Thu 18 Aug 2016 09:30:09 AM UTC, original submission:
Currently MakeCatalog can output the magnitude of an object, but it doesn't calculate the magnitude error. MakeCatalog does calculate the signaltonoise ratio (S/N) however.
Converting from the S/N to the error in estimating the magnitude is an easy operation (detailed below) which can be done with something like AWK, but it is common enough to justify its inclusion as a column in MakeCatalog.
To derive the error in measuring the magnitude from the S/N, let's take `F' as the flux, `Z' is the zeropoint, `M' is the magnitude, `S' is the S/N, and `D' to stand for capital delta (or error in a value) then from
`M = 2.5*log10(F) + Z'
we get the following equation after calculating the derivative with respect to F.
`dM/df = 2.5( 1/(Fln(10)) )'
From the Tailor series, `DM' can be written as:
`DM = dM/dF * DF'
So
`DM = 2.5/ln(10) * DF/F'
But `DF/F' is just the inverse of the Signal to noise ratio, or `1/S'. So
`DM = 2.5 / ( S * ln(10) )'
Note that this is just the raw flux measurement S/N, it does not include correlated noise, or aperture errors (which are also present in a realistic situation). This will be noted in the manual.
Maybe later, we can find ways of including these extra error sources directly in MakeCatalog (with the help of NoiseChisel in the form of FITS header values, ofcourse).
