theoretically zero, practically deflection would result from imperfections in ...
a) the straightness of the tube,
b) the cross-section of the tube,
c) the material properties of the tube,
d) the concentricity of the applied load,
e) the uniformity of the applied load,
f) the weather.
your stress is very low (if your dim'ns are inches, i get 150psi). euler is a start. maybe use modified column calcs to account for non-straight column, over a 1000" (?) the column is quite likely to have a wave, worst case might be a single sine wave, amplitude = allowable out-of-straight tolerance ? you could also account for the load being slightly eccentric.