Coronagraphic Observations of the Lunar Sodium Exosphere

We have designed, built and installed a small robotic coronagraph at the Winer Observatory in Sonoita, Arizona, in order to observe the sodium exosphere out to one-half degree around the Moon. Observations are obtained remotely every available clear night from our home base at Goddard Space Flight Center. Our data encompass lunations in 2015, 2016, and 2017, thus we have a long baseline of sodium exospheric calibrated images. We employ an Andover temperature-controlled 1.5 Å wide narrow-band filter centered on the sodium D2 line, and a similar 1.5 Å filter centered blueward of the D2 line by 5 Å. Exposures of 10 minutes are required to image the sodium corona at good signal to noise. Autoguiding is performed locking onto a small bright crater each night. Following each onband-offband exposure pair, on- and off-band images of the lunar surface are collected by taking a 0.1- 0.5 second exposures with the open filter. The sodium is calibrated using the counts in the open Moon images and the Hapke function. We use both dark and bright Hapke parameters for comparison check using Mare and highlands, respectively. In order to obtain the sodium profile around the entire limb, the images are transformed using a polar transform and the profiles are extracted automatically. We have derived zenith column abundances and surface abundances around the lunar limb for each observation and we fit these observations with a 3-dimensional model. We compare our lunar model derived from these observations with the data from the spectrograph onboard the LADEE spacecraft.