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.

the focal plane of a telescope is:

where F is the focal length of the telescope objective, d is the separation in pixels, and d jxel is the pixel width in the same units as the focal length.

7.7 Image Profiles

Profiles are one-dimensional slices through images, a graph of pixel values across a single line or column of pixels. Profiles are a huge help in sorting out and understanding brightness relationships in images. Although the human eye is good at extracting features, it is quite poor at estimating brightness. Humans perceive spiral galaxies, for example, as dominated by the spiral arms—but a profile through a typical spiral galaxy reveals that the nucleus overwhelms the arms, which are mere bumps superimposed on the radial decline in brightness from the nucleus. Profiling planetary images also reveals the importance of limb darkening, which observers accustomed to eyeballing images hardly notice.

• Tip: The Profile Tool in AIP4Win extracts a profile of pixel values along an arbitrary line drawn through an image.

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