Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. Instrument Science Report NICMOS 2004-002 New Exposure Time Calculator for NICMOS (imaging): Features, Testing and Recommendations S.Arribas, D. McLean, I. Busko, and M. Sosey February 26, 2004 ABSTRACT A new NICMOS ETC for imaging mode has been developed as part of the Astronomer’s Proposal Toolkit (APT) project. This new tool fully updates the NICMOS performance for Cycles 11+, expands the funtionality of the previous ETC, providing the user more options, and homogenizes the non-instrument specific parameters (i.e. sky background, extinction laws) with other HST-ETCs. This report summarizes its main characteristics, and gives some recommendations to potential users. Details about the tool itself can be found in the documentation linked to the ETC user interface, which can be accessed from the NICMOS web site at STScI. Introduction Exposure Time Calculators (ETCs) are used to obtain estimates of exposure time for a given signal to noise ratio (SNR) (or vice versa) for a simulated observation with a given telescope/instrument configuration under some particular sky background conditions. This requires a proper characterization of the instrument and the sky backgrounds. Two NICMOS ETCs have been supported at the STScI. A first tool was constructed by Skinner (1996), on the basis of pre-launch as well as early Cycle 7 throughput and quan- tum efficiency data. Some properties of the telescope and the instrument (e.g. thermal emission) were tuned to agree with the observations. Later, in preparation for the NIC-