When digiscoping with a Canon EOS body on aperture priority , you will notice the f number shows as f0.0 rather than the usual f11, f8, f4 etc. This is nothing to worry about, and the camera will still give correct exposure. It will still adjust the shutter speed accordingly. It shows as f0.0 simply because a spotting scope for digi-scoping or a telescope for astrophotography are not camera lenses. In other words the camera does not recognise them.
If using some Nikon DSLR camera bodies, you may only be able to use manual exposure for this type of photography. If you set them on aperture priority, the camera may say no lens attached and not allow you to take the photo, unless you use manual.