GMTIFS internal flexure budget and cold stop stability

As a faint-source cryogenic near-infrared spectrograph, GMTIFS requires a cold pupil stop, a Cold Stop, to reject parasitic thermal emission from outside the telescope pupil. For the GMT this requires a rotating segmented Cold Stop within the GMTIFS cryostat. The decentre accuracy achievable for the Cold Stop due to flexure under variable gravitational load at the GMT folded port Gregorian focus is a defining parameter for the Cold Stop under/oversizing. Under ideal circumstances, decentre accuracy should be within ±25 µm, the pupil image accuracy set by diffraction from the GMTIFS science field stop and relay foreoptics. The GMTIFS optical concept is based on elastic flexure of the optical table support structure suspended on trusses with global flexure corrected via telescope pointing. However, understanding the differential flexure residual between multiple internal focal and pupil planes requires modelling the motion of the full supported structure. This work explores the methods undertaken to simulate the mechanical stability of the Cold Stop in the context of the whole GMTIFS instrument, exploring how the kinematic mounting, cryostat, trusses, optical table and optics have a cumulative effect on Cold Stop motion. This analysis leads to a refinement of mechanical design across these subsystems, informing the final Cold Stop design and error budget.