function genoidSatSize, D, mag, dDiam=dDiam, dMag=dMag


  dSat = D * 10^( -0.2* mag ) 


  if keyword_set(dDiam) then begin
    deltaD = (dDiam/D)^2.
  endif else deltaD=0.

  if keyword_set(dMag) then deltaMag=dMag^2. else deltaMag=0.
  deltaSat = dSat*sqrt(  deltaD + deltaMag )


  return, [dSat, deltaSat]


end