The Saints came into the game on the back of a 24-15 win against Glasgow Warriors at Franklin's Gardens, but knew
Castres would prove tough opposition in windy conditions at the Stade Ernest Wallon.
Having only had five days to prepare after the Glasgow game, Northampton were well beaten this evening.
Although the loss disappointed Hartley, the 26-year-old claimed the Aviva Premiership leaders should still be considered as contenders for the trophy.
"We're not out of it," he said. "We've got Ulster next at our place, and we'll know where we are in terms of qualification a lot better after that one. There is still a long way to go in the competition.
"We knew that if we were to be considered contenders for the competition, we need to win games at home against the likes of Ulster and that's where our focus now turns to."
Although the Saints did not do enough for the win, a late rally saw them gain a bonus point - something which the visitors had had talked about during the interval.
"We said at half-time in the Glasgow game that we would target the bonus point, and it was the same again tonight," Hartley said. "We were pleased to come away with a bonus point. Our defence was pretty good but we just didn't do the basics particularly well tonight."
Saints lost away to Castres last season, and knew that they would be in for a battle in tonight's game.
Northampton coach Jim Mallinder was proud of his team's efforts but echoed Hartley's comments in that simple errors cost his side.
"We came up against a good side tonight on their own patch and knew it would be tough, just like it was last season," he said. "We fought well, but basic mistakes ultimately cost us the game.
"We must now look forward to Ulster, which will again be another tough game, but we've been in good form this season, and we know we can get a result against them at our place.
"We've got important games in the league before that, and our immediate attention will be on those matches."
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