In this answer I calculated the approximate angular separation between Mercury and the Sun as seen by the SOHO satellite using Skyfield's method of
.ecliptic_position() because I couldn't get true ICRF coordinates for SOHO from the Horizons web interface.
Is there a way I can get ICRF coordinates for SOHO that haven't been rotated into the ecliptic?
Coordinate system description: Ecliptic and Mean Equinox of Reference Epoch
Reference epoch: J2000.0
xy-plane: plane of the Earth's orbit at the reference epoch
x-axis : out along ascending node of instantaneous plane of the Earth's orbit and the Earth's mean equator at the reference epoch
z-axis : perpendicular to the xy-plane in the directional (+ or -) sense of Earth's north pole at the reference epoch.
You can see that x,y are indeed a circle, but z is just +/- a tiny fraction of an au - basically the vertical component of the halo orbit. That's confirmed by the 6-month periodicity.
So how can I get true ICRF coordinates instead of these coordinates flattened to the particular plane the earth moves in at epoch?
Horizons web interface setup: