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Help with the ITCs

Each new calculation will post results into the same page. The results page can be printed and re-sized e.g. so that the ITC form and results are both visible on your screen. 

Additional information is available for:


Image Size and Aperture

The derived image size (FWHM of the assumed Gaussian PSF) includes contributions from seeing, telescope diffraction, the performance of the tip-tilt secondary and many other components. See the image quality predictions in the observing condition constraints for more details of the wavelength dependence and frequency of occurrence. For the adaptive optics case both the FWHM of the AO-corrected core and the FWHM of the uncorrected halo are given, as is the Strehl ratio.

The "optimum aperture" size is described on the analysis methods page.

For most uniform surface brightness (USB) calculations, the (assumed circular) software aperture corresponds to an area of 1 arcsec^2 when the "optimum aperture" analysis option is selected. (A different value can be specified by the user and the point source size is also reported for reference). However in the case of the GMOS IFU, the "optimum aperture" option corresponds to a single IFU spatial element (not 1 arcsec^2) and the spectra from that IFU fiber (nominally 5 detector rows) are summed. No optimal extraction or deconvolution is performed at this time.

Format of imaging and spectroscopic results

In imaging calculations, the results page displays relevant derived properties on the top half page and key user parameters at the bottom. For spectroscopic calculations the relevant signal, noise and S/N characteristics are shown graphically. 

If the final and single exposure S/N are the same then only the former is visible. 

In the cross-dispersed mode with GNIRS, the source signal and sqrt(background) are color-coded for each order with dark and light shades of the same color. Only the final S/N plot (i.e. not the single-exposure chart) is shown.

To view (or save) the spectra as ASCII files with tab-separated columns, click (or shift+click) on the link(s) under each plot. The wavelength units are nm; the ordinate has the same units as the graphics plot. 

Signal to noise calculations

See the calculation and analysis methods for more details on how the S/N for the whole observation is derived.

Error messages - most are self explanatory; others:

  • Can't find sourceGeom - usually occurs when trying to use the ITC from Netscape 6.x which doesn't work with forms.
  • Please use a model line width > 1 nm to avoid undersampling of the line profile when convolved with the transmission response - the ITC currently samples the sky transmission spectrum, background and many other optical elements at 0.25-0.5nm resolution. Hence any model emission line that is narrower than 1nm may not be correctly propagated through the calculator. 


Last update January 30, 2004; Phil Puxley