Just when I thought I was done with building refractors, I got these cardboard tubes free from a neighbor who was going to throw them away. They are 76 mm ID, 85 mm OD, 1640 mm long.
These are by far the best cardboard tubes I have seen (including ones I purchased). Very smooth exterior, perfect round, and quite robust as the walls are 4 mm thick. Also quite long at 5'4".
So I wanted to see if a 70 mm achromat doublet or even singlet is available with long focal lengths, preferably > 1500 mm. Does such a lens exist?
Thanks for reading.