Nice one Kevin - this is a great quartet.
Try this technique in photoshop to reduce noise:
https://www.youtube.com/watch?v=32Mj2Ia-tC0
I love it - really helps reduce DSLR noise.
If you want a more "harsh" noise reduction - (I have attached a sample of your image) use median noise reduction. There is a balance between amount noise reduction and masking vs lost stars etc. but overall it helps smooth out the background. Personal taste I guess.
Tony.