A Space Mirage was a phenomenon encountered by some Space Corps astros whilst piloting their spaceships in Deep Space.
Warnings about Space Mirages were included in the Space Corps driving manual. Three million years later, they were mentioned in Kryten's pilot test aboard Starbug, as adjudicated by Rimmer.
The manual showed the racy silhouette of a naked, busty woman, which Kryten correctly took as an indication of a Space Mirage. ("Backwards", Series III)
- Space Mirages were presumably the hallucinations of astros suffering from some illness, such as cabin fever, in confined spaces and without female company for extended periods of time.