  We demonstrate the use of an optimized 5 core photonic lantern (PL) to
simultaneously measure tip/tilt errors at the telescope focal plane, while also
providing the input to an instrument. By replacing a single mode (SM) fiber
with the PL we show that it is possible to stabilize the input PSF to an
instrument due to non-common path tip/tilt aberrations in an adaptive optics
system. We show the PL in two different regimes, (i) using only the outer cores
for tip/tilt measurements while feeding an instrument with the central core
and, (ii) using all cores to measure tip/tilt when used in an instrument such
as a spectrograph. In simulations our PL displays the ability to retrieve
tip/tilt measurements in a linear range of +/- 55 milliarcseconds. At the
designed central wavelength of 1.55 microns, configuration (i) matches the
throughput of an on-axis SM fiber but declines as we move away from this
wavelength. In configuration (ii) we make use of the whole multimode input of
the PL resulting in a potential increase of overall throughput compared to a SM
fiber, while eliminating modal noise.
