FIR Optics Meeting – January Calibration issues related to the optical performances David Teyssier

D. TeyssierFIR Optics Meeting – January convert detector counts (voltages) into physical units as closely as possible related to the astronomical target  translates into a series of responses to be calibrated out The calibration task Objectives of the calibration  optical response (coupling, reflections, etc…)  electronic bandpass response

D. TeyssierFIR Optics Meeting – January we define a set of external (astronomical) and internal calibrators to rely on  primary calibrators provide the absolute scale through a series of coupling efficiencies The calibration philosophy How will we calibrate this ?  aperture efficiency (sensitivity to point sources)  main beam efficiency (coupling to a source filling the main beam)  primary calibrators are compact (vs beam) well-known planets  secondary calibrators, first calibrated against the primary ones, serve as backups in case planets are not visible at a given time  in our case, mainly asteroids, homogeneously visible in the course of the mission

D. TeyssierFIR Optics Meeting – January The calibration philosophy How to calibrate (cont’d)  transfer calibrators, used for the day-to-day calibration and to calibrate the system bandpass  most mm and submm instruments use internal reference loads (so-called hot and cold) expected to serve as black bodies (BB) at given temperatures  the astronomical target emission is then extrapolated from the instrument counts measured on the loads

D. TeyssierFIR Optics Meeting – January primary calibrator accuracy rely on planetary and asteroid models  these model still have many caveats The calibration issues Main issues (vs optical aspects) of this approach  have to account for season, surface (ice caps, albedo,..) effects  currently accurate only in the low frequency range (< 800 GHz)  modelling the expected beam + efficiencies may be more accurate  surface deviations  blockage  spillover  Simplified input assumptions: LIMITED ACCURACY

D. TeyssierFIR Optics Meeting – January non-optimal design of the Calibration Source Assembly (CSA) and its interface with the Common Optics Assembly (COA)  uncertain coupling (illumination from the mixers) on the loads The calibration issues Main issues (vs optical aspects) of this approach (cont’d)  effective BB reference temperatures a priori unknown  spillover + truncation at the interfaces and mirrors  direct consequences on the calibration error budget

D. TeyssierFIR Optics Meeting – January effects from reflections: standing waves  expected at numerous interfaces and mirrors in the system The calibration issues Main issues (vs optical aspects) of this approach (cont’d)  “ understood” to 1 st order BUT higher mode effects play an important role (AND are hard to predict !!)  result from Fabry-Perot-like resonances between surfaces in the path  consequences on science are of importance  degrade detection limit of weak lines  affect spectral baselines and accuracy of (e.g.) continuum measurements