ACD Airborne FTS Spectrometer

• History– Acquired in 1974 for measurements of

stratospheric composition– Flown 800-1000 hours on NCAR Sabreliner– Flown 800-1000 hours on NASA DC-8– Also flown on Electra, P-3– Extensively modified in 1999

ACD Airborne FTS Spectrometer

• Specifications– Aperture 5 cm, Cube corner reflectors– Scan length 16 cm OPD (~0.05 cm-1 resolution)– Scan time (normal) 6 sec– Range 1.6-20 mm (KBr beamsplitter)– Dual detectors

• InSb 2.5-6 mm• HgCdTe 6-13 mm

– Multiple bandpass filters, field stops– 16 bit A/D with gain switching (19 bit dynamic range)– 2 phase HeNe laser OPD reference– Air bearing moving mirror support

ACD Airborne FTS Spectrometer

• Normal operation– Observe sun near sunset (or sunrise)– Photoguided autogain solar tracker (~10”)– Average about 1 min. observations– Bandpass filter– SNR a few hundred (detector dependent)– Usually in shot noise regime

Challenges

• Low signals• Observing time• Guiding• Windows and observing portWill probably be operating in background noise

limited regime