Collimating an astronomical telescope usually includes a step where an image of a star is viewed, or imaged by a camera, out of focus. Adjustments are made to the optical elements by adjusting screws, typically 3 of them, to bring various parts of this out-of-focus image into perfect concentricity. This can be tricky as, although the eye is quite good at detecting lack of concentricity, it’s not infallible.
Collimation Aid is a simple app whose only purpose is to display three concentric circles on a transparent window. This window can be overlaid on an image, particularly an image from a camera attached to a telescope. The size of the window can be adjusted as required, and so can the size, and relative sizes, of the circles (via the Setup menu). The colour (and so brightness) of the circles can also be adjusted in the same way. A set of three numbers (representing the collimation screws) is shown around the outermost circle, and the rotational position of these (while remaining equally spaced from each other) can be simply adjusted.
Finally, Collimation Aid provides a pair of orthogonal, centred grid lines to aid in centring the pattern on the image.
By the way -- the second screenshot shows an image from a scope which is not particularly well collimated! One of the reasons I developed the app in the first place!
Up to six family members will be able to use this app with Family Sharing enabled.