The ACE-FTS is the main instrument on SCISAT. It is a high spectral resolution (0.02 cm-1) Fourier Transform Spectrometer (FTS) operating from 2.2 to 13.3 µm (750-4400 cm-1) with a Michelson interferometer that was custom designed and built by ABB-Bomem in Quebec City.
The interferometer uses two cube corners rotating on a central flex pivot to produce the optical path difference. An "end" mirror inside the interferometer is used to double pass the radiation and increase the optical path difference. The ACE-FTS design is fully compensated for tilt and shear of both moving and stationary optics inside the interferometer. A pointing mirror, controlled by a suntracker servo-loop, locks on the Sun center and tracks it while the instrument is taking measurements.
The FTS has a circular field of view (FOV) of 1.25 mrad, a mass of about 41 kg, and an average power consumption of 37 W. Double-sided interferograms are Fourier transformed on the ground to obtain the desired atmospheric spectra. The FTS uses two photovoltaic detectors (InSb and HgCdTe), aligned with a dichroic element to have the same field of view. The detectors are cooled to 80-100 K by a passive radiator pointing toward deep space.
Inside the ACE-FTS instrument is a visible/near infrared imager with two filtered channels at 0.525 and 1.02 µm, chosen to match two of the wavelengths monitored by the SAGE II satellite instrument. The imagers provide an important diagnostic for pointing and for detecting the presence of clouds in the FOV. The detectors in the imagers are filtered CMOS detector arrays, (effectively) 128 × 128 active pixel sensors made by Fill Factory of Mechelen, Belgium . The total FOV of the imagers is 30 mrad, to be compared to the 9 mrad angular diameter of the Sun. The SNR of each solar image is greater than 1000, but the main image suffers from overlap by weak secondary images from optical filters that were not tipped far enough off the optical axis.