The Effective Area
for all OM filters and corresponding Canned OM Response Matrices
can be downloaded here
. See notes below.
Canned response matrices
The OM response matrices
for all filters (UVW2, UVM2, UVW1, U, B, V) have been obtained by computing the effective area of the instrument from ground laboratory measurements of its various components (mirror reflectivity, filter transmission, photocathode quantum efficiency, microchannel plates and detector efficiency,...). In order to match the observed in-flight response, fudge factors were necessary. Those fudge factors were obtained by observing a series of standard stars. The stars used are the white dwarfs GD153, GD50, HZ2, HZ4, G93-48, LTT9491 and LB 227 and their absolutely calibrated energy distributions have been obtained from STScI CALSPEC database:
In addition to the broad band filters, OM has two grisms working respectively in the Visible and UV domains. We have produced as well response matrices for both grisms. The OM grisms have an absolute flux calibration in the form of an Inverse Sensitivity Function (ISF). The ISF provides a direct conversion from count rate in the extracted spectrum to absolute flux. This ISF can be easily converted into effective area for both grisms. Software to produce a grism response file and OGIP compliant spectral file from the output of omgchain is available and can be downloaded here
Generation and accuracy
OM data were processed with SAS to obtain the count rates for the stars in all filters. These count rates are corrected for time sensitivity degradation, and therefore all observations can be compared and averaged (in case of multiple observations).
Since the fluxes of the stars are well known, then the observed counts are compared with the on-ground predictions and in these way an average fudge factor can be obtained for each filter. The obtained fudge factors are then included into the effective area of each filter.
The need for the fudge factors comes from possible molecular contamination in the optical surfaces. It has been proven that the computed values agree with contamination models for a given concentration of contaminating material.
In the case of the grisms, the ISFs have been obtained from observations of the spectrophotometric standards GD153 and HZ2. Several spectra of both stars obtained with OM have been averaged and compared with their standard reference flux. The ratio between extracted count rate and flux gives us the ISF for each grism.
Since the fluxes of the standard stars are provided with better than 10% accuracy we can adopt this value as a limit for the errors of the response matrices.
The response files have the name om_effarea_<filter>_v2.0.rsp.
The response files should be used in conjunction with a spectral data file containing the filter count rate. Such a data file may be created by editing the template filter file om_filter_default.pi with a fits editor such as 'fv' and changing the "RATE" and "STAT_ERR" entries in the SPECTRUM extension to the filter count rate and error. The total error may be found by adding the statistical filter measurement error given by the OM processing, in quadrature with the absolute calibration error given above.
The OM grism data may be converted to OGIP format for use in spectral fitting packages such as XSPEC by:
- Run omgchain on the dataset to produce a spectral file with a name such as: p0125910501OMS005SPECTR0000.FIT
- Execute the program grismresp to produce a spectrum and response file in the required format. e.g.
Name of input spectrum
Name of output spectrum
Name of output response matrix
XSPEC> data mysrc_omgrismV.pi
Net count rate (cts/s) for file 1 264.0 +/- 0.4345
using response (RMF) file... mysrc_omgrismV.rsp
1 data set is in use
The grismresp software uses effective area datafiles, OM_UVgrism_efArea.txt and OM_Vgrism_efArea.txt which should be present in the current directory or held in a separate directory pointed to by the environment variable $OMGRISMCAL