For a point source, the count rate, C, can be expressed as the integral over the bandpass of the filter:
The peak counts/second/pixel/ from the point source, is given by:
Again, the integral is over the bandpass.
If the flux from your source can be approximated by a flat continuum (F = constant) and
f is roughly constant over the bandpass, then:
We can now define an equivalent bandpass of the filter (B) such that:
The count rate from the source can now be written as:
In Tables 9.1 to 9.3, we give the value of
for each of the filters.
Alternatively, we can write the equation in terms of V magnitudes:
where V is the visual magnitude of the source, the quantity under the integral sign is the mean sensitivity of the detector+filter combination, and is tabulated in Tables 9.1 to 9.3, and n is the filter-dependent correction for the deviation of the source spectrum from a constant F
spectrum. This latter quantity is tabulated for several different astronomical spectra in Tables 10.1 to 10.3 in Chapter 10.
For a diffuse source, the count rate, C, per pixel, due to the astronomical source can be expressed as
For a source where the flux is dominated by a single emission line, the count rate can be calculated from the equation
where C is the observed count rate in counts/second, (QT) is the system throughput at the wavelength of the emission line, F() is the emission line flux in units of erg/cm2/second, and
is the wavelength of the emission line in Angstroms. (QT)
can be determined by inspection of the plots in Chapter 10. See Section 9.6.4 for an example of emission-line imaging using ACS.
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