this is going to be an ignorant question, how do you zero a thermal during the day or do you have to do it at night with some type of heat source?
Lots of good solutions.
Thermal sees differences in heat and strong sunlight heats black up much more than white.
I have very good luck with black dots on white paper in strong direct sunlight.
Weak sun or overcast I use a AA battery wraped in aluminum foil shorting the battery. Gets up to about 150 degs and lasts about 30 minutes.
Slice an X in the target and insert the battery pointing at you. Very precise 1/2 inch diameter aim point.
One shot on the battery makes the battery go away but the thermal is zeroed.
