What filter size for Pentax 100SDUF telescope?

I am looking to use a Pentax 100SDUF telescope as a long focal length lens. I would like to protect the front element with a clear filter. There are no markings on the lens as to the size of filter required. There are some threads at the front of the front lens cell that I can thread onto. The distance between crests of the treads is 107.1mm measured to the best of my ability. My best internet searching has not revealed which filter size to which this corresponds. Since this is a telescope, there is no guarantee that the size of the lens cell actually corresponds to an available filter size. Since this scope is no longer being produced, there is no one, company wise, that I can ask.