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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.