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.
Often the millimeter size of the lens is written on the back of the lens cap. All of the spotting scopes currently being made by Pentax have the front lens size as part of their model number. If this applies to your Pentax 100 SDUF, then the filter size should be 100mm.
I went to B&H website (bhphotovideo.com) and discovered that they don't sell a 100mm circular screw-type filter. Since B&H pretty much carries everything, I assume nobody currently makes a 100mm UV filter. I have found one made by Olympus on eBay for $89. Here's the link https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313.TR0.TRC1.A0.H0.X%22100mm+uv+filter%22.TRS1&_nkw=%22100mm+uv+filter%22&_sacat=625&LH_TitleDesc=0&_osacat=625&_odkw=100mm+uv+filter