Nice first shot Mike
Pretty good for a single 18 sec sub.
Try to get it to 30 secs, push the ISO up a notch and take lots of shots, using the intervelometer on the D7100.
Looks like it was wide open at F2.8. If you can manage it, stop it down to F4 too, to reduce the CA and coma.
Was this taken from a back yard or a dark site?
Cheers
Bob