FORCAST Grism Observation Calculator

Web-based input form for FORCAST Grism Observation Calculator


This form can be used to estimate (1) the limiting fluxes of objects that can be observed with FORCAST for a given signal-to-noise and exposure time; (2) the integration time needed to reach a requested signal-to-noise for an input source flux and temperature; or (3) the signal-to-noise resulting from an input source flux and temperature and integration time. The output data file generated by this routine (forcast.plt...dat) can be saved to your machine from your browser window once the program has been run.

When estimating (1) the limiting flux, the user should specify the grism, the slit width, the desired signal-to-noise ratio (per resolution element), and the total on-source integration time for the observation (in sec). At each wavelength point over the observed range, the limiting flux is calculated for the input parameters.

When estimating (2) the on-source integration time needed to reach a specified signal-to-noise ratio (per resolution element), the user should specify the grism, the slit size, the desired signal-to-noise ratio, the source flux at a reference wavelength in microns, the source spectral shape (blackbody or power law) and either the effective blackbody temperature of the source or the power law index (depending on the shape chosen). The power law index is alpha, and F_lambda ~ lambda^-alpha.

When estimating (3) the signal-to-noise ratio per resolution element, the user should specify the slit size, the the source flux at a reference wavelength in microns, the source spectral shape (blackbody or power law), either the effective blackbody temperature of the source or the power law index (depending on the shape chosen), and the total on-source integration time for the observation (in sec). The power law index is alpha, and F_lambda ~ lambda^-alpha.

In all cases, the calculator accounts for the additional noise resulting from the chop and nod subtractions necessary for FORCAST observations. Therefore, the results are strictly valid only for nod-match-chop observations in which the source is moved off the slit for the chop. Observations made in the NXCAC mode will have a S/N that is smaller by a factor of sqrt(2) for the same on-source time.

The conversion between the continuum flux and line flux, under the assumption of an unresolved Gaussian line, is given by F_line = 1.06*lambda*F_peak/R = 1.06*lambda*chi_line*F_cont/R where F_line is the line flux in units of W/m2, F_peak is the flux density at the peak of the line and F_cont is the continuum flux density in units of W/m2/micron, lambda is the wavelength of the line in microns, R is the resolving power, and chi_line is the line contrast factor (peak line intensity relative to the continuum intensity). Alternatively, F_line = 3.19e-15*F_peak/(lambda*R) = 3.19e-15*chi_line*F_cont/(lambda*R) for F_peak and F_cont in units of mJy.

This form and the program to estimate the desired quantities was written by Bill Vacca. It incorporates the latest information on the grism throughputs, as estimated from previous grism observations, as well as the SOFIA/FORCAST image quality. The program uses a model of the atmospheric transmission and emission as a function of wavelength for an altitude of 41000 ft, an elevation angle of 45 deg (airmass of 1.4), and a zenith water vapor content of 7.3 microns. The model is smoothed to the requested resolution (which depends on the slit width and grism chosen). The calculations assume nominal instrument behavior. Note that the plate scale for FORCAST is 0.768 arcsec/pixel.

Questions about FORCAST and its expected performance should be directed to the SOFIA Help Desk. If you have problems with this form, please contact the SOFIA Help desk (sofia_help@sofia.usra.edu).



Input Observing Parameters

Select the quantity to be estimated:

Choose a grism:

Choose a slit size (arcsec):

Required Signal-to-Noise ratio:

Total on-source integration time (sec):

Source type:

Source Flux:

at

microns
Source spectral shape:

Source blackbody temperature (K) or Power Law Index:



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