The distance between two points on a CCD image is the very simplest form of as-trometry, the science of measuring the positions of heavenly bodies. The points can be integer pixel positions or the non-integer centroids of star images. The distance d between two points on the surface of a CCD is:
where (xv jj) and (x2, y2) represent the two locations. The x-axis is conventionally taken as the sample direction, and the y-axis as the line direction on the CCD.
On a sensor with square pixels (i.e., equal pixel width and pixel height), the distance is in pixels. On one with differing pixel width and height, the raw pixel positions must be corrected for the pixel aspect ratio of the chip. The pixel aspect ratio is defined as the pixel width divided by the pixel height, so the distance is:
with the result in units of pixel width.
The angular separation, $, in radians, between the two measured points in
Figure 7.6 The Distance Tool in AIP4Win computes the distance between two star images. If you know the focal length of the telescope and the orientation ol the CCD image, you can measure the angular separation and the position angle of the second star with respect to the first.
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