___________________ What is our plate-scale and pixel-to-pixel spacing ? ___________________ We'd like to know the distance in arcminutes from the center of one pixel to the center of an adjacent pixel. The plate-scale at the SPARO detector arrays is then obtained by dividing by the physical separation of the pixels, which is 3.05 millimeters from the center of one pixel to the center of the adjacent pixel. Finally, the reimaging in SPARO is 1:1, so the plate scale at the detectors should equal the plate scale at SPARO's focal plane in the snout (e.g. at "field lens"). This last quantity can be directly calculated from the Viper ray tracing, providing a sanity check on SPARO's optics. Also, we want to confirm the 22 degree "rotation" of the arrays on the sky that is part of SPARO design. We observed the SPASE absorber in Jan/Feb 2000, and centered this source in each of SPARO pixels 2, 4, 5, 6, and 8. (See logbook 6, p. 22). Thus we determined these pixels' "boresights" (e.g. found pointing locations of each pixel). The El center was not very well determined, but the az center was accurately determined, by finding the half-power points. A quick analysis gives a pixel-to-pixel spacing of 3.2 arcmin. A more careful analysis (see "boresight analysis" below) takes advantage of the fact that the az positions are well determined but the El positions are not, and gives: -> pixel-to-pixel spacing = 3.5 arcmin -> Inferred plate scale at the detectors = 1.16 arcmin/mm -> Rotation of array: 25 degrees In Fall 2001, again using the SPASE source, we measured the following (log. 6, page 130): -> pixel-to-pixel spacing = 3.28 arcmin -> Inferred plate scale at the detectors = 1.08 arcmin/mm -> Rotation of array: 29.5 degrees Compare these with: -> Predicted plate scale (ray-tracing) = 0.98 arcmin/mm -> Rotation of array (design): 22 degrees ____________ Boresight analysis (using data from Jan/Feb 2000) ____________ First, assume a 22 degree array rotation. Based on the separation in azimuth of R4 (0.056 degrees) and R6 (0.053 deg.) to the average (from three scans) of R5, the plate scale is 1.16'/mm. R2 and R8 should then be 0.022 degrees away from R5 in azimuth. In actuality, R2 is 0.021 degrees off and R8 is 0.029 degrees off. This means R2 has a 21 degree rotation and R8 has a 29 degree rotation, which averages to a 25 degree rotation (from normal Az-el axes). Use this array rotation to determine a new plate scale from the R4-5-6 azimuth separations. When this is done, you get 1.18'/mm for the plate scale. In the first case the focal length is 2.972m. (In the second case the focal length is 2.906m.) ____________ Theoretical plate scale: ___________ After checking out the plate scale on beam3, we get a ps = 0.98'/mm. This would give a 2.94' pixel spacing, with a focal length of 3.516m. ___________