Summer Projects

Simulating The High Energy Solar Imager
How it works
A rock polisher model

Grid Characterization
What is it?
Why do it?


The High Energy Spectroscopic Solar Imaging (HESSI) satellite will employ a new technique for imaging X-rays emitted from the Sun during solar flare events. This technology was developed and tested by the scientists of the NASA Goddard Space Flight Center using a balloon launch vehicle known as HEIDI. Fourier Transform Imaging utilizes an interference pattern produced by the rotation of coupled pairs of grids. The grids are similar to picket fences with slits and slats on the order of 30 microns or larger. Many pairs of grids will be incorporated in HESSI and will be of various sizes. As the grids rotate, the X-ray photons either pass through both grids and reach the detector or they are blocked by at least one of the slats. At a given instant in time, the photon that reaches the detector did so by passing through any combinations of slits on the two grids, such as slit one on grid one and slit one on grid two, or slit one on grid one and slit two on grid two. This results in bands of areas from which the X-ray could have originated. In the next instant, the telescope will have rotated and any photons that are detected, also went through a finite number of slit combinations that again result in bands of areas from which the X-ray could have originated. Since there has been a rotation, the two bands of areas are not the same. Some areas will be eliminated by the fact that they don't overlap each other. When the next set of data is added to the first two sets, even more areas are eliminated. This process is continued until all the data are added and the true image of the source is produced.

Rock Polisher Model

The rock polisher model of HESSI consists of a light source (a projector), a pinhole, a tube fitted with grids at either end, and a rock polisher (to provide a constant rotation). The light source is diverging so the grids must be scaled to account for the spread of the light as it travels through the tube. The best design for the tube is a 2 pound coffee can with plastic lids on each end. The center of the lid is cut away and the grids are attached to the edge of the lid. The grids are a photocopy of an actual grid used in the HEIDI project. The size was varied by using the enlarger option on the copier and overlay transparencies were made. The ratio of the grid sizes was 1:0.864 with the larger grid farthermost from the light source. The distance between the light source and the grids is critical in order to produce the interference pattern. When the distance is too large or too small, the pattern never reaches a state of complete darkness.

What is grid characterization?

Grid characterization is a process whereby all the characteristics of the grid are tested. These characteristics include the evenness of the spacing between the slats, the uniformity of the slat thickness, and the deviation in normality (running parallel to each other without waves) The grid characterization facility consists of a laser and CCD camera mounted parallel to each other and perpendicular to the table. The grid is mounted parallel to the table and the image from the CCD camera is analyzed by a computer program.

Why is the Grid Characterized?

In order to form the image, the characteristics of the grid must be well understood since the interference pattern obtained depends on the characteristics of the grid. Interpretation of the interference pattern requires a knowledge of the grid. This is very similar to what happens when an ordinary, everyday object is enlarged to such an extent that details often missed become very prominent. Until you know what it is you are looking at, interpretation of the image is nearly impossible. A very course grid will give the general location of an event but will not be able to define details. A finer grid will be able to provide the details.

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Sun-Centered Physics

Last Modified July 26, 1996,