  The Atacama Large Millimeter/submillimeter Array (ALMA) was used in 2021 to
image the carbon-rich evolved star R Lep in Bands 8-10 (397-908 GHz) with
baselines up to 16 km. The goal was to validate the calibration, using
band-to-band (B2B) phase referencing with a close phase calibrator J0504-1512,
1.2 deg from R Lep in this case, and the imaging procedures required to obtain
the maximum angular resolution achievable with ALMA. Images of the continuum
emission and the hydrogen cyanide (HCN) maser line at 890.8 GHz, from the
J=10-9 transition between the (1110) and (0400) vibrationally excited states,
achieved angular resolutions of 13, 6, and 5 mas in Bands 8-10, respectively.
Self-calibration (self-cal) was used to produce ideal images as to compare with
the B2B phase referencing technique. The continuum emission was resolved in
Bands 9 and 10, leaving too little flux for self-cal of the longest baselines,
so these comparisons are made at coarser resolution. Comparisons showed that
B2B phase referencing provided phase corrections sufficient to recover 92%,
83%, and 77% of the ideal image continuum flux densities. The HCN maser was
sufficiently compact to obtain self-cal solutions in Band 10 for all baselines
(up to 16 km). In Band 10, B2B phase referencing as compared to the ideal
images recovered 61% and 70% of the flux density for the HCN maser and
continuum, respectively.
