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• W93767519 abstract "This report describes the latest version of the NICMOS Exposure Time Calculator (ETC) developed to support NCS-era HST NICMOS planning. We added a thermal optics library to the ETC, and model various detector behaviors seen during the NICMOS end-of-life period. There are no ad-hoc parameter values in the thermal optics library describing the telescope and the NICMOS camera. This ETC supports imaging through all filters except the grisms. Exposure Time Calculators (ETCs) are used to estimate expected signal to noise ratios given a source brightness and size, and an expected sky background. In the infrared, a knowledge of the thermal properties of the optics is also required. There are two older NICMOS ETCs: a private one by Thompson (1996) and an STScI-supported one by Skinner (1996). The previous STScI-maintained version, written in FORTRAN by C. J. Skinner, and modified by L. Colina, D. Calzetti and D. Daou, uses pre-launch characteristics of the NICMOS camera, and its own collection of throughput and quantum efficiency data. In consequence, it will be hard to maintain when the NICMOS instrument is revived by the NCS cryocooler. Furthermore, instrument and telescope parameters in this ETC were tuned to match observations, especially in the thermal infrared. This replacement ETC uses measured values of all quantities except for Point Spread Function (PSF) information. In particular, the measured emissivities of the HST primary and secondary are used. These result in good agreement with observations (Robberto et al., 2000). Here we describe the user interface of this new NICMOS ETC, which uses Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. instrument throughput and data files maintained and updated by the Space Telescope Science Institute (STScI) and data from NICMOS end-of-life observations (Boeker et al., 1999). The NICMOS detector showed an unexpected bump in dark current during warmup: it is not known whether this bump will recur if the instrument is cooled down again. The bump peaks at around 7e per second per pixel at about 82K. Thus cycle 10 proposals should use the ‘dark current bump present’ case when planning observations, as this is the worst-case scenario. The observer can also look at the more optimistic ‘bump absent’ case if they wish. Offsetting this predicted increased dark current is the fact that the DQE gets better with higher temperatures. The expected operating temperature for the detector in the cryo-cooler era is about 75K (cycle 7 temperatures were between 61 and 64 K). This version of the ETC will be available for planning cycle 10 observations with NICMOS. We also describe the core algorithms used by the ETC. For software details see the ISR devoted to maintenance, support and installation (Sivaramakrishnan et al., 2000a): all the source code is made available to anyone interested in inspecting or modifying the implementation details on their own, though STScI does not provide development support for such activity. Description of the Calculator This version of the NICMOS ETC uses throughput data in the Calibration Data Base System (CDBS) at STScI instead of separately maintained private data files. It uses the STSDAS package SYNPHOT as its throughput calculator. SYNPHOT uses the CDBS instrument data files, and thus brings NICMOS exposure time calculations into line with other HST instruments (notably STIS, ACS and WFC3). It is hoped than this unification will make for easier data management. All the code used is non-proprietary, and can be distributed without concern for licensing issues. Copies of the CDBS data base are also freely available (eg., through STScI’s Starview software package). While this NICMOS ETC derives its high level software architecture and execution thread from a STIS ETC (Hack et al., 1996, with subsequent modifications and redesign by B. Simon), it differs from the STIS and other optical ETCs in that it has to estimate a noise component due to thermal contributions from the telescope and the NICMOS foreoptics. SYNPHOT is not capable of handling internal sources of radiation yet. In order to accomplish this a library of NICMOS-specific thermal and optical transfer routines were added to the ETC. The NICMOS thermal model implemented in this library differs from the one described in the NICMOS ISR-14 in that it contains no ad-hoc adjustment factors to match observations, and it models various components of the HST pupil as seen from NICMOS, following the work of Krist et al. (1999). Background from sky is presumed to be zodiacal in origin. A model of zodiacal light from the previous STScI NICMOS ETC is used (cf. the one described in NICMOS ISR14). This is an average of zodiacal light at 45 degrees from ecliptic. Table 1 indicates the values used in photons per second per steradian per unit area per unit wavelength interval." @default.
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• W93767519 date "2000-05-01" @default.
• W93767519 modified "2023-10-17" @default.
• W93767519 title "The NICMOS Exposure Time Calculator: Algorithms and User Interface" @default.
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