Nick Large:
Hi Andrew, welcome to the hobby and welcome to astrobin. Always good to see new faces.
Clip-on filters have always been difficult to find for Nikon cameras. The main reasons are that Canon are more popular for ASTRO and that Nikon camera body vary. This means that filter maker would have to make a specific filter for each camera type.. So not really a good investment. I have a D5300 and I never found one for that camera.
AS for the d850 i believe there was a filter but if I'm not mistaken Nikon modified something on the camera at some point so the clip-in filters won't fit anymore on some D850.
Do you use telephoto lenses or a telescope?
Hi. Both. 14mm for nightscapes and a 72ED for DSOs and I live in a Bortle 5. It was the Astronomic filter that did one but after a firmware upgrade on the D850 it no longer worked. I think Kase only supply for mirrorless. Before buying a dedicated Astro camera I want to become competent with the setup I have. I have a planetary camera that so far I have only used for guiding so lunar and planets are on the menu this winter.