MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) is a dual, diode-array spectrometer meaning it is comprised of two independent spectrometers with diode-array detectors. Each spectrometer is referred to as a channel. One of the channels measures ultraviolet (UV) wavelengths from 285 to 565 nm with a spectral resolution of 1.5 nm. The second channel measures VIS and near infrared (NIR) wavelengths from 515 to 1015 nm with a spectral resolution of 2 nm.
MAESTRO makes measurements in the solar occultation mode. In this mode the satellite looks through the limb of the Earth to the sun and measures the intensity of the sunlight that is going through the atmosphere of Earth at different tangent heights. The diagram below shows the geometry of the measurements where, “h” denotes the tangent height which is defined as the shortest distance between the limb view and the Earth’s surface. When the satellite is moving behind the Earth it will be measuring a “sunset” and when it is moving towards the front of the Earth it will be measuring a “sunrise”. This mode of measurement provides a higher vertical resolution (≈3 km) but a low spatial resolution (≈300 km).
MAESTRO also measures in nadir backscatter mode. In this mode it measures the sunlight scattered back from the Earth’s atmosphere into space. This mode is ideal to map the total column amounts of ozone \(O_3\) and nitrogen dioxide \(NO_2\).
The MAESTRO instrument has dimensions of approximately 0.5 m cube and weighs about 8 kg. Each individual spectrometer that comprise MAESTRO are quite simple in design. They consist of a lens, slit, concave holographic grating, and photodiode detector arrays. The diagram on the right shows the optical schematics of MAESTRO.
In the solar occultation mode MAESTRO shares the input beam from the sun with the FTS instrument. Approximately 7% of the beam from the FTS instrument is directed into the MAESTRO input optics. A beam splitter reflects 70% of the input beam to the UV channel and 30% is transmitted to the second beam splitter which in turn reflects 90% of that beam to the VIS channel and the remaining 10% is transmitted to an exit port which is used for alignment during pre-launch activities. A telescope in each channel magnifies the beam by a factor of 7. The backscatter ports are combined with the solar view with a beam splitter that reflects 80% of the backscatter beam towards the two channels. Each channel has its own backscatter port which is open all the time. During sunrise and sunset when the instrument is measuring in solar occultation mode, the contribution from the backscatter port is minimal and therefore ignored.
To avoid anomalous diffraction features the input beam is put through a Glan-Taylor Polarizing Prism. An OG515 filter is placed at the entrance of the VIS channel to reject light from higher orders of diffraction. A 22 mm lens projects the image of the sun on the entrance slits which are 50 micrometers wide and 2.5 mm long. After passing through the slits the light is diffracted by a concave holographic grating surface with dimensions of 5 by 5 cm and is detected by a Reticon 1024-element photodiode array. To read more about the instrument design refer to Tom McElroy’s paper published in Optical Society of America in 2007 here.
References
McElroy, C. T., Nowlan, C. R., Drummond, J. R., Bernath, P. F., Barton, D. V., Dufour, D. G., Midwinter, C., Hall, R. B., Ogyu, A., Ullberg, A., Wardle, D. I., Kar, J., Zou, J., Nichitiu, F., Boone, C. D., Walker, K. A., and Rowlands, N.: The ACE-MAESTRO instrument on SCISAT: Description, performance, and preliminary results, Appl. Optics., 46, 4341–4356, https://doi.org/10.1364/AO.46.004341, 2007.