rat156, no thermodynamic laws are being broken for an object's temperature to fall below that of its surroundings! The heat is radiated away from surfaces like metal faster than it is radiated/conducted to the metal. If the radiation heads upwards, much of this longwave infrared radiation then ultimately escapes to space (far from the immediate surroundings of the telescope), though some is intercepted by CO2 and H2O molecules in the atmosphere as part of the greenhouse effect. Because the lost heat isn't returned so effectively through radiation or conduction by the warmer surrounding air, the object cools a little below its surroundings. This is the reason you get most visible frosts, especially on cars or telescopes, and also the reason meteorologists distinguish between air frost (air temperature below 0C) and ground frost (air temperature above 0C).
From Wiki: "Hoar frost (also called radiation frost or hoarfrost or pruina) refers to the white ice crystals, loosely deposited on the ground or exposed objects, that form on cold clear nights when heat losses into the open skies cause objects to become colder than the surrounding air."
"Other objects on which frost tends to form are those with low specific heat or high thermal emissivity, such as blackened metals; hence the accumulation of frost on the heads of rusty nails."