EXES Slit Position Angles

I calculated the EXES slit position angle (PA) on the sky from an existing observation that was done in nod-on-slit mode. The telescope pointing of the two nod positions gives the PA, considering that we do see both stellar spectra. The telescope pointings were taken from the MCCS keywords sibs.ra and sibs.dec from housekeeping data on the "SOFIA Engineering Data Products" page at https://solar.sofia.usra.edu/products/SOFIAEngDataProducts.html.

For the observation listed in the Figure below, sibs.ra and sibs.dec were obtained at 1 second intervals. The top two panels show the RA and Dec as a function of time. The first pointing is defined as 'B' in the data acquistion software (also mentioned in a FITS header keyword). Subsequently, four A+B nodding pairs are observed. At the end, the telescope moves to the middle of the slit, followed by a short pointing to 'A' where no data are taken, after which the telescope moves to 'B' again for another 4 nodding pairs. 

The nod throw is shown in the bottom left panel. It is 15 arcsec for this observation. The calculated PA, defined as degrees from North through East, in the bottom right panel varies from 146 to 143 degrees The jumps are due to LOS rewinds. It was assumed that the B position is defined as where the telescope is at the top of the slit, so where the source is at the bottom (source: Matt, Curtis, Mike). A few comments and conclusions from this effort:

• Comparing the calculated slit PA's with those from the SOFIA Visibility Tool (VT; https://dcs.sofia.usra.edu/observationPlanning/installVT/) at the time of the observations gives this relation: PA(measured)=PA(VT)-30. I have informed the SOFIA DCS team about this discrepancy, and they will correct it. Developer Sean Colgan was not surprised as VT was never formally verified.
• The FITS keyword TELVPA in the EXES data files is the same as the calculated PA's, but flipped: PA(measured)=TELVPA-180. I also checked that TELVPA is not just the nodding PA, by looking at off-source nodding observations. It is not, TELVPA is the EXES slit PA minus 180 degrees.
• PA's were calculated for a number of other observations in flight 288 as well as for September 2015, and the above conclusions hold.
• Using the same observations, I looked at the relation between  the slit PA and the SOFIA field rotation ('ROF') reported in the flight plans: PA(calculated)=ROF+93. This means that the EXES slit is mounted horizontally, because ROF is defined as the angle of the Zenith from North through East.