  The SKYSURF project constrained extragalactic background light (EBL) and
diffuse light with the vast archive of Hubble Space Telescope (HST) images.
Thermal emission from HST itself introduces an additional uncertain background
and hinders accurate measurement of the diffuse light level. Here, we use
archival WFC3/IR engineering data to investigate and model changes in the
temperature of various components in HST's optical path as a function of time
(solar cycle) and time of the year (Earth-Sun distance). We also specifically
investigate changes in temperature with HST's orbital phase and time since
Earth occultation. We investigate possible correlations between HST component
temperature and year, and temperature and month. The thermal background changes
by less than one Kelvin in the WFC3 pick-off mirror, one of the most important
contributors to the thermal background. We model these data to describe the
impact that orbital phase, year, and time of year have on the HST and WFC3
component temperatures, and use this to derive the impact on the thermal dark
signal and the resulting diffuse light measurements. Based on this improved
modeling, we provide new upper limits on the level of diffuse light of 21 nW
m-2 sr-1, 32 nW m-2 sr-1, and 25 nW m-2 sr-1 for F125W, F140W, and F160W.
Additionally, by accounting for all known sources of measurement uncertainty,
we report lower limits on the level of diffuse light of 12 nW m-2 sr-1, 20 nW
m-2 sr-1, and 2 nW m-2 sr-1 for F125W, F140W, and F160W.
