CIRIM uses a 256 256 HgCdTe NICMOS 3 array. CIRIM's array appears to have better than average cosmetic properties and uniformity. It is operated using double correlated sampling to reduce its read noise as illustrated in Figure 1 and is not shuttered. During a typical observing cycle the array is first reset. Then the array is read for the first time. The initial read of the array is not immediately after the array is reset but rather is delayed by an amount of time referred to as ''fdly''. This time is set in software and allows the array to stabilize so that it loses its memory of how it was operated before the first reset. After a delay equal to the ''exposure time'' the array is read the second time. The image that is displayed is the difference between the second and first read. Due to timing considerations fdly can not be set to less than about 0.3 seconds which is also equal to the minimum exposure time. For exposures with an integration time less than the set value of fdly the actual fdly will be set equal to the exposure time. Like all NICMOS arrays the CIRIM array does have traps which cause the array to have ``memory'' of previous bright sources. So it is best to take a few trash images if changing from a high background to a low background filter (i.e. K band to a narrow band filter).
We normally operate the array with a bias of 900 mv. This provides a gain of 9 e /ADU and a read noise of about 37 e for double-correlated sampling. With a 900 mv bias the array is 2% nonlinear at 15,000 ADU, 4% at 25,000 ADU and 8% at 39,000 ADU. We would recommend that you operate the array at less than 15,000 ADU. This is particularly critical for short exposures since there is a time delay of about 0.3 seconds between the array reset and the first read of the double-correlated sampling. Thus, for a minimum exposure time of 0.4 seconds the array has already collected signal for 0.3 seconds before the first read and this charge is not indicated in the final difference image. If you use irlincor in the CTIO package of IRAF the linearity coefficients are; coeff1=0.99893, coeff2=0.0288 and coeff3=0.0233. The delay between the reset and the first read can be extended to give better array stability but we have found this to have little beneficial effect for most observing (but see section on coadds vs. pics) and it will lead to more linearity problems with short exposures.
For very high signal levels the ADC hits its rail at about 45000ADU. At or above this level there is no way to recover the true sky intensity from the difference image, since the second read is held at a constant level. This creates the odd phenomenon that brighter sources actually cause lower signals since only the signal level at the first read increases and the thus the difference becomes smaller.