Query about the definition of photometric zeropoints

Question

In stscl page, apparent magnitude is defined as $$m=-2.5\log{\frac{{\rm DN}}{{\rm EXPTIME}}}+{\rm ZEROPOINT}\ ,$$, DN is Data number (I cannot understand what is meant by 'Data number'... they said it is 'one count', but count of what?).

Because I want to know why the magnitude is defined like this, I tried to derive it from Pogson's formula: $$m_1 - m_2 = -2.5log_{10}{\frac{F_1}{F_2}}, m_1 = m, m_2 = {\rm ZEROPOINT}\ , $$ $$m = -2.5log_{10}{\frac{F}{F_{\rm ZEROPOINT}}} + {\rm ZEROPOINT}\ .$$

So I want to ask, Why $F = DN\times F_{\rm ZEROPOINT}={\rm EXPTIME}$?

I think it's strange because I thought $F_1$ is the flux of object 1 and $F_2$ is flux of the calibration object (like Vega's flux).

Answer 1

The zeropoint is defined in any way you want. But this definition is quite conventional. It says that a source with a measured count (or data number) rate of one per second has a magnitude given by the zeropoint.

The zeropoint is then calibrated by measuring the count rate of stars of known magnitude.

The data number is just the number of counts measured for that source, presumably in some sort of analogue to digital units.