Abstract: Generation and use of a focus quality metric that is intensity independent is described. In one example, the focus quality metric is generated by acquiring an image, such as an image of a patterned surface of a flow cell, and processing all or part (e.g., a sub-region or sub-image) to generate a Fourier transform of the respective image data. By way of example, in one embodiment a discrete Fourier transform may be applied to a sub-region of an image of a patterned flow cell surface. A focus quality metric that is intensity independent may be derived from the Fourier transform of the image data.

Abstract: A gas turbine engine exhaust nozzle includes a row of laterally sinuous chevrons extending from an aft end of an exhaust duct. The chevrons have radially outer and inner surfaces bound by a laterally sinuous trailing edge extending between a base of the chevrons adjoining the duct and an axially opposite apex of the chevrons. Each chevron has a compound arcuate contour both axially and laterally, and the sinuous trailing edge of the chevrons further compounds the arcuate configuration of each chevron.

Abstract: A method of assembling a gas turbine engine includes coupling an annular exhaust duct to the gas turbine engine, coupling a plurality of chevrons to the annular exhaust duct, and coupling a chevron actuation system to the annular exhaust duct such that selective operation of the chevron actuation system repositions the plurality of chevrons to adjust an amount convergence of the annular exhaust duct.