Space Telescope Science Institute   9.2.1 Imaging  9.3 Computing Exposure Times

9.2.2 Spectroscopy


Point Source

For a point source spectrum with a continuum flux distribution, the count rate, C, is per pixel in the dispersion direction, and is integrated over a fixed extraction height in the spatial direction perpendicular to the dispersion:

Where:

For an unresolved emission line at with a flux of in
erg/second/cm2 the total counts recorded over the Nspix extraction height is:

These counts will be distributed over pixels in the wavelength direction according to the instrumental line spread function.

In contrast to the case of imaging sensitivity , the spectroscopic point source sensitivity calibration ( ) for a default extraction height of Nspix is measured directly from observations of stellar flux standards after insertion of ACS into HST. Therefore, the accuracy in laboratory determinations of for the ACS prisms and grisms is NOT crucial to the final accuracy of their sensitivity calibrations.

The peak counts/second/pixel from the point source, is given by:

Where:


 9.2.1 Imaging  9.3 Computing Exposure Times
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