Innovative compact coronagraph approach for Balloon-borne Investigation of Temperature and Speed of Electrons in the corona (BITSE)

We are developing an innovative compact coronagraph for studying the physical conditions in the solar wind acceleration region. This paper presents the new development of the compact coronagraph for the investigation of temperature and speed of electrons in the solar corona. The proposed compact coronagraph is a one stage externally occulted coronagraph without internal occulter or Lyot stop mask. The key aspect of the new idea is to set the inner field cutoff at External Occulter (EO) much smaller than the specified inner field cutoff. A second occulter on the surface of the detector array removes the remaining diffraction. The occulter on the detector surface functions similar to an internal occulter with the Inner Field of View Cutoff (IFoVC) exactly the same as specified. For BITSE, the desired inner field cutoff is 3 R⊕, but the cutoff at EO is only 1.5 R. The diffraction analysis shows that in the sensor plane, the diffraction intensity at the 3 R is not sensitive to the EO cutoff, either at 1.5 R or close to 3 R. The advantage of having a smaller EO cutoff is that the vignetting decreased for the Field of View (FoV) near 3 R, therefore, the signal increases. Meanwhile, the diffraction of Point Spread Function is much less in the radial direction, which not only increases the image quality around 3 R, but also increases the encircled energy and signal to noise ratio. In other words, the data is useful right at 3 R! The BITSE optical design and diffraction analysis will be presented in detail. The simulation shows the signal to noise ratio obtained from the diffraction and vignetting data enables corona temperature and speed measurement.