Why can we see only 2 rings in Teltron electron beam tube?

Question

http://en.wikipedia.org/wiki/Teltron_Tube

Answer 1

I assume you are using it for a diffraction experiment so you have rings and you are trying to estimate the "slit" size.

The things that could be happening are there is very little intensity in the 2nd order ring. There is something in the way so the 2nd order ring never makes it to the screen. The second order ring would be so big that the tube isnt big enough for you to see it.

Those are all the reasons that I can think of.