Want to shoot with an extremely long lens? A reflecting telescope may be your cheapest option. In this video Bohus shows you how he rigged his Sony A7R II to shoot with a Meade ETX-90EC 1250mm telescope!
Click here to get your T-mount to Sony E-mount adapter: https://www.fotodioxpro.com/products/t2-snye
These are sometimes called Maksutov lenses, after their inventor, Dmitri Dmitrievich Maksutov. Check out this link to learn more: https://en.wikipedia.org/wiki/Maksutov_telescope