Problem: One of the instruments on board the Unreal X-ray Observatory (UXO) has a detector with 3600 pixels x 1800 pixels and a field of view of 30 ar-cmin x 15 arcmin. If each pixel is 24 μm x 24 μm in size, calculate the plate scale, in arcsec mm-1, and hence determine the focal length of the UXO telescope.