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HESSI Demonstration Telescope
Introduction The HESSI Demonstration Telescope is intended to show how HESSI will make images in hard x-rays and gamma rays. The demonstration telescope will show that a pair of rotating grids modulates light from an off-axis point source. Experimental data of the type in Boxes 3 and 4 in The HESSI Online Brochure are obtained with visible light. Further experimentation is recommended to produce back projection patterns (Box 6 in The HESSI Online Brochure). The demonstration
telescope was designed by fifteen talented 4-8th grade students and mentored by Belinda
Lowe-Schmahl, Ken Schmahl,
Tom Mengel, and Dr. Ed Schmahl in the Schmahl Science Workshop in San Jose, California [1], [2]. Click
here to link to their HESSI Telescope Demonstration
web page.
Procedure A point source of light is directed at the front of the demonstration telescope, at an angle from the telescope's axis of rotation. Images are recorded at small angular increments over one revolution of the telescope. These images show how the intensity of the light passing through the telescope varies as the telescope rotates. The intensity of the light in the recorded images is plotted against roll angle. It is expected that these plots can be approximated by the equation P(a)
= P0 + A cos(2pr sin(a-b)/p
+ L) (Equation 1),
where P is the light intensity, Equation 1 indicates that the number of peaks in a graph of P( Apparatus
Click here for a description and photos of the demonstration telescope and equipment.
Results
Ten sets of images were recorded. The first five sets were recorded using grids with a pitch of 0.90 cm;
the remaining five sets were recorded using grids with a pitch of 0.59 cm. In both sets the light source was
at offset angles of 0.034, 0.048, 0.063, 0.095, and 0.161 radians. Each set contains 176 images (two per mark
on the telescope's rim - one at every mark, and one halfway between every two marks). This corresponds to angular
increments between images of about two degrees.
For each image, the average light intensity was calculated over an 80 pixel diameter disk in the center of the image
(each image is 173 x 173 pixels). The light intensity is the average pixel value, which ranges from 0 (black) to 255
(white). The average light intensity is plotted against roll angle.
Below are plots of the ten sets of data. Click on any plot to link to a page that contains a larger plot of
the experimental data and a plot of Equation 1 with the appropriate values of P0, A, r,
p = 9.0 mm  
;
p = 5.9 mm
A few notes on the determination of the parameters in Equation 1:
There is always some uncertainty involved in physical measurements. In this experiment, uncertain measurements
were made of r, p, and The uncertainties in measuring p and The angular pitch, p, is the angle subtended by one pitch of the front grids with respect to the detector (or in
this case, the first layer of diffuser). For all sets of data, the distance between front grid and diffuser was
9 cm; the angular pitches are therefore 0.9/9 = 0.1 and 0.59/9 = 0.07, approximately.
Ideally, the variable
HESSI Home Page
Laboratory for Astronomy and Solar Physics Home Page
Sally House, summer intern, 2000
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