  The wavelength dependent refraction of light in the atmosphere causes the
chromatic dispersion of a target on the focal plane of an instrument. This is
known as atmospheric dispersion, with one of the consequences being wavelength
dependent flux losses which are difficult to minimise, requiring analysis in
both instrument design and operations. We present Atmosphyre, a novel python
package developed to characterise the impact of atmospheric dispersion on a
spectrograph, with a focus on fibre multi-object spectrographs (MOS) which will
be at the forefront of ground-based astronomy for the next few decades.
  We show example simulations and provide recommendations for minimising fibre
MOS flux losses. We conclude that the guiding wavelength should typically be
bluer than the observing band mid-wavelength, around 25-45% of the way through
the band. The aperture should be centred on this wavelength's location on the
focal plane. This wavelength/position remains constant for all reasonable
declinations and target hour angles. We also present an application of the
package to MOSAIC, the ELT's multi-object spectrograph. We find that
differential losses greater than 10% are unavoidable for 1h observations that
are a) after a local hour angle of 2.5h, or b) at declinations below -60
degrees and above 10 degrees. We identify that the introduction of an
atmospheric dispersion corrector (ADC) would result in the significant
reduction of spectral distortions, a gain in survey speed for many
observations, and enable the implementation of wider visible observing bands;
as a result, there has been a proposal to adopt ADCs at a positioner level for
MOSAIC. Future work includes adding field differential refraction to
Atmosphyre, important for future wide-field multi-object spectrograph projects
such as the proposed WST.
