The ETC tool for NICMOS imaging can be found on the NICMOS web site at
http://stsdas.stsci.edu/ETC/NIC/nic_img_etc.html.
Note that at the time of preparing this handbook, a new ETC (APT-ETC) is under development. The user interface of this new ETC will look somewhat different to the one explained in this section, although it will keep the same format.
Figure 9.1 through Figure 9.5 below show the graphical user interface.
Figure 9.1: ETC Web interfaceSection 1: Select a detector and available filter.
Section 2: Specify wether you want your observation parameters to be calculated for a given exposure time or a required signal to noise limit.
Figure 9.2: ETC web interfaceSection 3: Specify the parameters for the source you wish to observe.
User Supplied Spectrum
: This is accomplished by placing the user input spectrum in an ftp staging area, which the program will load for the simulation. For sources with emission lines that fall in the wings of the filter, this is the optimum way of estimating the SNR. The "staging area" is the anonymous ftp directory:
ftp.stsci.edu:/outside-access/in.coming
Kurucz Model
: These model spectra are calculated from the Kurucz database (Dr. R. Kurucz, CD-ROM No. 13, GSFC) which have been installed in the Calibration Database System (CDBS).
HST Standard Star Spectra
: These spectra are available in CDBS and were chosen from the paper Turnshek et al., 1990, An Atlas of HST Photometric, Spectrophotometric, and Polarimetric Calibration Objects.
Real Object Templates:
Observed object spectra that are on-line.
Section 4: Normalizing the source flux. Whether supplying your own spectra or using one of the supplied model spectra, the source's continuum flux needs to be normalized at some wavelength. This wavelength needs to be within the wavelength range of the input spectrum. The ETC will use it only for normalization and calculate the appropriate flux values for the wavelength range of the observations. If your object is point-like, it can be normalized to a magnitude at a particular Johnson band, or it can be normalized to a flux [in Janskys] value at a given wavelength.
If you supply your own spectrum or use one of the HST calibration sources, you can either normalize this spectrum to a fixed value, or you can use the "Do not renormalize" option on the form. In this case the spectrum must be in a form acceptable to SYNPHOT. The simplest form is an ASCII file with two columns, wavelength in Ångstroms, and a flux in ergs-1cm-2 Å-1. For an extended source, you must specify the surface brightness.
E(B-V): The flux is normalized after the extinction is taken into account so that it always corresponds to the observed flux. The ETC supports two different extinction laws:
Section 5: Specify the expected background levels. Zodiacal light levels may be independently varied between 'low', 'average', and 'high'. Earthshine levels may be varied between 'shadow', 'average', and 'high'.
Figure 9.5 shows an example output page that is returned to the user. It contains the suggested exposure time, target signal to noise and the chosen observation parameters.
Figure 9.5: Returned output to the user. The S/N is only given for the brightest pixel.
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