of the narrow-band filter centered around the He\,{\sc i} D$_{3}$ line for prominence and coronal mass ejection (CME) observations with the ASPIICS ({\it Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun}) coronagraph onboard the PROBA-3 ({\it Project for On-board Autonomy}) satellite, to be launched in 2020. We analyze He\,{\sc i} D$_{3}$ line intensities for three representative non-LTE prominence models at temperatures 8, 30 and 100~kK computed by the radiative transfer code and the prominence visible-light (VL) emission due to Thomson scattering on the prominence electrons. We compute various useful relations at prominence line-of-sight (LOS) velocities of 0, 100, and 300~km~s$^{-1}$ for 20~\AA~wide flat filter and three Gaussian filters with full width at half maximum (FWHM) equal to 5, 10, and 20~\AA~to show the relative brightness contribution of the He\,{\sc i} D$_{3}$ line and the prominence VL to the visibility in a given narrow-band filter. We also discuss possible signal contamination by Na\,{\sc i} D$_{1}$ and D$_{2}$ lines which otherwise may be useful to detect comets. Results mainly show: i) an optimal narrow-band filter should be flat or somewhere between flat and Gaussian with FWHM of 20~\AA~in order to detect fast moving prominence structures, ii) the maximum emission in the He\,{\sc i} D$_3$ line is at 30~kK and the minimal at 100~kK, and iii) the ratio of emission in the He\,{\sc i} D$_3$ line to the VL emission can provide a useful diagnostic for the temperature of prominence structures. This ratio is up to 10 for hot prominence structures, up to 100 for cool structures and up to 1000 for warm structures.