So I got to looking at the frames I took the other night. The way this works is that there is software that stacks the exposures and uses three additional types of calibration frames which are used to calibrate the sensor. Going back over the frames I took of the Horsehead Nebula I found this one. It has an airplane flying through it. Pretty cool. FYI-the frame is about 5.3 X3.7 degrees. For reference: the sun and the moon are about 0.5 degrees in diameter. The stacking software realizes it is not supposed to be there because all the other frames don't have it.