By December, 1985, the long-anticipated brightening of Halley's comet was well under way, and early that month a faint tail was detected as it approached the sun, and is seen in this AAT photograph. The picture alongside was taken on the UK Schmidt Telescope after it had rounded the sun, and is the blue-light component of a colour photograph.  A colour photograph of a comet (or any astronomical object) does not always reveal every detail, and photograph here was made from the UK Schmidt plate taken in blue light for the colour picture. The turbulent structure evident on plate was emphasised by a darkroom process known as unsharp masking and is hardly visible on the colour picture.

Comet tails form two distinct types, a yellowish dust tail, made of tiny solid particles released as the icy, dusty cometary nucleus is evaporated by sunlight, and the blue ion tail, made from volatile compounds released along with the dust, then ionised by the sun's ultraviolet light. Both tails point away from the Sun, irrespective of the direction of the comet, but the ion tail's structure reflects the effect of the solar wind.