The instrumental signature for NICMOS data can be divided into two categories, bias and dark, according to whether or not the signal is noiseless and purely electronic in origin (bias), or noisy and arising from thermal or luminous sources (dark). During detector reset, a net DC bias with a large, negative value (of value -25000 ADU) is introduced. This bias is different in each readout quadrant, but essentially constant within each quadrant.
In addition to the net quadrant bias introduced at array reset, there is some additional offset which is time-variable and, to some degree, stochastic. This variable quadrant bias has been described as the “
pedestal effect” in many discussions of NICMOS data, although we note here that the term “pedestal” has also been applied to other aspects of NICMOS array behavior. The variable quadrant bias is usually constant over a given array quadrant, but different from one quadrant to another. Its amplitude varies from readout to readout, sometimes drifting gradually, but occasionally with sharp changes from one readout to another (not always seen in all quadrants simultaneously).
The IRAF scripts pedsky and
pedsub have been developed to remove the pedestal effect. For a more detailed discussion of the pedestal effect and how to remove it, please see the latest version of the NICMOS Data Handbook, Chapter 4:
Anomalies and Error Sources, at URL:
