- 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.
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.
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.
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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Last Modified July 26, 1996,