Filter novice for DSLR

[Okeydokey]

I have bought a Chinon f1.7 55mm lens and an M42 adapter ring for my Pentax K-m which I am very happy with and would like to put a filter on the front end to protect the lens itself.  Tonight... with luck I will try to capture my first DSO.  I have an EQ3-2 mount with RA tracking and would love any further attatchments to compliment my ambitions...  Are there any filter guru's out there who would suggest a lens filter that will both protect my lens and add value to my DSO efforts at the same time and in the near future?

I have no idea whether polarising, IR, red, blue or green would be any use, a budget version I guess would be clear but I am sure I push it one step further 

Many thanks