Tech Briefs

Readout is not materially affected by drifts in analog circuitry.

In the electronic absolute Cartesian autocollimator, the target is a coded Cartesian grid (see Figure 2) and the viewing plane is occupied by an image sensor. Vertical lines in the target image encode azimuthal deflections of the datum mirror from the optical axis, while horizontal lines encode elevational deflections. The planar array of pixels of the image sensor intrinsically constitutes a fixed high-resolution coordinate grid. The outputs from the pixels are digitized, and the resulting digital data are processed to decipher the codes in the target image and to determine locations of centroids of grid lines, which provide angular measurement with a granularity nearly one thousand times finer than the angular extent of a single pixel. Each centroid produces an independent position measurement. Averaging measurements together naturally increases readout accuracy and sensitivity.

The combination of the intrinsic grid structure of the image sensor and the Cartesian grid of the target image ensures linearity of output and a high degree of immunity to any non-uniformity among responses of individual sensor pixels. The coding of the grid ensures unambiguous position readout.

Processing of the target image is not subject to drift as a result of weakness of signals on the image sensor. At worst, weakness of signals increases the proportion of noise. Therefore, the electronic absolute Cartesian autocollimator includes a servo loop that regulates the brightness of illumination to keep signal levels optimum. Finally, the electronic absolute Cartesian autocollimator offers one major additional advantage over a conventional electronic autocollimator: The cells of the Cartesian grid effectively constitute a multiplicity of targets that, collectively, makes the field of regard of this apparatus much larger than that of a conventional electronic autocollimator.

This work was done by Douglas B. Leviton of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at under the Physical Sciences category.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Goddard Space Flight Center, (301) 286-7351. Refer to GSC-14718-1.

This Brief includes a Technical Support Package (TSP).

Electronic Absolute Cartesian Autocollimator (reference GSC-14718-1) is currently available for download from the TSP library.

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