HIPO in-flight performance improvements

The High-speed Imaging Photometer for Occultations (HIPO) is a special purpose science instrument for SOFIA. HIPO can be co-mounted with FLITECAM in the so-called FLIPO configuration for stellar occultation or extrasolar planet transit observations. We gained some flight experience with HIPO and FLITECAM in 2011 as described in a previous publication (Dunham, et al., Proc SPIE, 8446-42, 2012). Since that time a number of improvements to HIPO have been made and a deeper understanding of the airborne environment's impact on photometric precision at optical wavelengths has been obtained. The improvements to HIPO include an improved beamsplitter for the FLIPO configuration, adding deep depletion CCDs as a detector option, expanding the filter set to include a Sloan Digital Sky Survey filter set as well as two custom filters for transit work, and an ability to guide the SOFIA telescope using HIPO data being acquired for science purposes. We now understand that variations in PSF size due to varying static air density has a noticeable impact on photometric stability while the related effect of Mach number is unimportant. The seriousness of ozone absorption in the Chappuis band is now understood and an approach to avoid this has been found. Finally we present demonstration transit data to illustrate our current transit photometry capability.