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Image name: Straylight_Grills_I.png (click image to enlarge)
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Date submitted: 14-Nov-1995
L. Acton
14-Nov-95
SXT THIN ALUMINUM STRAYLIGHT GRILL PATTERNS - I
Except for the 2 inset plots this figure is the same as I prepared in 1994
to examine the subtle artifacts of the Al.1 images. It was put on SHOW_PIX
as file /calibration/Filter_mesh_effects. At that time I identified the
square pattern as x-ray shadowing and the grill pattern at 45 degrees to it
as a pattern produced by visible straylight penetrating the filter.
Now I doubt the interpretation of the square pattern. Since the additional
entrance filter failure of 16-August-95 the grill of lines that run SE-NW
at 32 degrees are strongly enhanced (see Straylight_Grills_II) while the
grill that runs at 90 degrees to this and forms the square pattern is
relatively much weaker. The grill of lines that run at 45 degrees to the
square pattern are about the same.
I have rotated this image to put the grills of interest horizontal and
then have done a one-directional sum in the X direction to provide a vector
of intensities that can be FFT analyzed to determine the period of the
grill pattern. The vectors thus prepared were summed modulo the period to
form the small plots in the lower left.
Rotation of 31.7 degrees puts one grill of the square pattern horizontal
and the period turns out to be about 20.83 FR pixels. The other grill of
the square pattern has the same period -- it indeed is a square pattern.
The tilt of the NE-SW grill pattern is 13.1 degrees and the period is 14.9
FR pixels. As the two patterns are at 45 degrees and appear to be in phase
we should expect the NE-SW spacing to be 20.83/sqrt(2) or 14.7 pixels. The
numbers do not quite match but it is difficult to assess the errors so the
agreement may be OK.
Note that the NE-SW grill pattern is quite different from the square
pattern.
See SHOW_PIX /calibration/Straylight_Grills_II to learn how the entrance
filter failure of 16-Aug-95 affected this pattern.
