Integrations and exposures

This page is part of the ING document INS-DAS-29: Operations manual for UltraDAS

The detailed handling of integration timing is quite subtle.  This section tells you what to expect in the various records for the different kinds of observation that UltraDAS can do.

Integration vs. exposure

For a CCD, the integration starts when the CCD is cleared, and the integration is counted from the end of clearing. The integration is deemed to end when the CCD starts to read out. The exposure starts when the shutter opens and ends when the shutter closes. The exposure starts after the integration and finishes before the integration; the exposed time is slightly shorter than the integrated time. The actual exposure and the effective exposure are identical for this case.

For cameras that don't have a shutter, the actual exposure is continuous through all integrations. Effective exposure times are identical to the matching integration times. This mode applies to cameras that have a shutter if the shutter is opened explicitly before the observation: for this case, UltraDAS treats the camera as shutterless.

For CCDs, readout destroys the charge pattern on the detector by moving the charge into the output amplifier. This means that readout ends the integration. IR devices (e.g. INGRID) typically do non-destructive readout where the stored charge is measured in situ. This means that the effective exposure stops at readout, but that the actual exposure goes on through readout and afterwards.

"Idling" vs. integrating

Charge from light leaks and thermal noise builds up on the detector between observations. If this charge is integrated by the detector, it may not be completely cleared away by the clear cycle of the next observation. In that case, the observation will be contaminated by extra counts. (Often, this appears as a ramp in the background leading up to a saturated region in the low-numbered rows.)

To avoid this problem, the detector is made to clear itself continuously between observations. This is called "idling", and is reported as such on the mimic.

Reported times

In the mimic: This detail affects both the start times and the durations. One result of this arrangement is that the displayed integration for INGRID and similar IR cameras will continue to count up throughout the observation. The final integration time at the end of observation will be higher than the effective integration time of the last readout in that observation.

In the FITS header:

Source and accuracy of timing

Exposures delimited by shutter movements are timed by the clock in the SDSU detector-controller. This clock only measures the length of the exposure, not the absolute start-time. The exposure is considered to start when the shutter starts to open and to end when the shutter finishes closing.

Start-times of exposures and integrations and lengths of integrations (and by extension, lengths of shutterless exposures) are measured from the clock of the DAS computer. This clock gives absolute time. The computer clock is regulated by network time protocol (NTP) and its accuracy is limited by the length of the network path to the primary time-server.

All time measurements are precise to the nearest millisecond.

Accuracy of timing is difficult to test. We believe that these are the current limits:

If funds are made available, it is possible to add a GPS time-service to each DAS computer such that the absolute timings are accurate to better than a microsecond.