>>>>>>> Do you know WHY this is happening? The actual physical phenomena involved?
The physical parameter is known as coefficient of expansion. I am sure you know that things expand when they get warm and contract when cold. This is what makes asphalt streets crack, what makes a can of soda bulge when stuck in a freezer, etc. the coefficient of expansion is how much a material changes per degree of temperature change.
The material in the tube of your telescope is not immune to expansion and contraction in the changing temperature. If you have 2.000 feet of warm metal between your primary optic and your sensor at the beginning of the night, that same tube will shrink to 1.997 feet of metal (or something) when it gets cold. Depending on how far the temperature drops and what material is in the tube that number will be different for different types of "metal" (or non metallic) tube.
As the distance from the optic to the sensor changes, the sensor moves out of the "critical focus zone."
Of course, the CFZ itself can change position, as the optics change. The optics, after all, may also contract and expand with the cold.
But it is the simple expansion and contraction of materials during the change of temperature which is at fault.
Edited by Alex McConahay, 14 February 2018 - 09:15 AM.