NICMOS Detectors

NICMOS employs three low-noise, high QE, 256x256 pixel HgCdTe arrays. Active cooling by NCS will keep the detectors at a temperature between 75K and 86K.

Like CCDs, NICMOS detectors have read-noise, and substantial dark current. Additionally, they demonstrate an effect called "shading" which is a time-variable bias from the last read, affecting the readout amplifiers. Unlike CCDs, the individual pixels of the NICMOS arrays are strictly independent and can be read non-destructively. Thus they do not suffer from smearing and bleeding. However they can suffer from image persistence (memory) if they are illuminated to saturation for a sustained time period.

The NICMOS detectors have expected dark current values between 0.4 e-/sec (best case scenario) and 2.0 e-/sec (worst case scenario). Additionally, they are capable of very high dynamic range observations and have no count-rate limitations in terms of detector safety. Unlike CCDs, NICMOS detectors do not have a linear regime for the accumulated signal. The low- and intermediate-count regime can be described by a quadratic curve and deviations form this quadratic behavior is what we define as "strong non-linearity." This strong non-linearity is the limiting factor on the dynamic range since it limits the number of electron which can usefully be accumulated in a pixel during an exposure. Current estimates under NCS operations give a value of ~145,000 electrons (NIC1 and NIC2) or 185,000 electrons (NIC3)for 2% deviation from quadratic non-linearity.

NICMOS has three detector read-out modes that may be used to take data, plus a target acquisition mode: ACCUM, MULTIACCUM, BRIGHTOBJ, and ACQ. However, BRIGHTOBJ will not be supported during Cycle 11, but is available to determine the centroid of very bright targets for coronagraphy. ACCUM is the simplest mode, and it provides a single integration on a source. BRIGHT-OBJ is designed to observe very bright objects, which might saturate the detector. This mode reads out a single pixel at a time. MULTIACCUM provides intermediate read-outs during an integration, which can be subsequently analyzed on the ground. It should be used for most observations, since this mode provides the best dynamic range and correction for cosmic rays.

There are three target acquisition options for coronagraphic observing. The ACQ mode (which is one of the pre-defined modes in the Phase II proposals) commands NICMOS to position the brightest object within a restricted field of view behind the coronagraph. The RE-USE TARGET OFFSET special requirement can be used to accomplish a positioning relative to an earlier acquisition image. And a real time acquisition, INT-ACQ can be obtained as well. Although this is costly in time and resources.