Spitzer Documentation & Tools
IRS Instrument Handbook

6.6.2             Exposure Time

There are several integration-time-related keywords. Of greatest interest to the observer is the “effective integration time”, which is the time on-chip between the first and last non-destructive reads for each pixel.  It is called

RAMPTIME = Total integration time for the current DCE.

The value of RAMPTIME gives the usable portion of the integration ramp, occurring between the beginning of the first read and the end of the last read.  It excludes detector array pre-conditioning time. It may also be of interest to know the exposure time at other points along the ramp. The SUR sequence consists of the time taken at the beginning of a SUR sequence to condition the array (header keyword DEADTIME), the time taken to complete one read and one spin through the array (GRPTIME), and the non-destructive reads separated by uniform wait times. The wait consists of “clocking” through the array without reading or resetting. The time it takes to clock through the array once is given by the SAMPTIME keyword. So, for an N-read ramp:


Equation 6.1



Equation 6.2

Note that peak-up data is not obtained in SUR mode. It is obtained in Double Correlated Sampling (DCS) mode. In that case, RAMPTIME gives the time interval between the 2nd sample and the preceeding reset. The on-source exposure time is given by GRPTIME:


Equation 6.3

where the spincount is 7 for all peak-up acquisition observations.