NASA has revealed that the cameras on the Transiting Exoplanet Survey Satellite (TESS) become slightly out of focus when they are cooled to -75 Celsius, the temperature they will experience in space.

NASA has also decided that the fuzziness is not enough to require a fix, and is proceeding with the mission as is, despite concerns expressed by scientists.

Alan Boss, an astronomer with the Carnegie Institution, brought up the issue in a summary of a meeting last week of the Astrophysics Advisory Committee, of which he is a member. “That could have some big effects on the photometry,” he said of the focus problem. “This is certainly a concern for the folks who know a lot about photometry.”

TESS will use those cameras to monitor the brightness of the nearest and brightest stars in the sky, an approach similar to that used by Kepler, a spacecraft developed originally to monitor one specific region of the sky. Both spacecraft are designed to look for minute, periodic dips in brightness of those stars as planets pass in front of, or transit, them. Chou said that since TESS is designed to conduct photometry, measuring the brightness of the stars in its field of view, “resolution is less important compared to imaging missions like Hubble.” However, astronomers are concerned that there will be some loss of sensitivity because light from the stars will be spread out onto a slightly larger area of the detector.

“The question is how much science degradation will there be in the results,” Boss said. “The TESS team thinks there will be a 10 percent cut in terms of the number of planets that they expect to be able to detect.”

It could be that NASA has decided that the cost and delay required to fix this is not worth that 10% loss of data.