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How can I convert from OM count rates to fluxes?

Any XMM-Newton users, who need to convert OM count rates in the optical and UV filters into fluxes (expressed, e.g., in erg/cm/cm/s/A), can do it by following any of the methods we outline below.

OM count rates can be obtained with SAS. The chain omichain allows any user to process OM image data. Among other products, it produces source list files where the count rates for all objects detected in an image are given. These rates, as from SAS 6.5, are corrected from all known instrumental effects, mainly coincidence losses, dead time and time sensitivity degradation. The count rates can also be obtained from the PPS pipeline products delivered by the SSC at Leicester.

In case a user wants to measure the count rates by him/herself, we provide a recipe at the end of this document.

Method 1 (flux conversion based in white dwarf standard stars observations)

  1. multiply the count rate obtained with SAS (files *SWSRLI*, corrected count rate in CORR_RATE column) by the corresponding average conversion factors listed in the following Table.
    Filter Effective
    Conversion factor
    V 543 2.49E-16
    B 450 1.29E-16
    U 344 1.94E-16
    UVW1 291 4.76E-16
    UVM2 231 2.20E-15
    UVW2 212 5.71E-15

    These conversion factors have been derived from observations of white dwarf standard stars. They give an average transformation which can be used in all cases where no information is available on the spectral characteristics of the source. The error of the fluxes obtained by using these factors is less than 10%.

    In case of stellar objects for which the spectral type is known, the user may prefer to use Method 2.

Method 2 (spectral type dependent flux conversion)

  1. choose the most appropriate spectral type (in case of white dwarf, refer to Method 1)
  2. convert from count rate, obtained with SAS (files *SWSRLI*, corrected count rate in CORR_RATE column), to flux, selecting the most appropriate factor for the chosen spectral type and filter from the following Tables:
    Filter A0V B0V F0V G0V K0V M0V Vega
    V 2.50E-16 2.48E-16 2.52E-16 2.54E-16 2.56E-16 2.65E-16 2.50E-16
    B 1.36E-16 1.16E-16 1.41E-16 1.53E-16 1.60E-16 1.81E-16 1.34E-16
    U 1.71E-16 1.94E-16 1.80E-16 1.83E-16 1.88E-16 2.01E-16 1.70E-16
    UVW1 4.96E-16 4.72E-16 4.96E-16 4.51E-16 3.88E-16 1.09E-16 4.86E-16
    UVM2 2.20E-15 2.14E-15 2.10E-15 1.84E-15 1.66E-15    n.a. 2.19E-15
    UVW2 6.06E-15 5.56E-15 7.15E-15 6.05E-15 5.76E-16    n.a. 5.88E-15


These numbers have been obtained by folding the spectral library of Pickles with the in-flight response curves of the OM. For Vega we have used the alpha_lyr_stis_002 calibrated spectrum provided by the HST Calspec database.

If we compare these factors with the ones derived from white dwarfs (Method 1) we see that they are very similar except for the UV filters in cool stars. These is mostly due to the fact that the library spectra for these stars have very low or zero flux at ultraviolet wavelengths.

Similar results are obtained with a different library: the Bruzual-Persson-Gunn-Stryker Spectra:


Filter A0V B0Ib F0IV G0V K0V M0V
V 2.48E-16 2.50E-16 2.50E-16 2.55E-16 2.56E-16 2.61E-16
B 1.29E-16 1.17E-16 1.38E-16 1.44E-16 1.55E-16 1.80E-16
U 1.66E-16 1.97E-16 1.77E-16 1.88E-16 1.85E-16 1.94E-16
UVW1 4.79E-16 4.76E-16 4.84E-16 5.02E-16 5.15E-16 3.14E-16
UVM2 2.15E-15 2.17E-15 2.18E-15 2.27E-15 2.02E-15 1.42E-15
UVW2 5.56E-15 5.25E-15 6.14E-15 6.50E-15 6.34E-15 2.46E-15


As in the case of Pickles library, the UV part of the spectra for cool stars is not reliable.

