  The Dark Energy Spectroscopic Instrument (DESI) is under construction to
measure the expansion history of the universe using the Baryon Acoustic
Oscillation (BAO) technique. The spectra of 35 million galaxies and quasars
over $14000 \,\text{deg}^2$ will be measured during the life of the experiment.
A new prime focus corrector for the KPNO Mayall telescope will deliver light to
5000 robotically positioned optic fibres. The fibres in turn feed ten broadband
spectrographs. Proper alignment of focal plate structure, mainly consisting of
a focal plate ring (FPR) and ten focal plate petals (FPP), is crucial in
ensuring minimal loss of light in the focal plane. A coordinate measurement
machine (CMM) metrology-based approach to alignment requires comprehensive
characterisation of critical dimensions of the petals and the ring, all of
which were 100% inspected. The metrology data not only served for quality
assurance (QA), but also, with careful modelling of geometric transformations,
informed the initial choice of integration accessories such as gauge blocks,
pads, and shims. The integrated focal plate structure was inspected again on a
CMM, and each petal was adjusted according to the updated focal plate metrology
data until all datums were extremely close to nominal positions and optical
throughput nearly reached the theoretically best possible value. This paper
presents our metrology and alignment methodology and complete results for
twelve official DESI petals. The as-aligned, total RMS optical throughput for
6168 positioner holes of twelve production petals was indirectly measured to be
$99.88 \pm 0.12 \%$, well above the 99.5% project requirement. The successful
alignment fully demonstrated the wealth of data, reproducibility, and
micron-level precision made available by our CMM metrology-based approach.
