Is the E-term correction in PyAST and SLALIB the 'reference' implementation?

[astrofrog]

This is a question for @timj, @dsberry, and @scottransom - digging into the SLALIB source code (which I think AST relies on for the actual transformations?) it looks like the correction for the e-terms of aberration is linear in time. However, the Explanatory Supplement to the Astronomical Almanac suggests a 3rd order polynomial correction, which Kapteyn (and Astropy, in #551) implement. Should the version in SLALIB be updated, or is there a good reason for keeping it the way it is? (just trying to understand whether I should be using AST as a reference for conversions involving FK4 with the correction applied.

[timj]

AST relies on PAL which relies on SOFA and a port of the Fortran SLA to C. The e-terms code is therefore identical to the SLA code. The SLA code cites Smith, C.A. et al., 1989. Astr.J. 97, 265 and Yallop, B.D. et al., 1989. Astr.J. 97, 274. Pat Wallace is probably the person to ask rather than us.

[eteq]

@astrofrog - did you get a resolution to this? Is this just a matter of the explanatory supplement suggesting a higher-order approximation to the Earth's orbit, or is there actually an official "right" answer?