The brighter the object the longer the diffraction spikes. You will not get diffraction spikes from normal dirt on the mirror; you only get them from sharp edges in the light path. One is the ring around the Airy disk, from the sharp edge of the mirror itself, then there is the bright spikes from the spider vanes, more larger (but fainter) rings from the edge of the secondary, and there are still more spikes from the edges of the mirror clamps. I know it's a fair bit of self-pollution.
