How to calculate the total flux of a source from a fits file?

Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 05/30/2016 - 08:34

 

 

My fits file is a spire 250micron image that has data units of Jy/beam . I injected a source into it with a peak intensity 20Jy and fwhm(3,3). How can I calculate the total flux of the source? Please help