DIYbanter - View Single Post - Question of TV technology, if anyone can answer two questions

isw

In article
,
wrote:

--snippety-snip--

Now, to address the question of electrostatic deflection, I realize it
is a pain in the ass. Instead of four parameters, two more which could
almost be called axes night be focus and astigmatism.

And I do not understand how it could be true that the same small spot
size cannot be achieved elecrostatically.

If not for the requirement to deflect the beam, it could be.

With electrostatic (but not magnetic) deflection, there is an increase
in the momentum of the electrons -- they are accelerated sideways while
traversing the deflection plates.

Recall that inside the tube, the "beam" is not a constant small diameter
"rod" the size of the spot; it's really two cones, base-to-base, with
the largest diameter located at the focus electrode. The spot on the
screen is really an image of the cathode.

Because of the momentum change in the electrostatic case, the position
of an electron within the beam as it transits the deflection plates
determines how much it is deflected (the electrons are not "running
parallel", they are converging), and so the spot becomes more oval the
more it is deflected (and that is why electrostatic tubes must be so
long). But it's even worse than that, because the ellipticity of the
beam caused by the first set of plates makes the distortion caused by
the second set very much worse (which is why things are so bad in the
corners).

The distortion of the beam (astigmatism) caused by deflection is not
correctable, even in theory. There is another source of astigmatism in
CRTs, caused by cathode asymmetry, which *is* correctable, by a properly
asymmetric lens.

Isaac