My Telescope.

[Anomalocaris]

Yes, telecomverter is a cheap way to get greater focal length, but sacrifices aperture and some image quality.    Camera telephoto lenses are not really well suited nor economical for deep sky photography because even long telephoto lenses normally don’t have the focal length to resolve fine details in planets or deep sky objects, and their costs are high for the focal length they provide because ordinary photography demands many type of image correction that would be of little consequence to amateur astrophotography.

But I am a hobby photographer so I already have the lenses, so it made sense to use them for the purpose.