Sorry I missed your original post and welcome to the select band of astrophotographers on the forum. As you've found out, post-processing isn't optional for this sort of work.
There are a number of commercial programs out there tuned for astronomical images but, if you are happy with a steepish learning curve, there's also the excellent, powerful and free IRIS software which you can download from here
. A handy primer can be read here
15 second exposure times are always going to be a bit limiting. If you can run to a tracking mount then all of a sudden your challenge will not be how to capture enough stars but how to keep those pesky street lights from ruining the image!