WFC3 Instrument Handbook for Cycle 25 |

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Wide Field Camera 3 Instrument Handbook for Cycle 25 > Chapter 9: WFC3 Exposure-Time Calculation > 9.5 Count Rates (Slitless Spectroscopy)

For a point source with a continuous spectrum, the count rate, C, is calculated per pixel along the dispersed spectral trace, and is integrated over a fixed extraction height in the spatial direction perpendicular to the dispersion. The count rate is:

•

• d is the spectral dispersion in Å pixel−1.

• For a source with an unresolved emission line with a flux of in units of

erg cm−2 s−1, the total count rate recorded over the Nspix extraction height is:In contrast to the case of imaging sensitivity , the spectroscopic point source sensitivity calibration () for a default extraction height of Nspix was measured directly from observations of stellar flux standards during SMOV and Cycle 17 calibration programs 11934, 11936, and 11552. Therefore, the accuracy in laboratory determinations of for the WFC3 grisms are not crucial to the final accuracy of their sensitivity calibrations.

Wide Field Camera 3 Instrument Handbook for Cycle 25 > Chapter 9: WFC3 Exposure-Time Calculation > 9.5 Count Rates (Slitless Spectroscopy)