A calibration observing campaign is currently being carried out with OM in order to obtain the most reliable conversion factors for each stellar type. The users are invited to check periodically the content of this page, to ensure that the most updated calibrations are always being employed.


Method 3 (AB magnitude system)

The OM Team has implemented the AB magnitude system for OM. Data processed with SAS in the SSC general re-processing, or using a version higher than 6.5 will contain these magnitudes and also the corresponding fluxes in the combined source list file.

The AB system is defined for OM as:

mAB(filter) = Zero_point(filter) - 2.5 log10(Count_Rate)


where the Zero points for each filter come from the original definition of AB magnitude as:

Zero_point(filter) = -48.60 - 2.5 log10(1/n_phot)


with n_phot being the count rate measured in a filter for a constant incoming flux of one erg/sec/cm2/hz. The calculated zero points for the OM AB system are then:

Filter Zero-point
V 17.923
B 19.081
U 19.189
UVW1 18.566
UVM2 17.412
UVW2 16.572


The inverse of n_phot provides a count rate to flux conversion factor (in frequency space) for each filter:

Filter Conversion factor
V 2.46E-27
B 8.47E-28
U 7.66E-28
UVW1 1.36E-27
UVM2 3.94E-27
UVW2 8.54E-27


These conversion factors are obtained by folding a constant flux in frequency space of one erg/sec/cm2/hz with the in-flight response curves of the OM.
The equivalent count rate to flux conversion factors in wavelength space for each filter:  

Filter Conversion factor
V 2.50E-16
B 1.25E-16
U 1.94E-16
UVW1 4.82E-16
UVM2 2.21E-15
UVW2 5.70E-15


Method 4 (General method. Flux based on Vega flux scale)

This describes the whole process. Steps 1 to 7 are currently performed by SAS. The magnitudes of the detected objects are written in the source lists for each exposure (*SWSRLI* file) and in the final combined list (*OBSMLI* file)

  1. determine the source+background counts within an aperture of 12 pixels radius (unbinned) for U, B and V filters. For UVW1, UVM2 and UVW2, the counts measured within the 12 pixels aperture are extrapolated to 35 by using the PSF definition. Users doing this "by hand" should take an aperture of 35 pixels. In case of binned images (2x2) aperture sizes shoudl be reduced by half.
  2. determine the background counts within an annulus of radii 14 and 25 pixels (unbinned) for U, B and V filters (8 and 13 in binned images). For UVW1, UVM2 and UVW2 the inner radius should be at least 37
  3. apply the coincidence loss and deadtime corrections to the source+background counts as specified in the XMM-Newton User Handbook
  4. subtract the background counts
  5. correct for time sensitivity degradation as described in these Evergreen tips and tricks
  6. calculate the instrumental magnitude, using the formula:
    mag = -2.5 log10 (count rate) + zero-point

    where the zero-points are reported in the following table:
    Filter Zero-point
    V 17.963
    B 19.266
    U 18.259
    UVW1 17.204
    UVM2 15.772
    UVW2 14.867


    This is the way SAS uses to derive the magnitude of any detected star in an OM image. The use of SAS is strongly recommended.

  7. Vega is used as a reference to determine the magnitude to flux conversion. The flux of Vega should correspond to a magnitude 0.03 in OM B,V filters, and 0.025 in OM U and UV filters. The Vega fluxes in the OM filters are listed in the following Table:
    Filter Effective wavelength (nm) Flux (erg/cm2/s/A)
    V 543 3.70E-09
    B 450 6.40E-09
    U 344 3.20E-09
    UVW1 291 3.68E-09
    UVM2 231 4.33E-09
    UVW2 212 5.03E-09


    The quoted fluxes for Vega have been obtained by interpolation in its calibrated spectrum provided by the HST Calspec database.

  8. apply the following formula for a given filter
    mag (Vega) - mag (source) = -2.5 log [Flux(Vega)/Flux(source)]