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(D. Band 4/6/07) I concur with removing this column.
(Ormes, from an e-mail message 4/14/07) The deadtime comes naturally from properties of the electronics and is normally easy to track, but as you have assumed in making the above decision, not what we want. We need live time to compute fluxes. Live time cannot reliably be obtained by subtracting dead time from clock time, no matter how much attention is paid to removing time in SAA, spacecraft anomalies, etc. I pushed for a requirement that the electronic contain a clock that measure the livetime between events. One should be able, from this, to plot an "interval distribution" which should be exponential, with a mean that reflect the inverse of the counting rate. If the live time distribution, for any given type of event is not exponential, there is trouble.
However, this said, if there is trouble, we should have the deadtime, too, for cross checks. It should be filled with the number of events times the deadtime per event, of if the deadtime per event is not exactly constant but varies with the amount of readout or something else, the sum of the deadtimes over the events obtained during the accumulation time in question. I recommend not dropping the deadtime column but filling it with something useful.
(Digel, from reply 4/15/07) I'm not sure what you mean by trouble, whether if the interval distribution does not turn out to be exponential (because of a hardware problem) we'll be able to do anything about it. As far as I know, even if we filled the DEADTIME column with the number of events times the average deadtime per event in each interval, the average deadtime would be derived from the livetime counter values. So I'm not sure that the DEADTIME column would give us any independent information that would be useful for
correcting live times.
We'll certainly want to make sanity checks on the livetimes vs. the numbers of triggers, but I think that we don't need to carry the DEADTIME column in the FT2 file. We'll either decide that everything is ok with the LIVETIME determinations or that there's a problem. If there's a problem, I think we won't distribute FT2 files until it is solved, rather than make FT2 files for the user in the street to potentially misinterpret.
(Ormes, from e-mail 4/16/07) I don't know how the electronics was finally done. I do know these numbers, live and dead were supposed to be measured independently in the electronics. Maybe it was not done in the end.
3. McIlwain coordinates
(Digel) These are geomagnetic coordinates and presumably useful in some way for studying variations in residual background. The actual model we are using for the background depends on geomagnetic latitude, however, rather than the McIlwain L, B. I'm not entirely sure that we need (i.e., that anyone will use) L, B, but I won't propose removing them. I would like to propose including geomagnetic latitude, however, as an additional column.
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