Converting Counts to Flux or Magnitude

[When I tried to use the flux from catalog, the unit is count/s !! You have to convert it to flux or magnitude considering the zero point! Let me show you here.]

All calibrated HST images record signal in units of counts or Data Numbers (DN)¹-NICMOS data is DN s-

1. The pipeline calibration tasks do not alter the units of the pixels in the image. Instead they calculate and write the inverse sensitivity conversion factor (PHOTFLAM) and the ST magnitude scale zero point (PHOTZPT) into header keywords in the calibrated data. WF/PC-1 and WFPC2 observers should note that the four chips are calibrated individually, so these photometry keywords belong to the group parameters for each chip.

For all instruments other than NICMOS, PHOTFLAM is defined to be the mean flux density F in units of erg cm^-2 s-1 Å-1 that produces 1 count per second in the HST observing mode (PHOTMODE) used for the observation. If the F spectrum of your source is significantly sloped across the bandpass or contains prominent features, such as strong emission lines, you may wish to recalculate the inverse sensitivity using synphot, described below. WF/PC-1 observers should note that the PHOTFLAM value calculated during pipeline processing does not include a correction for temporal variations in throughput owing to contamination buildup. Likewise, FOC observers should note that PHOTFLAM values determined by the pipeline before May 18, 1994 do not account for sensitivity differences in formats other than 512 x 512 (see "Format-Dependent Sensitivity" on page 7-10).

To convert from counts or DN to flux in units of erg cm^-2 s-1 Å-1, multiply the total number of counts by the value of the PHOTFLAM header keyword and divide by the value of the EXPTIME keyword (exposure time). You can use the STSDAS task imcalc to convert an entire image from counts to flux units. For example, to create a flux-calibrated output image outimg.fits from an input image inimg.fits[1] with header keywords PHOTFLAM = 2.5E-18 and EXPTIME = 1000.0, you could type:

st> imcalc inimg.fits[1] outimg.fits "im1*2.5E-18/1000.0" 

Calibrated NICMOS data are in units of DN s-1, so the PHOTFLAM values in their headers are in units of erg cm^-2 Å-1. You can simply multiply these images by the value of PHOTFLAM to obtain fluxes in units of erg cm^-2 s-1 Å-1. NICMOS headers also contain the keyword PHOTFNU in units of Jy s. Multiplying your image by the PHOTFNU value will therefore yield fluxes in Janskys.



From:     http://www.stsci.edu/documents/dhb/web/c03_stsdas.fm3.html


Here is another website for the ACS header: 

Photometric Systems:

• Flux : The average flux F in erg cm^-2 s^-1 Ang^-1 over an ACS bandpass is F=N*PHOTFLAM, where N is the count rate in infinite aperture. For count rates N(ap) in smaller apertures, N=N(ap)/EE, where EE is the fractional encircled energy. (See the ACS Data Handbook and ISR-IV.)
• VEGAmag : Magnitude system where Vega has magnitude 0 at all wavelengths by definition. The vega magnitude of a star with flux F is -2.5 log10 (F/F_vega) where F_vega is the current flux spectrum of Vega from the CALSPEC archive.
• STmag and ABmag: Both systems define the absolute physical flux density for a point source. The conversion is chosen so that the magnitude at V corresponds roughly to that in the Johnson system. In the STmag system, the flux density is expressed per unit wavelength, while in the ABmag system, the flux density is expressed per unit frequency. The definitions are:
  • STmag = -2.5 Log F_lam -21.10
  • ABmag = -2.5 Log F_nu - 48.60

where F_nu is expressed in erg cm^-2 s^-1 Hz^-1, and F_lam in erg cm^-2 s^-1 Ang^-1. An object with a constant flux distribution F_nu = 3.63 x 10^-20 erg cm^-2 s^-1 Hz^-1 at all wavelengths will have ABmag=0 at all wavelengths, and similarly an object with F_lam = 3.63 x 10^-9 erg cm^-2 s^-1 Ang^-1 will have STmag=0.

Photometric Keywords in the SCI extention of ACS images:

(keywords affected by the sensitivity curve update are highlighted)

• PHOTMODE: Observation configuration for photometric calibration.
• PHOTFLAM: inverse sensitivity (erg cm^-2 s^-1 Ang^-1).
• PHOTZPT: ST magnitude zeropopint (= -21.10).
• PHOTPLAM: pivot wavelength.

The header keywords PHOTFLAM and PHOTPLAM relate to the STMAG and ABMAG zeropoints through the formulae:

• STMAG_ZEROPOINT = -2.5 Log (PHOTFLAM) - 21.10
• ABMAG_ZEROPOINT=-2.5 Log(PHOTFLAM)-5 Log(PHOTPLAM)-2.408

From:  http://www.stsci.edu/hst/acs/analysis/zeropoints