Just gone to list one and see that there is no category for them so could it be added next to the binoculars and telescopes.
Photography > Binoculars & Telescopes > Binoculars
Photography > Binoculars & Telescopes > Telescopes
That is how it looks at the moment but as monoculars are different you'll probably need to change the words Binoculars & Telescopes to
Photography > Binoculars & Telescopes Monoculars > Binoculars
Photography > Binoculars & Telescopes Monoculars > Monoculars
Photography > Binoculars & Telescopes Monoculars > Telescopes