When he did, just before midnight last Monday, he immediately sent an electronic message to the International Astronomical Union in Harvard - which confirmed by the morning that he was the first in the world to spot it, beating the professionals and becoming only the second Briton ever to discover such an event.
Mr Laurie, 38, an actuary from Church Stretton, Shropshire, had spent just six weeks using the pounds 3,500, 10in telescope on his patio, linked to a pounds 2,000 device which contains photoelectric sensors capable of picking up light far fainter than the human eye can discern. That data was passed for processing to a computer inside, then to another computer, which compared the images with earlier ones. And in one, there was a significantly brighter spot - the supernova.
"It's known as a 1A type - a double star system in which they orbit each other, and the bigger star had accreted more mass from the smaller one." Then, the star reached a critical mass and exploded into light. Supernovae are rare and unpredictable: professional astronomers have been known to spend 20 years looking for them without success.
Mr Laurie, a keen astronomer since acquiring his first telescope at the age of 12, had to hide his excitement in order not to disturb his wife, Angela, who was asleep. "I didn't wake her up, but told her the next morning," he said. "I was very lucky to find a supernova so quickly. It's a bit like winning the lottery."
But his patience has paid off before: he has already discovered 50 asteroids.
He was inspired to hunt supernovae after fellow amateur Mark Armstrong became the first British astronomer - amateur or professional - to discover an exploding star last October.
He programmed his telescope to scour the heavens systematically, focusing on up to 60 galaxies an hour. The supernova stood out as a white dot in galaxy NGC 3147, which he realised was missing from an earlier image of the same galaxy.