The first point of every line is under-exposed. Four commissioning scans
put a number on it, against the 2e-3 head start that was in place:
scan exp_time first/near lost
324 15 ms 0.655 5.18 ms
254 20 ms 0.732 5.36 ms
411 50 ms 0.881 5.93 ms
450 50 ms 0.879 6.04 ms
A fixed time, not a fixed fraction: it varies by 17% across a 3.3x range
of exposure while the fraction varies by 2.9x. The same deficit appears
on the integrated scattering, a different detector behind a different
gate, so the cause is upstream of both readout chains rather than in
either of them. 2e-3 allowed and ~5.6e-3 still lost means the shutter
needs about 7.6 ms, rounded up to 8 ms: the spread across the four scans
is 0.9 ms, so the third digit is not meaningful, and overshooting costs
only the difference in dead time at the start of each line.
The trigger scheme was already right -- the shutter fires on cd at t0
and the acquisition on ab is held back by _shutter_to_open_delay, with
the widths, burst_period and cont_grid's acc_time and premove all
derived from it. Only the value was wrong, and it was a literal in two
places, so setting one and not the other would have been silently undone
by keep_shutter_open_during_scan.
Lifts it to DEFAULT_SHUTTER_TO_OPEN_DELAY next to the other defaults,
with the measurement recorded, and adds set_shutter_to_open_delay to
USER_ACCESS so the value can be converged from the client instead of by
redeploying the device server. It is bounded, because a delay is paid on
every line and a fat-fingered value would stretch the scan rather than
fail.
Cost at the new value is 6 ms per line -- 0.26 s over scan 450 -- and
0.8 um of extra premove.
The existing stage test asserted the 2e-3 literal and now asserts the
constant. New tests cover the default, the bound, the USER_ACCESS entry,
that a set value actually reaches the ab channel while cd still fires at
t0, and that keep_shutter_open_during_scan discards a tuned value, which
is a sharp edge worth pinning rather than leaving to be rediscovered.
menzel
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