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Full Aperture Seeing Sensor
(FASS)

The FASS is a novel turbulence profiling technique that exploits a low noise EMCCD to image the full aperture of the telescope in order to measure the atmospheric turbulence profile. This technique could replace the current MASS instrument since it is not only easier to implement, but it also aims to produce higher resolution turbulence profiles. The FASS instrument is a collaborative development between PUC, Chile and Durham University, UK. The concept and analysis was developed by Andrés Guesalaga at PUC, and the design, manufacturing and testing was completed at Durham University.

Forest
Figure 2: This is the old DIMM tower where the FASS-SHIMM installed at Paranal Observatory.
Snow
Figure 1: The FASS-SHIMM mounted on an 11" telescope.

As the altitude of a turbulent layer increases the structure of the scintillation pattern in the pupil image (flying shadows or speckles) changes i.e. the characteristic size of the Fresnel zones associated with the layer increase. By extracting the speckle size and intensity information from 2-D spatial power spectra obtained from the pupil image (Guesalaga+16).

By calculating the power spectra of each linear vector, and averaging for a large number of images, spectra are obtained which are used for the profile estimation. Smaller speckles will populate higher spatial frequencies whereas large speckles will tend to concentrate energy at low spatial frequencies. Weighting functions can be calculated assuming Kolmogorov turbulence. These can then be used to estimate the optical turbulence profile via a fit to the measured data.

The FASS when conjugated to the ground can not detect the ground layer turbulence. As a result the FASS was combined with the SHIMM, to make the FASS-SHIMM (figure 1). Since the SHIMM measures the total integrated turbulence, the strength of the ground layer can be found by differencing these measurements from the two instruments. The FASS and SHIMM are an ideal combination since both instruments require the full unmasked aperture of the telescope. This is in contrast to the MASS-DIMM, where aperture masks for both instruments are used. In 2018 I installed the FASS-SHIMM at Paranal Observatory on the old DIMM tower (figure 2).

Currently, this work is being extended negatively conjugate the FASS such that it is sensitive to the ground layer and therefore will not need a SHIMM branch, this will increase the amount of light given to the FASS.



References