Last night I was on the thruway in NY with my sister and niece in the car. I had my cruise control set on just under 70 so it would be 68 - 69 mph. The speed limit was 65 mph. I was pulled over by a state trooper. The officer asked if I knew why he pulled me over. I said no. He asked if I knew how fast I was traveling. I told him yes, that I had my cruise set on just under 70. He said he tagged me going almost 80. I said that wasn't possible, repeating what the cruise was set on. He said that I slowed down after I saw him but that also is absolutely not true. I never touched the brake or canceled my cruise until after I saw his lights on to pull me over. I know absolutely without a doubt that I was not doing 80 mph and that I did not slow up after spotting the police car (I retained my set speed). Until he put his lights. What should I do to fight this?

Tell the same story to the judge. But sometimes the judge doesn't allow any testimony in my experience. You have a witness which might help.
A similar thing happeed to my sister in western MD. She told the trooper the same thing you did and he gave her a ticket. Twenty miles down the interstate she got pulled over by the same trooper. He stopped her to tell her that he had found that his radar was incorrectly calibrated and he couldn't rescind the ticket but he would testify in court that it was an error, which he did.
But since this is after the fact. Maybe it would be worthwhile to hire a lawyer or at least get an opinion. Good luck.

You need to hire an attorney, with that said, you have to request every bit of evidence.
1. Is his radar license up to date
2, Is all of his Post training up to date
3. was the radar calibrated at the start of his shift ( required most places)
And a lot of other info