> er,not quite. If the phosphor was long persistance every sweep would
> remain on the screen. There is some sort of mesh inside the tube which
> can preserve the scan when energised. Gradually the electrons leak away
> and the trace fades. When storage mode is de-energised, the scope
> reverts to normal display.


I think the theory is that a very high voltage screen is placed near the
phosphor, and that if a photon leaves the phosphor and hits the high volt-
age screen it will knock off an electron (which in effect "recharges" that
spot on the phosphor).

I have actually used an analog storage scope once, but it was a bit trick-
y to adjust.  Basically, there was a persistance knob which would set how
fast the trace would fade.  If the knob was set too high, however, then
the trace would start to "bloom" until eventually the whole screen was a
solid glow.

I think the thing must have had two screens, though, one inside the other
since if memory serves it had a "Save" switch which would blank the display
without disturbing the image on it; when you turned off the "save" switch
the last-saved image would reappear.  I think the idea there was that the
image would last longer if it didn't have to be swapping enough electrons
for photons and vice versa to produce a visible picture.

Kinda a fun gizmo, being able to set the persistence.  Personally, I sorta
wish people could make digital scopes that did a better job of emulating
analog ones.  For example, I'd like to see a scope that could (assuming
moderate resolution, e.g., 10uS/div), store all of the following values
for each pixel:

[1] The minimum and maximum values observed during that time period, as
    sampled.
[2] The minimum and maximum values observed during that time period, if
    run through a moderately low-pass filter (e.g. if each pixel is 500ns,
    I'd like the filter's "effective" RC time constant to be about 100ns.
    if the minima and maxima are logged every 10ns.
[3] The average value observed during that time period, Gaussian-weighted
    into the preceding and following ones.

If the scope logged these five values, it could use them to show a waveform
that was much more informative than that given by any mode on a normal DSO.
Peaks would still be shown, but they wouldn't drown out the rest of the wave-
form.  Also, since peaks would be stored in filtered as well as unfiltered
form, it would be possible to estimate how much energy they actually cont-
ained.

Does anyone know if any scopes do anything like that?